From 8bd01d2d9bbda1a2e044223db4b2272e7c5020a0 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sat, 4 Nov 2023 13:57:29 -0700
Subject: [PATCH 01/11] zig reduce: add transformation for inlining file-based
 `@import`

One thing is missing for it to be useful, however, which is dealing with
ambiguous reference errors introduced by the inlining process.
---
 lib/std/zig/render.zig | 32 +++++++++++++++++++++---
 src/reduce.zig         | 57 ++++++++++++++++++++++++++++++++++--------
 src/reduce/Walk.zig    | 39 ++++++++++++++++++++++++-----
 3 files changed, 108 insertions(+), 20 deletions(-)

diff --git a/lib/std/zig/render.zig b/lib/std/zig/render.zig
index 45aaf1bd28..ce63c08533 100644
--- a/lib/std/zig/render.zig
+++ b/lib/std/zig/render.zig
@@ -24,8 +24,11 @@ pub const Fixups = struct {
     gut_functions: std.AutoHashMapUnmanaged(Ast.Node.Index, void) = .{},
     /// These global declarations will be omitted.
     omit_nodes: std.AutoHashMapUnmanaged(Ast.Node.Index, void) = .{},
-    /// These expressions will be replaced with `undefined`.
-    replace_nodes: std.AutoHashMapUnmanaged(Ast.Node.Index, void) = .{},
+    /// These expressions will be replaced with the string value.
+    replace_nodes: std.AutoHashMapUnmanaged(Ast.Node.Index, []const u8) = .{},
+    /// All `@import` builtin calls which refer to a file path will be prefixed
+    /// with this path.
+    rebase_imported_paths: ?[]const u8 = null,
 
     pub fn count(f: Fixups) usize {
         return f.unused_var_decls.count() +
@@ -277,8 +280,8 @@ fn renderExpression(r: *Render, node: Ast.Node.Index, space: Space) Error!void {
     const main_tokens = tree.nodes.items(.main_token);
     const node_tags = tree.nodes.items(.tag);
     const datas = tree.nodes.items(.data);
-    if (r.fixups.replace_nodes.contains(node)) {
-        try ais.writer().writeAll("undefined");
+    if (r.fixups.replace_nodes.get(node)) |replacement| {
+        try ais.writer().writeAll(replacement);
         try renderOnlySpace(r, space);
         return;
     }
@@ -1515,6 +1518,7 @@ fn renderBuiltinCall(
     const tree = r.tree;
     const ais = r.ais;
     const token_tags = tree.tokens.items(.tag);
+    const main_tokens = tree.nodes.items(.main_token);
 
     // TODO remove before release of 0.12.0
     const slice = tree.tokenSlice(builtin_token);
@@ -1609,6 +1613,26 @@ fn renderBuiltinCall(
         return renderToken(r, builtin_token + 2, space); // )
     }
 
+    if (r.fixups.rebase_imported_paths) |prefix| {
+        if (mem.eql(u8, slice, "@import")) f: {
+            const param = params[0];
+            const str_lit_token = main_tokens[param];
+            assert(token_tags[str_lit_token] == .string_literal);
+            const token_bytes = tree.tokenSlice(str_lit_token);
+            const imported_string = std.zig.string_literal.parseAlloc(r.gpa, token_bytes) catch |err| switch (err) {
+                error.OutOfMemory => return error.OutOfMemory,
+                error.InvalidLiteral => break :f,
+            };
+            defer r.gpa.free(imported_string);
+            const new_string = try std.fs.path.resolvePosix(r.gpa, &.{ prefix, imported_string });
+            defer r.gpa.free(new_string);
+
+            try renderToken(r, builtin_token + 1, .none); // (
+            try ais.writer().print("\"{}\"", .{std.zig.fmtEscapes(new_string)});
+            return renderToken(r, str_lit_token + 1, space); // )
+        }
+    }
+
     const last_param = params[params.len - 1];
     const after_last_param_token = tree.lastToken(last_param) + 1;
 
diff --git a/src/reduce.zig b/src/reduce.zig
index 878f99a0fa..82e4bc9031 100644
--- a/src/reduce.zig
+++ b/src/reduce.zig
@@ -109,8 +109,11 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
     var rendered = std.ArrayList(u8).init(gpa);
     defer rendered.deinit();
 
-    var tree = try parse(gpa, arena, root_source_file_path);
-    defer tree.deinit(gpa);
+    var tree = try parse(gpa, root_source_file_path);
+    defer {
+        gpa.free(tree.source);
+        tree.deinit(gpa);
+    }
 
     if (!skip_smoke_test) {
         std.debug.print("smoke testing the interestingness check...\n", .{});
@@ -159,7 +162,7 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
             subset_size = @max(1, subset_size / 2);
 
             const this_set = transformations.items[start_index..][0..subset_size];
-            try transformationsToFixups(gpa, this_set, &fixups);
+            try transformationsToFixups(gpa, arena, root_source_file_path, this_set, &fixups);
 
             rendered.clearRetainingCapacity();
             try tree.renderToArrayList(&rendered, fixups);
@@ -171,7 +174,8 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
             });
             switch (interestingness) {
                 .interesting => {
-                    const new_tree = try parse(gpa, arena, root_source_file_path);
+                    const new_tree = try parse(gpa, root_source_file_path);
+                    gpa.free(tree.source);
                     tree.deinit(gpa);
                     tree = new_tree;
 
@@ -241,6 +245,8 @@ fn runCheck(arena: std.mem.Allocator, argv: []const []const u8) !Interestingness
 
 fn transformationsToFixups(
     gpa: Allocator,
+    arena: Allocator,
+    root_source_file_path: []const u8,
     transforms: []const Walk.Transformation,
     fixups: *Ast.Fixups,
 ) !void {
@@ -254,20 +260,51 @@ fn transformationsToFixups(
             try fixups.omit_nodes.put(gpa, decl_node, {});
         },
         .replace_with_undef => |node| {
-            try fixups.replace_nodes.put(gpa, node, {});
+            try fixups.replace_nodes.put(gpa, node, "undefined");
+        },
+        .inline_imported_file => |inline_imported_file| {
+            defer gpa.free(inline_imported_file.imported_string);
+            const full_imported_path = try std.fs.path.join(gpa, &.{
+                std.fs.path.dirname(root_source_file_path) orelse ".",
+                inline_imported_file.imported_string,
+            });
+            defer gpa.free(full_imported_path);
+            var other_file_ast = try parse(gpa, full_imported_path);
+            defer {
+                gpa.free(other_file_ast.source);
+                other_file_ast.deinit(gpa);
+            }
+            var other_source = std.ArrayList(u8).init(gpa);
+            defer other_source.deinit();
+            var inlined_fixups: Ast.Fixups = .{};
+            defer inlined_fixups.deinit(gpa);
+            try other_source.appendSlice("struct {\n");
+            try other_file_ast.renderToArrayList(&other_source, .{
+                .rebase_imported_paths = std.fs.path.dirname(inline_imported_file.imported_string),
+            });
+            try other_source.appendSlice("}");
+
+            try fixups.replace_nodes.put(
+                gpa,
+                inline_imported_file.builtin_call_node,
+                try arena.dupe(u8, other_source.items),
+            );
         },
     };
 }
 
-fn parse(gpa: Allocator, arena: Allocator, root_source_file_path: []const u8) !Ast {
-    const source_code = try std.fs.cwd().readFileAllocOptions(
-        arena,
-        root_source_file_path,
+fn parse(gpa: Allocator, file_path: []const u8) !Ast {
+    const source_code = std.fs.cwd().readFileAllocOptions(
+        gpa,
+        file_path,
         std.math.maxInt(u32),
         null,
         1,
         0,
-    );
+    ) catch |err| {
+        fatal("unable to open '{s}': {s}", .{ file_path, @errorName(err) });
+    };
+    errdefer gpa.free(source_code);
 
     var tree = try Ast.parse(gpa, source_code, .zig);
     errdefer tree.deinit(gpa);
diff --git a/src/reduce/Walk.zig b/src/reduce/Walk.zig
index 5ef341fcbf..c68e58a4fa 100644
--- a/src/reduce/Walk.zig
+++ b/src/reduce/Walk.zig
@@ -2,6 +2,7 @@ const std = @import("std");
 const Ast = std.zig.Ast;
 const Walk = @This();
 const assert = std.debug.assert;
+const BuiltinFn = @import("../BuiltinFn.zig");
 
 ast: *const Ast,
 transformations: *std.ArrayList(Transformation),
@@ -16,6 +17,11 @@ pub const Transformation = union(enum) {
     delete_node: Ast.Node.Index,
     /// Replace an expression with `undefined`.
     replace_with_undef: Ast.Node.Index,
+    /// Replace an `@import` with the imported file contents wrapped in a struct.
+    inline_imported_file: struct {
+        builtin_call_node: Ast.Node.Index,
+        imported_string: []const u8,
+    },
 };
 
 pub const Error = error{OutOfMemory};
@@ -437,16 +443,16 @@ fn walkExpression(w: *Walk, node: Ast.Node.Index) Error!void {
 
         .builtin_call_two, .builtin_call_two_comma => {
             if (datas[node].lhs == 0) {
-                return walkBuiltinCall(w, main_tokens[node], &.{});
+                return walkBuiltinCall(w, node, &.{});
             } else if (datas[node].rhs == 0) {
-                return walkBuiltinCall(w, main_tokens[node], &.{datas[node].lhs});
+                return walkBuiltinCall(w, node, &.{datas[node].lhs});
             } else {
-                return walkBuiltinCall(w, main_tokens[node], &.{ datas[node].lhs, datas[node].rhs });
+                return walkBuiltinCall(w, node, &.{ datas[node].lhs, datas[node].rhs });
             }
         },
         .builtin_call, .builtin_call_comma => {
             const params = ast.extra_data[datas[node].lhs..datas[node].rhs];
-            return walkBuiltinCall(w, main_tokens[node], params);
+            return walkBuiltinCall(w, node, params);
         },
 
         .fn_proto_simple,
@@ -680,10 +686,31 @@ fn walkContainerDecl(
 
 fn walkBuiltinCall(
     w: *Walk,
-    builtin_token: Ast.TokenIndex,
+    call_node: Ast.Node.Index,
     params: []const Ast.Node.Index,
 ) Error!void {
-    _ = builtin_token;
+    const ast = w.ast;
+    const gpa = w.gpa;
+    const main_tokens = ast.nodes.items(.main_token);
+    const builtin_token = main_tokens[call_node];
+    const builtin_name = ast.tokenSlice(builtin_token);
+    const info = BuiltinFn.list.get(builtin_name).?;
+    switch (info.tag) {
+        .import => {
+            const operand_node = params[0];
+            const str_lit_token = main_tokens[operand_node];
+            const token_bytes = ast.tokenSlice(str_lit_token);
+            const imported_string = std.zig.string_literal.parseAlloc(gpa, token_bytes) catch
+                unreachable;
+            if (std.mem.endsWith(u8, imported_string, ".zig")) {
+                try w.transformations.append(.{ .inline_imported_file = .{
+                    .builtin_call_node = call_node,
+                    .imported_string = imported_string,
+                } });
+            }
+        },
+        else => {},
+    }
     for (params) |param_node| {
         try walkExpression(w, param_node);
     }

From 802c82b0720ff2b7c59ba96effbdd1eed7491df4 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sat, 4 Nov 2023 15:46:30 -0700
Subject: [PATCH 02/11] std.ArrayHashMap: add init function

for when you have keys and values array
---
 lib/std/array_hash_map.zig | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/lib/std/array_hash_map.zig b/lib/std/array_hash_map.zig
index 75a86f63f6..bf5f6581ac 100644
--- a/lib/std/array_hash_map.zig
+++ b/lib/std/array_hash_map.zig
@@ -574,6 +574,19 @@ pub fn ArrayHashMapUnmanaged(
             };
         }
 
+        pub fn init(allocator: Allocator, key_list: []const K, value_list: []const V) !Self {
+            var self: Self = .{};
+            try self.entries.resize(allocator, key_list.len);
+            errdefer self.entries.deinit(allocator);
+            @memcpy(self.keys(), key_list);
+            if (@sizeOf(V) != 0) {
+                assert(key_list.len == value_list.len);
+                @memcpy(self.values(), value_list);
+            }
+            try self.reIndex(allocator);
+            return self;
+        }
+
         /// Frees the backing allocation and leaves the map in an undefined state.
         /// Note that this does not free keys or values.  You must take care of that
         /// before calling this function, if it is needed.

From fc1e7a56441aea4a9394c4e8c0142110891b4a1a Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sat, 4 Nov 2023 15:48:28 -0700
Subject: [PATCH 03/11] zig reduce: rename identifiers when inlining an
 `@import`

---
 lib/std/zig/render.zig | 18 +++++++++-
 src/reduce.zig         | 42 ++++++++++++++++++-----
 src/reduce/Walk.zig    | 78 +++++++++++++++++++++++++++++++++---------
 3 files changed, 112 insertions(+), 26 deletions(-)

diff --git a/lib/std/zig/render.zig b/lib/std/zig/render.zig
index ce63c08533..348b75159f 100644
--- a/lib/std/zig/render.zig
+++ b/lib/std/zig/render.zig
@@ -26,6 +26,9 @@ pub const Fixups = struct {
     omit_nodes: std.AutoHashMapUnmanaged(Ast.Node.Index, void) = .{},
     /// These expressions will be replaced with the string value.
     replace_nodes: std.AutoHashMapUnmanaged(Ast.Node.Index, []const u8) = .{},
+    /// Change all identifier names matching the key to be value instead.
+    rename_identifiers: std.StringArrayHashMapUnmanaged([]const u8) = .{},
+
     /// All `@import` builtin calls which refer to a file path will be prefixed
     /// with this path.
     rebase_imported_paths: ?[]const u8 = null,
@@ -34,7 +37,9 @@ pub const Fixups = struct {
         return f.unused_var_decls.count() +
             f.gut_functions.count() +
             f.omit_nodes.count() +
-            f.replace_nodes.count();
+            f.replace_nodes.count() +
+            f.rename_identifiers.count() +
+            @intFromBool(f.rebase_imported_paths != null);
     }
 
     pub fn clearRetainingCapacity(f: *Fixups) void {
@@ -42,6 +47,9 @@ pub const Fixups = struct {
         f.gut_functions.clearRetainingCapacity();
         f.omit_nodes.clearRetainingCapacity();
         f.replace_nodes.clearRetainingCapacity();
+        f.rename_identifiers.clearRetainingCapacity();
+
+        f.rebase_imported_paths = null;
     }
 
     pub fn deinit(f: *Fixups, gpa: Allocator) void {
@@ -49,6 +57,7 @@ pub const Fixups = struct {
         f.gut_functions.deinit(gpa);
         f.omit_nodes.deinit(gpa);
         f.replace_nodes.deinit(gpa);
+        f.rename_identifiers.deinit(gpa);
         f.* = undefined;
     }
 };
@@ -2833,6 +2842,13 @@ fn renderIdentifier(r: *Render, token_index: Ast.TokenIndex, space: Space, quote
     const token_tags = tree.tokens.items(.tag);
     assert(token_tags[token_index] == .identifier);
     const lexeme = tokenSliceForRender(tree, token_index);
+
+    if (r.fixups.rename_identifiers.get(lexeme)) |mangled| {
+        try r.ais.writer().writeAll(mangled);
+        try renderSpace(r, token_index, lexeme.len, space);
+        return;
+    }
+
     if (lexeme[0] != '@') {
         return renderToken(r, token_index, space);
     }
diff --git a/src/reduce.zig b/src/reduce.zig
index 82e4bc9031..d8af4dddc3 100644
--- a/src/reduce.zig
+++ b/src/reduce.zig
@@ -148,7 +148,7 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
 
     var transformations = std.ArrayList(Walk.Transformation).init(gpa);
     defer transformations.deinit();
-    try Walk.findTransformations(&tree, &transformations);
+    try Walk.findTransformations(arena, &tree, &transformations);
     sortTransformations(transformations.items, rng.random());
 
     fresh: while (transformations.items.len > 0) {
@@ -162,12 +162,19 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
             subset_size = @max(1, subset_size / 2);
 
             const this_set = transformations.items[start_index..][0..subset_size];
+            std.debug.print("trying {d} random transformations: ", .{subset_size});
+            for (this_set[0..@min(this_set.len, 20)]) |t| {
+                std.debug.print("{s} ", .{@tagName(t)});
+            }
+            std.debug.print("\n", .{});
             try transformationsToFixups(gpa, arena, root_source_file_path, this_set, &fixups);
 
             rendered.clearRetainingCapacity();
             try tree.renderToArrayList(&rendered, fixups);
             try std.fs.cwd().writeFile(root_source_file_path, rendered.items);
 
+            //std.debug.print("trying this code:\n{s}\n", .{rendered.items});
+
             const interestingness = try runCheck(arena, interestingness_argv.items);
             std.debug.print("{d} random transformations: {s}. {d} remaining\n", .{
                 subset_size, @tagName(interestingness), transformations.items.len - start_index,
@@ -179,7 +186,7 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
                     tree.deinit(gpa);
                     tree = new_tree;
 
-                    try Walk.findTransformations(&tree, &transformations);
+                    try Walk.findTransformations(arena, &tree, &transformations);
                     // Resetting based on the seed again means we will get the same
                     // results if restarting the reduction process from this new point.
                     rng = std.rand.DefaultPrng.init(seed);
@@ -263,7 +270,6 @@ fn transformationsToFixups(
             try fixups.replace_nodes.put(gpa, node, "undefined");
         },
         .inline_imported_file => |inline_imported_file| {
-            defer gpa.free(inline_imported_file.imported_string);
             const full_imported_path = try std.fs.path.join(gpa, &.{
                 std.fs.path.dirname(root_source_file_path) orelse ".",
                 inline_imported_file.imported_string,
@@ -274,14 +280,34 @@ fn transformationsToFixups(
                 gpa.free(other_file_ast.source);
                 other_file_ast.deinit(gpa);
             }
-            var other_source = std.ArrayList(u8).init(gpa);
-            defer other_source.deinit();
+
             var inlined_fixups: Ast.Fixups = .{};
             defer inlined_fixups.deinit(gpa);
+            if (std.fs.path.dirname(inline_imported_file.imported_string)) |dirname| {
+                inlined_fixups.rebase_imported_paths = dirname;
+            }
+            for (inline_imported_file.in_scope_names.keys()) |name| {
+                // This name needs to be mangled in order to not cause an
+                // ambiguous reference error.
+                var i: u32 = 2;
+                const mangled = while (true) : (i += 1) {
+                    const mangled = try std.fmt.allocPrint(gpa, "{s}{d}", .{ name, i });
+                    if (!inline_imported_file.in_scope_names.contains(mangled))
+                        break mangled;
+                    gpa.free(mangled);
+                };
+                try inlined_fixups.rename_identifiers.put(gpa, name, mangled);
+            }
+            defer {
+                for (inlined_fixups.rename_identifiers.values()) |v| {
+                    gpa.free(v);
+                }
+            }
+
+            var other_source = std.ArrayList(u8).init(gpa);
+            defer other_source.deinit();
             try other_source.appendSlice("struct {\n");
-            try other_file_ast.renderToArrayList(&other_source, .{
-                .rebase_imported_paths = std.fs.path.dirname(inline_imported_file.imported_string),
-            });
+            try other_file_ast.renderToArrayList(&other_source, inlined_fixups);
             try other_source.appendSlice("}");
 
             try fixups.replace_nodes.put(
diff --git a/src/reduce/Walk.zig b/src/reduce/Walk.zig
index c68e58a4fa..d62cc4e905 100644
--- a/src/reduce/Walk.zig
+++ b/src/reduce/Walk.zig
@@ -7,7 +7,9 @@ const BuiltinFn = @import("../BuiltinFn.zig");
 ast: *const Ast,
 transformations: *std.ArrayList(Transformation),
 unreferenced_globals: std.StringArrayHashMapUnmanaged(Ast.Node.Index),
+in_scope_names: std.StringArrayHashMapUnmanaged(u32),
 gpa: std.mem.Allocator,
+arena: std.mem.Allocator,
 
 pub const Transformation = union(enum) {
     /// Replace the fn decl AST Node with one whose body is only `@trap()` with
@@ -18,25 +20,39 @@ pub const Transformation = union(enum) {
     /// Replace an expression with `undefined`.
     replace_with_undef: Ast.Node.Index,
     /// Replace an `@import` with the imported file contents wrapped in a struct.
-    inline_imported_file: struct {
+    inline_imported_file: InlineImportedFile,
+
+    pub const InlineImportedFile = struct {
         builtin_call_node: Ast.Node.Index,
         imported_string: []const u8,
-    },
+        /// Identifier names that must be renamed in the inlined code or else
+        /// will cause ambiguous reference errors.
+        in_scope_names: std.StringArrayHashMapUnmanaged(void),
+    };
 };
 
 pub const Error = error{OutOfMemory};
 
 /// The result will be priority shuffled.
-pub fn findTransformations(ast: *const Ast, transformations: *std.ArrayList(Transformation)) !void {
+pub fn findTransformations(
+    arena: std.mem.Allocator,
+    ast: *const Ast,
+    transformations: *std.ArrayList(Transformation),
+) !void {
     transformations.clearRetainingCapacity();
 
     var walk: Walk = .{
         .ast = ast,
         .transformations = transformations,
         .gpa = transformations.allocator,
+        .arena = arena,
         .unreferenced_globals = .{},
+        .in_scope_names = .{},
     };
-    defer walk.unreferenced_globals.deinit(walk.gpa);
+    defer {
+        walk.unreferenced_globals.deinit(walk.gpa);
+        walk.in_scope_names.deinit(walk.gpa);
+    }
 
     try walkMembers(&walk, walk.ast.rootDecls());
 
@@ -49,14 +65,18 @@ pub fn findTransformations(ast: *const Ast, transformations: *std.ArrayList(Tran
 
 fn walkMembers(w: *Walk, members: []const Ast.Node.Index) Error!void {
     // First we scan for globals so that we can delete them while walking.
-    try scanDecls(w, members);
+    try scanDecls(w, members, .add);
 
     for (members) |member| {
         try walkMember(w, member);
     }
+
+    try scanDecls(w, members, .remove);
 }
 
-fn scanDecls(w: *Walk, members: []const Ast.Node.Index) Error!void {
+const ScanDeclsAction = enum { add, remove };
+
+fn scanDecls(w: *Walk, members: []const Ast.Node.Index, action: ScanDeclsAction) Error!void {
     const ast = w.ast;
     const gpa = w.gpa;
     const node_tags = ast.nodes.items(.tag);
@@ -80,9 +100,27 @@ fn scanDecls(w: *Walk, members: []const Ast.Node.Index) Error!void {
 
             else => continue,
         };
+
         assert(token_tags[name_token] == .identifier);
         const name_bytes = ast.tokenSlice(name_token);
-        try w.unreferenced_globals.put(gpa, name_bytes, member_node);
+
+        switch (action) {
+            .add => {
+                try w.unreferenced_globals.put(gpa, name_bytes, member_node);
+
+                const gop = try w.in_scope_names.getOrPut(gpa, name_bytes);
+                if (!gop.found_existing) gop.value_ptr.* = 0;
+                gop.value_ptr.* += 1;
+            },
+            .remove => {
+                const entry = w.in_scope_names.getEntry(name_bytes).?;
+                if (entry.value_ptr.* <= 1) {
+                    assert(w.in_scope_names.swapRemove(name_bytes));
+                } else {
+                    entry.value_ptr.* -= 1;
+                }
+            },
+        }
     }
 }
 
@@ -567,12 +605,12 @@ fn walkLocalVarDecl(w: *Walk, var_decl: Ast.full.VarDecl) Error!void {
         try walkExpression(w, var_decl.ast.section_node);
     }
 
-    assert(var_decl.ast.init_node != 0);
-    if (!isUndefinedIdent(w.ast, var_decl.ast.init_node)) {
-        try w.transformations.append(.{ .replace_with_undef = var_decl.ast.init_node });
+    if (var_decl.ast.init_node != 0) {
+        if (!isUndefinedIdent(w.ast, var_decl.ast.init_node)) {
+            try w.transformations.append(.{ .replace_with_undef = var_decl.ast.init_node });
+        }
+        try walkExpression(w, var_decl.ast.init_node);
     }
-
-    return walkExpression(w, var_decl.ast.init_node);
 }
 
 fn walkContainerField(w: *Walk, field: Ast.full.ContainerField) Error!void {
@@ -582,7 +620,9 @@ fn walkContainerField(w: *Walk, field: Ast.full.ContainerField) Error!void {
     if (field.ast.align_expr != 0) {
         try walkExpression(w, field.ast.align_expr); // alignment
     }
-    try walkExpression(w, field.ast.value_expr); // value
+    if (field.ast.value_expr != 0) {
+        try walkExpression(w, field.ast.value_expr); // value
+    }
 }
 
 fn walkBlock(
@@ -690,7 +730,6 @@ fn walkBuiltinCall(
     params: []const Ast.Node.Index,
 ) Error!void {
     const ast = w.ast;
-    const gpa = w.gpa;
     const main_tokens = ast.nodes.items(.main_token);
     const builtin_token = main_tokens[call_node];
     const builtin_name = ast.tokenSlice(builtin_token);
@@ -700,12 +739,17 @@ fn walkBuiltinCall(
             const operand_node = params[0];
             const str_lit_token = main_tokens[operand_node];
             const token_bytes = ast.tokenSlice(str_lit_token);
-            const imported_string = std.zig.string_literal.parseAlloc(gpa, token_bytes) catch
-                unreachable;
-            if (std.mem.endsWith(u8, imported_string, ".zig")) {
+            if (std.mem.endsWith(u8, token_bytes, ".zig\"")) {
+                const imported_string = std.zig.string_literal.parseAlloc(w.arena, token_bytes) catch
+                    unreachable;
                 try w.transformations.append(.{ .inline_imported_file = .{
                     .builtin_call_node = call_node,
                     .imported_string = imported_string,
+                    .in_scope_names = try std.StringArrayHashMapUnmanaged(void).init(
+                        w.arena,
+                        w.in_scope_names.keys(),
+                        &.{},
+                    ),
                 } });
             }
         },

From 31ad3af956231357ffa2c0fcfaf96a95c2fe1e8b Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sat, 4 Nov 2023 17:37:34 -0700
Subject: [PATCH 04/11] std.zig.render: support fixing unused parameter

---
 lib/std/zig/render.zig | 82 ++++++++++++++++++++++++++++++++++--------
 1 file changed, 67 insertions(+), 15 deletions(-)

diff --git a/lib/std/zig/render.zig b/lib/std/zig/render.zig
index 348b75159f..f8ad44353f 100644
--- a/lib/std/zig/render.zig
+++ b/lib/std/zig/render.zig
@@ -133,6 +133,7 @@ fn renderMember(
 ) Error!void {
     const tree = r.tree;
     const ais = r.ais;
+    const node_tags = tree.nodes.items(.tag);
     const token_tags = tree.tokens.items(.tag);
     const main_tokens = tree.nodes.items(.main_token);
     const datas = tree.nodes.items(.data);
@@ -194,6 +195,45 @@ fn renderMember(
                 ais.popIndent();
                 try ais.insertNewline();
                 try renderToken(r, tree.lastToken(body_node), space); // rbrace
+            } else if (r.fixups.unused_var_decls.count() != 0) {
+                ais.pushIndentNextLine();
+                const lbrace = tree.nodes.items(.main_token)[body_node];
+                try renderToken(r, lbrace, .newline);
+
+                var fn_proto_buf: [1]Ast.Node.Index = undefined;
+                const full_fn_proto = tree.fullFnProto(&fn_proto_buf, fn_proto).?;
+                var it = full_fn_proto.iterate(&tree);
+                while (it.next()) |param| {
+                    const name_ident = param.name_token.?;
+                    assert(token_tags[name_ident] == .identifier);
+                    if (r.fixups.unused_var_decls.contains(name_ident)) {
+                        const w = ais.writer();
+                        try w.writeAll("_ = ");
+                        try w.writeAll(tokenSliceForRender(r.tree, name_ident));
+                        try w.writeAll(";\n");
+                    }
+                }
+                var statements_buf: [2]Ast.Node.Index = undefined;
+                const statements = switch (node_tags[body_node]) {
+                    .block_two,
+                    .block_two_semicolon,
+                    => b: {
+                        statements_buf = .{ datas[body_node].lhs, datas[body_node].rhs };
+                        if (datas[body_node].lhs == 0) {
+                            break :b statements_buf[0..0];
+                        } else if (datas[body_node].rhs == 0) {
+                            break :b statements_buf[0..1];
+                        } else {
+                            break :b statements_buf[0..2];
+                        }
+                    },
+                    .block,
+                    .block_semicolon,
+                    => tree.extra_data[datas[body_node].lhs..datas[body_node].rhs],
+
+                    else => unreachable,
+                };
+                return finishRenderBlock(r, body_node, statements, space);
             } else {
                 return renderExpression(r, body_node, space);
             }
@@ -1069,7 +1109,7 @@ fn renderVarDecl(
     space: Space,
 ) Error!void {
     try renderVarDeclWithoutFixups(r, var_decl, ignore_comptime_token, space);
-    if (r.fixups.unused_var_decls.contains(var_decl.ast.mut_token)) {
+    if (r.fixups.unused_var_decls.contains(var_decl.ast.mut_token + 1)) {
         // Discard the variable like this: `_ = foo;`
         const w = r.ais.writer();
         try w.writeAll("_ = ");
@@ -1967,7 +2007,6 @@ fn renderBlock(
     const tree = r.tree;
     const ais = r.ais;
     const token_tags = tree.tokens.items(.tag);
-    const node_tags = tree.nodes.items(.tag);
     const lbrace = tree.nodes.items(.main_token)[block_node];
 
     if (token_tags[lbrace - 1] == .colon and
@@ -1976,22 +2015,35 @@ fn renderBlock(
         try renderIdentifier(r, lbrace - 2, .none, .eagerly_unquote); // identifier
         try renderToken(r, lbrace - 1, .space); // :
     }
-
     ais.pushIndentNextLine();
     if (statements.len == 0) {
         try renderToken(r, lbrace, .none);
-    } else {
-        try renderToken(r, lbrace, .newline);
-        for (statements, 0..) |stmt, i| {
-            if (i != 0) try renderExtraNewline(r, stmt);
-            switch (node_tags[stmt]) {
-                .global_var_decl,
-                .local_var_decl,
-                .simple_var_decl,
-                .aligned_var_decl,
-                => try renderVarDecl(r, tree.fullVarDecl(stmt).?, false, .semicolon),
-                else => try renderExpression(r, stmt, .semicolon),
-            }
+        ais.popIndent();
+        try renderToken(r, tree.lastToken(block_node), space); // rbrace
+        return;
+    }
+    try renderToken(r, lbrace, .newline);
+    return finishRenderBlock(r, block_node, statements, space);
+}
+
+fn finishRenderBlock(
+    r: *Render,
+    block_node: Ast.Node.Index,
+    statements: []const Ast.Node.Index,
+    space: Space,
+) Error!void {
+    const tree = r.tree;
+    const node_tags = tree.nodes.items(.tag);
+    const ais = r.ais;
+    for (statements, 0..) |stmt, i| {
+        if (i != 0) try renderExtraNewline(r, stmt);
+        switch (node_tags[stmt]) {
+            .global_var_decl,
+            .local_var_decl,
+            .simple_var_decl,
+            .aligned_var_decl,
+            => try renderVarDecl(r, tree.fullVarDecl(stmt).?, false, .semicolon),
+            else => try renderExpression(r, stmt, .semicolon),
         }
     }
     ais.popIndent();

From 75efb736ce704fc43bdbe2410989c2b275d855e6 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sat, 4 Nov 2023 17:38:10 -0700
Subject: [PATCH 05/11] zig reduce: run results through astgen

and use that to fix up usused variable declarations and parameters that
are caused by transformations.

also add a transformation to replace global variable init with
`undefined`.
---
 src/reduce.zig      | 56 ++++++++++++++++++++++++++++++++++++++++++---
 src/reduce/Walk.zig |  9 +++++---
 2 files changed, 59 insertions(+), 6 deletions(-)

diff --git a/src/reduce.zig b/src/reduce.zig
index d8af4dddc3..0339eb004a 100644
--- a/src/reduce.zig
+++ b/src/reduce.zig
@@ -5,6 +5,8 @@ const assert = std.debug.assert;
 const fatal = @import("./main.zig").fatal;
 const Ast = std.zig.Ast;
 const Walk = @import("reduce/Walk.zig");
+const AstGen = @import("AstGen.zig");
+const Zir = @import("Zir.zig");
 
 const usage =
     \\zig reduce [options] ./checker root_source_file.zig [-- [argv]]
@@ -39,8 +41,6 @@ const Interestingness = enum { interesting, unknown, boring };
 // - add support for parsing the module flags
 // - more fancy transformations
 //   - @import inlining of modules
-//   - @import inlining of files
-//   - deleting unused functions and other globals
 //   - removing statements or blocks of code
 //   - replacing operands of `and` and `or` with `true` and `false`
 //   - replacing if conditions with `true` and `false`
@@ -109,6 +109,9 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
     var rendered = std.ArrayList(u8).init(gpa);
     defer rendered.deinit();
 
+    var astgen_input = std.ArrayList(u8).init(gpa);
+    defer astgen_input.deinit();
+
     var tree = try parse(gpa, root_source_file_path);
     defer {
         gpa.free(tree.source);
@@ -129,6 +132,10 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
 
     var fixups: Ast.Fixups = .{};
     defer fixups.deinit(gpa);
+
+    var more_fixups: Ast.Fixups = .{};
+    defer more_fixups.deinit(gpa);
+
     var rng = std.rand.DefaultPrng.init(seed);
 
     // 1. Walk the AST of the source file looking for independent
@@ -171,8 +178,51 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
 
             rendered.clearRetainingCapacity();
             try tree.renderToArrayList(&rendered, fixups);
-            try std.fs.cwd().writeFile(root_source_file_path, rendered.items);
 
+            // The transformations we applied may have resulted in unused locals,
+            // in which case we would like to add the respective discards.
+            {
+                try astgen_input.resize(rendered.items.len);
+                @memcpy(astgen_input.items, rendered.items);
+                try astgen_input.append(0);
+                const source_with_null = astgen_input.items[0 .. astgen_input.items.len - 1 :0];
+                var astgen_tree = try Ast.parse(gpa, source_with_null, .zig);
+                defer astgen_tree.deinit(gpa);
+                if (astgen_tree.errors.len != 0) {
+                    @panic("syntax errors occurred");
+                }
+                var zir = try AstGen.generate(gpa, astgen_tree);
+                defer zir.deinit(gpa);
+
+                if (zir.hasCompileErrors()) {
+                    more_fixups.clearRetainingCapacity();
+                    const payload_index = zir.extra[@intFromEnum(Zir.ExtraIndex.compile_errors)];
+                    assert(payload_index != 0);
+                    const header = zir.extraData(Zir.Inst.CompileErrors, payload_index);
+                    var extra_index = header.end;
+                    for (0..header.data.items_len) |_| {
+                        const item = zir.extraData(Zir.Inst.CompileErrors.Item, extra_index);
+                        extra_index = item.end;
+                        const msg = zir.nullTerminatedString(item.data.msg);
+                        if (mem.eql(u8, msg, "unused local constant") or
+                            mem.eql(u8, msg, "unused local variable") or
+                            mem.eql(u8, msg, "unused function parameter") or
+                            mem.eql(u8, msg, "unused capture"))
+                        {
+                            const ident_token = item.data.token;
+                            try more_fixups.unused_var_decls.put(gpa, ident_token, {});
+                        } else {
+                            std.debug.print("found other ZIR error: '{s}'\n", .{msg});
+                        }
+                    }
+                    if (more_fixups.count() != 0) {
+                        rendered.clearRetainingCapacity();
+                        try astgen_tree.renderToArrayList(&rendered, more_fixups);
+                    }
+                }
+            }
+
+            try std.fs.cwd().writeFile(root_source_file_path, rendered.items);
             //std.debug.print("trying this code:\n{s}\n", .{rendered.items});
 
             const interestingness = try runCheck(arena, interestingness_argv.items);
diff --git a/src/reduce/Walk.zig b/src/reduce/Walk.zig
index d62cc4e905..c740ac4f0b 100644
--- a/src/reduce/Walk.zig
+++ b/src/reduce/Walk.zig
@@ -581,9 +581,12 @@ fn walkGlobalVarDecl(w: *Walk, decl_node: Ast.Node.Index, var_decl: Ast.full.Var
         try walkExpression(w, var_decl.ast.section_node);
     }
 
-    assert(var_decl.ast.init_node != 0);
-
-    return walkExpression(w, var_decl.ast.init_node);
+    if (var_decl.ast.init_node != 0) {
+        if (!isUndefinedIdent(w.ast, var_decl.ast.init_node)) {
+            try w.transformations.append(.{ .replace_with_undef = var_decl.ast.init_node });
+        }
+        try walkExpression(w, var_decl.ast.init_node);
+    }
 }
 
 fn walkLocalVarDecl(w: *Walk, var_decl: Ast.full.VarDecl) Error!void {

From 212aa717d73866863c593033a1c8bd3c853150ed Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sat, 4 Nov 2023 17:57:43 -0700
Subject: [PATCH 06/11] zig reduce: add transformation for removing container
 fields

---
 src/reduce/Walk.zig | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/reduce/Walk.zig b/src/reduce/Walk.zig
index c740ac4f0b..fc64b0541c 100644
--- a/src/reduce/Walk.zig
+++ b/src/reduce/Walk.zig
@@ -165,7 +165,10 @@ fn walkMember(w: *Walk, decl: Ast.Node.Index) Error!void {
         .container_field_init,
         .container_field_align,
         .container_field,
-        => try walkContainerField(w, ast.fullContainerField(decl).?),
+        => {
+            try w.transformations.append(.{ .delete_node = decl });
+            try walkContainerField(w, ast.fullContainerField(decl).?);
+        },
 
         .@"comptime" => {
             try w.transformations.append(.{ .delete_node = decl });

From 88acdb9aa6cdb8ea20e02c4d0f85be81f696897c Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sat, 4 Nov 2023 18:06:11 -0700
Subject: [PATCH 07/11] zig reduce: delete statements from blocks

---
 lib/std/zig/render.zig | 7 ++++++-
 src/reduce/Walk.zig    | 7 +++++--
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/lib/std/zig/render.zig b/lib/std/zig/render.zig
index f8ad44353f..acd4a6e002 100644
--- a/lib/std/zig/render.zig
+++ b/lib/std/zig/render.zig
@@ -2043,7 +2043,12 @@ fn finishRenderBlock(
             .simple_var_decl,
             .aligned_var_decl,
             => try renderVarDecl(r, tree.fullVarDecl(stmt).?, false, .semicolon),
-            else => try renderExpression(r, stmt, .semicolon),
+
+            else => {
+                if (!r.fixups.omit_nodes.contains(stmt)) {
+                    try renderExpression(r, stmt, .semicolon);
+                }
+            },
         }
     }
     ais.popIndent();
diff --git a/src/reduce/Walk.zig b/src/reduce/Walk.zig
index fc64b0541c..49730e9085 100644
--- a/src/reduce/Walk.zig
+++ b/src/reduce/Walk.zig
@@ -134,8 +134,8 @@ fn walkMember(w: *Walk, decl: Ast.Node.Index) Error!void {
             const body_node = datas[decl].rhs;
             if (!isFnBodyGutted(ast, body_node)) {
                 try w.transformations.append(.{ .gut_function = decl });
+                try walkExpression(w, body_node);
             }
-            try walkExpression(w, body_node);
         },
         .fn_proto_simple,
         .fn_proto_multi,
@@ -648,7 +648,10 @@ fn walkBlock(
             .aligned_var_decl,
             => try walkLocalVarDecl(w, ast.fullVarDecl(stmt).?),
 
-            else => try walkExpression(w, stmt),
+            else => {
+                try w.transformations.append(.{ .delete_node = stmt });
+                try walkExpression(w, stmt);
+            },
         }
     }
 }

From 9e81222d9281f8d26e32578fe3cef54e0c7d5232 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sat, 4 Nov 2023 18:48:08 -0700
Subject: [PATCH 08/11] zig reduce: some adjustments to make it go faster

* don't reset the rng. it seems like it was getting stuck trying the
  same transforms over and over again.
* slide the window over after failing a large transform set, idea being
  that transformations in the failed set are more likely to be
  problematic.
---
 src/reduce.zig | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/src/reduce.zig b/src/reduce.zig
index 0339eb004a..bd7ab54bf1 100644
--- a/src/reduce.zig
+++ b/src/reduce.zig
@@ -166,7 +166,10 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
         var start_index: usize = 0;
 
         while (start_index < transformations.items.len) {
-            subset_size = @max(1, subset_size / 2);
+            const prev_subset_size = subset_size;
+            subset_size = @max(1, subset_size * 3 / 4);
+            if (prev_subset_size > 1 and subset_size == 1)
+                start_index = 0;
 
             const this_set = transformations.items[start_index..][0..subset_size];
             std.debug.print("trying {d} random transformations: ", .{subset_size});
@@ -237,9 +240,6 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
                     tree = new_tree;
 
                     try Walk.findTransformations(arena, &tree, &transformations);
-                    // Resetting based on the seed again means we will get the same
-                    // results if restarting the reduction process from this new point.
-                    rng = std.rand.DefaultPrng.init(seed);
                     sortTransformations(transformations.items, rng.random());
 
                     continue :fresh;
@@ -249,6 +249,11 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
                     // If we tested only one transformation, move on to the next one.
                     if (subset_size == 1) {
                         start_index += 1;
+                    } else {
+                        start_index += subset_size;
+                        if (start_index + subset_size > transformations.items.len) {
+                            start_index = 0;
+                        }
                     }
                 },
             }

From e93d3a3cdb276c8ed3e50466f348e6a4b9de6899 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sat, 4 Nov 2023 19:27:46 -0700
Subject: [PATCH 09/11] zig reduce: don't try to remove discard statements

those are handled separately
---
 src/reduce.zig      | 4 ++--
 src/reduce/Walk.zig | 6 +++++-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/reduce.zig b/src/reduce.zig
index bd7ab54bf1..7013c70674 100644
--- a/src/reduce.zig
+++ b/src/reduce.zig
@@ -229,8 +229,8 @@ pub fn main(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
             //std.debug.print("trying this code:\n{s}\n", .{rendered.items});
 
             const interestingness = try runCheck(arena, interestingness_argv.items);
-            std.debug.print("{d} random transformations: {s}. {d} remaining\n", .{
-                subset_size, @tagName(interestingness), transformations.items.len - start_index,
+            std.debug.print("{d} random transformations: {s}. {d}/{d}\n", .{
+                subset_size, @tagName(interestingness), start_index, transformations.items.len,
             });
             switch (interestingness) {
                 .interesting => {
diff --git a/src/reduce/Walk.zig b/src/reduce/Walk.zig
index 49730e9085..23935398af 100644
--- a/src/reduce/Walk.zig
+++ b/src/reduce/Walk.zig
@@ -649,7 +649,11 @@ fn walkBlock(
             => try walkLocalVarDecl(w, ast.fullVarDecl(stmt).?),
 
             else => {
-                try w.transformations.append(.{ .delete_node = stmt });
+                // Don't try to remove `_ = foo;` discards; those are handled separately.
+                switch (categorizeStmt(ast, stmt)) {
+                    .discard_identifier => {},
+                    else => try w.transformations.append(.{ .delete_node = stmt }),
+                }
                 try walkExpression(w, stmt);
             },
         }

From c4dddcbadbcce587e0bd8593636373712294b5b2 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sat, 4 Nov 2023 20:26:39 -0700
Subject: [PATCH 10/11] std.zig.render: ability to omit variable declarations

---
 lib/std/zig/render.zig | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/lib/std/zig/render.zig b/lib/std/zig/render.zig
index acd4a6e002..c2a8ac9926 100644
--- a/lib/std/zig/render.zig
+++ b/lib/std/zig/render.zig
@@ -2037,6 +2037,7 @@ fn finishRenderBlock(
     const ais = r.ais;
     for (statements, 0..) |stmt, i| {
         if (i != 0) try renderExtraNewline(r, stmt);
+        if (r.fixups.omit_nodes.contains(stmt)) continue;
         switch (node_tags[stmt]) {
             .global_var_decl,
             .local_var_decl,
@@ -2044,11 +2045,7 @@ fn finishRenderBlock(
             .aligned_var_decl,
             => try renderVarDecl(r, tree.fullVarDecl(stmt).?, false, .semicolon),
 
-            else => {
-                if (!r.fixups.omit_nodes.contains(stmt)) {
-                    try renderExpression(r, stmt, .semicolon);
-                }
-            },
+            else => try renderExpression(r, stmt, .semicolon),
         }
     }
     ais.popIndent();

From a9002156a09130038abcf418609fea725ce71bc2 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sat, 4 Nov 2023 20:27:15 -0700
Subject: [PATCH 11/11] zig reduce: add reduction for removing var decls

---
 src/reduce.zig      |  6 ++++
 src/reduce/Walk.zig | 73 ++++++++++++++++++++++++++++++++++-----------
 2 files changed, 62 insertions(+), 17 deletions(-)

diff --git a/src/reduce.zig b/src/reduce.zig
index 7013c70674..f11b2a6ae1 100644
--- a/src/reduce.zig
+++ b/src/reduce.zig
@@ -321,6 +321,12 @@ fn transformationsToFixups(
         .delete_node => |decl_node| {
             try fixups.omit_nodes.put(gpa, decl_node, {});
         },
+        .delete_var_decl => |delete_var_decl| {
+            try fixups.omit_nodes.put(gpa, delete_var_decl.var_decl_node, {});
+            for (delete_var_decl.references.items) |ident_node| {
+                try fixups.replace_nodes.put(gpa, ident_node, "undefined");
+            }
+        },
         .replace_with_undef => |node| {
             try fixups.replace_nodes.put(gpa, node, "undefined");
         },
diff --git a/src/reduce/Walk.zig b/src/reduce/Walk.zig
index 23935398af..94ef0eeb26 100644
--- a/src/reduce/Walk.zig
+++ b/src/reduce/Walk.zig
@@ -8,6 +8,7 @@ ast: *const Ast,
 transformations: *std.ArrayList(Transformation),
 unreferenced_globals: std.StringArrayHashMapUnmanaged(Ast.Node.Index),
 in_scope_names: std.StringArrayHashMapUnmanaged(u32),
+replace_names: std.StringArrayHashMapUnmanaged(u32),
 gpa: std.mem.Allocator,
 arena: std.mem.Allocator,
 
@@ -17,6 +18,13 @@ pub const Transformation = union(enum) {
     gut_function: Ast.Node.Index,
     /// Omit a global declaration.
     delete_node: Ast.Node.Index,
+    /// Delete a local variable declaration and replace all of its references
+    /// with `undefined`.
+    delete_var_decl: struct {
+        var_decl_node: Ast.Node.Index,
+        /// Identifier nodes that reference the variable.
+        references: std.ArrayListUnmanaged(Ast.Node.Index),
+    },
     /// Replace an expression with `undefined`.
     replace_with_undef: Ast.Node.Index,
     /// Replace an `@import` with the imported file contents wrapped in a struct.
@@ -48,10 +56,12 @@ pub fn findTransformations(
         .arena = arena,
         .unreferenced_globals = .{},
         .in_scope_names = .{},
+        .replace_names = .{},
     };
     defer {
         walk.unreferenced_globals.deinit(walk.gpa);
         walk.in_scope_names.deinit(walk.gpa);
+        walk.replace_names.deinit(walk.gpa);
     }
 
     try walkMembers(&walk, walk.ast.rootDecls());
@@ -133,6 +143,7 @@ fn walkMember(w: *Walk, decl: Ast.Node.Index) Error!void {
             try walkExpression(w, fn_proto);
             const body_node = datas[decl].rhs;
             if (!isFnBodyGutted(ast, body_node)) {
+                w.replace_names.clearRetainingCapacity();
                 try w.transformations.append(.{ .gut_function = decl });
                 try walkExpression(w, body_node);
             }
@@ -187,7 +198,15 @@ fn walkExpression(w: *Walk, node: Ast.Node.Index) Error!void {
     const node_tags = ast.nodes.items(.tag);
     const datas = ast.nodes.items(.data);
     switch (node_tags[node]) {
-        .identifier => try walkIdentifier(w, main_tokens[node]),
+        .identifier => {
+            const name_ident = main_tokens[node];
+            assert(token_tags[name_ident] == .identifier);
+            const name_bytes = ast.tokenSlice(name_ident);
+            _ = w.unreferenced_globals.swapRemove(name_bytes);
+            if (w.replace_names.get(name_bytes)) |index| {
+                try w.transformations.items[index].delete_var_decl.references.append(w.arena, node);
+            }
+        },
 
         .number_literal,
         .char_literal,
@@ -646,13 +665,31 @@ fn walkBlock(
             .local_var_decl,
             .simple_var_decl,
             .aligned_var_decl,
-            => try walkLocalVarDecl(w, ast.fullVarDecl(stmt).?),
+            => {
+                const var_decl = ast.fullVarDecl(stmt).?;
+                if (var_decl.ast.init_node != 0 and
+                    isUndefinedIdent(w.ast, var_decl.ast.init_node))
+                {
+                    try w.transformations.append(.{ .delete_var_decl = .{
+                        .var_decl_node = stmt,
+                        .references = .{},
+                    } });
+                    const name_tok = var_decl.ast.mut_token + 1;
+                    const name_bytes = ast.tokenSlice(name_tok);
+                    try w.replace_names.put(w.gpa, name_bytes, @intCast(w.transformations.items.len - 1));
+                } else {
+                    try walkLocalVarDecl(w, var_decl);
+                }
+            },
 
             else => {
-                // Don't try to remove `_ = foo;` discards; those are handled separately.
                 switch (categorizeStmt(ast, stmt)) {
+                    // Don't try to remove `_ = foo;` discards; those are handled separately.
                     .discard_identifier => {},
-                    else => try w.transformations.append(.{ .delete_node = stmt }),
+                    // definitely try to remove `_ = undefined;` though.
+                    .discard_undefined, .trap_call, .other => {
+                        try w.transformations.append(.{ .delete_node = stmt });
+                    },
                 }
                 try walkExpression(w, stmt);
             },
@@ -905,6 +942,7 @@ fn isFnBodyGutted(ast: *const Ast, body_node: Ast.Node.Index) bool {
 }
 
 const StmtCategory = enum {
+    discard_undefined,
     discard_identifier,
     trap_call,
     other,
@@ -930,8 +968,14 @@ fn categorizeStmt(ast: *const Ast, stmt: Ast.Node.Index) StmtCategory {
         },
         .assign => {
             const infix = datas[stmt];
-            if (isDiscardIdent(ast, infix.lhs) and node_tags[infix.rhs] == .identifier)
-                return .discard_identifier;
+            if (isDiscardIdent(ast, infix.lhs) and node_tags[infix.rhs] == .identifier) {
+                const name_bytes = ast.tokenSlice(main_tokens[infix.rhs]);
+                if (std.mem.eql(u8, name_bytes, "undefined")) {
+                    return .discard_undefined;
+                } else {
+                    return .discard_identifier;
+                }
+            }
             return .other;
         },
         else => return .other,
@@ -951,26 +995,21 @@ fn categorizeBuiltinCall(
 }
 
 fn isDiscardIdent(ast: *const Ast, node: Ast.Node.Index) bool {
-    const node_tags = ast.nodes.items(.tag);
-    const main_tokens = ast.nodes.items(.main_token);
-    switch (node_tags[node]) {
-        .identifier => {
-            const token_index = main_tokens[node];
-            const name_bytes = ast.tokenSlice(token_index);
-            return std.mem.eql(u8, name_bytes, "_");
-        },
-        else => return false,
-    }
+    return isMatchingIdent(ast, node, "_");
 }
 
 fn isUndefinedIdent(ast: *const Ast, node: Ast.Node.Index) bool {
+    return isMatchingIdent(ast, node, "undefined");
+}
+
+fn isMatchingIdent(ast: *const Ast, node: Ast.Node.Index, string: []const u8) bool {
     const node_tags = ast.nodes.items(.tag);
     const main_tokens = ast.nodes.items(.main_token);
     switch (node_tags[node]) {
         .identifier => {
             const token_index = main_tokens[node];
             const name_bytes = ast.tokenSlice(token_index);
-            return std.mem.eql(u8, name_bytes, "undefined");
+            return std.mem.eql(u8, name_bytes, string);
         },
         else => return false,
     }