sema: use real zig caches and fix thread pool lifetime

- Use Directories.init() to resolve ~/.cache/zig (global) and
  .zig-cache (local) instead of a temp dir, re-using the same
  resolution logic as src/main.zig (introspect module).

- Point zig_lib at zig-out/lib/zig/ (installed copy) instead of lib/
  (source tree) to avoid "file exists in modules 'root' and 'std'"
  when compiling files that live under lib/.

- Heap-allocate the thread pool to keep its address stable. Worker
  threads reference the Pool's condvar/mutex; a by-value copy into
  ZigSemaResult left them waiting on the original stack address while
  deinit broadcast on the copy, causing a deadlock.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

stage0/sema.zig

@@ -8,19 +8,19 @@ const Compilation = zig_internals.Compilation;
 const Package = zig_internals.Package;
 
 /// Result of running the real Zig sema pipeline via Compilation.
-/// Owns the Compilation, Directories, thread pool, temp dir, and arena.
+/// Owns the Compilation, Directories, thread pool, and arena.
 pub const ZigSemaResult = struct {
     comp: *Compilation,
     dirs: Compilation.Directories,
-    tmp_dir: std.testing.TmpDir,
     arena_state: std.heap.ArenaAllocator,
-    thread_pool: std.Thread.Pool,
+    thread_pool: *std.Thread.Pool,
+    gpa: Allocator,
 
     pub fn deinit(self: *ZigSemaResult) void {
         self.comp.destroy();
         self.dirs.deinit();
         self.thread_pool.deinit();
-        self.tmp_dir.cleanup();
+        self.gpa.destroy(self.thread_pool);
         self.arena_state.deinit();
     }
 };
@@ -32,25 +32,18 @@ pub fn zigSema(gpa: Allocator, src_path: []const u8) !ZigSemaResult {
     errdefer arena_state.deinit();
     const arena = arena_state.allocator();
 
-    // Set up temp dir for Compilation cache.
-    var tmp_dir = std.testing.tmpDir(.{});
-    errdefer tmp_dir.cleanup();
-
-    // Resolve paths.
-    const cwd_path = try std.fs.cwd().realpathAlloc(arena, ".");
-    const zig_lib_path = try std.fs.path.join(arena, &.{ cwd_path, "lib" });
-    const zig_lib_handle = try std.fs.cwd().openDir("lib", .{});
-    const tmp_path = try tmp_dir.dir.realpathAlloc(arena, ".");
-    const cache_path = try std.fmt.allocPrint(arena, "{s}/.cache", .{tmp_path});
-    try tmp_dir.dir.makeDir(".cache");
-    const cache_handle = try tmp_dir.dir.openDir(".cache", .{});
-
-    var dirs = Compilation.Directories{
-        .cwd = cwd_path,
-        .zig_lib = .{ .path = zig_lib_path, .handle = zig_lib_handle },
-        .global_cache = .{ .path = cache_path, .handle = cache_handle },
-        .local_cache = .{ .path = cache_path, .handle = cache_handle },
-    };
+    // Use the real Zig cache directories: ~/.cache/zig (global) and .zig-cache (local).
+    // Point zig_lib at zig-out/lib/zig/ (the installed copy) rather than lib/ (the source
+    // tree) to avoid "file exists in modules 'root' and 'std'" when compiling source files
+    // that live under lib/.
+    var dirs: Compilation.Directories = .init(
+        arena,
+        "zig-out/lib/zig",
+        null,
+        .search,
+        {},
+        "",
+    );
     errdefer dirs.deinit();
 
     // Hardcode x86_64-linux-musl target.
@@ -96,14 +89,21 @@ pub fn zigSema(gpa: Allocator, src_path: []const u8) !ZigSemaResult {
         .parent = null,
     });
 
-    var thread_pool: std.Thread.Pool = undefined;
+    // Heap-allocate the thread pool so its address stays stable after zigSema
+    // returns. The worker threads hold references to the Pool's internal
+    // condvar/mutex; a by-value copy would leave them waiting on a stale address.
+    const thread_pool = try gpa.create(std.Thread.Pool);
+    thread_pool.* = undefined;
     try thread_pool.init(.{
         .allocator = gpa,
         .n_jobs = 1,
         .track_ids = true,
         .stack_size = 60 << 20,
     });
-    errdefer thread_pool.deinit();
+    errdefer {
+        thread_pool.deinit();
+        gpa.destroy(thread_pool);
+    }
 
     var create_diag: Compilation.CreateDiagnostic = undefined;
     const comp = Compilation.create(gpa, arena, &create_diag, .{
@@ -112,7 +112,7 @@ pub fn zigSema(gpa: Allocator, src_path: []const u8) !ZigSemaResult {
         .config = config,
         .root_mod = root_mod,
         .emit_bin = .no,
-        .thread_pool = &thread_pool,
+        .thread_pool = thread_pool,
         .cache_mode = .whole,
     }) catch |err| switch (err) {
         error.CreateFail => {
@@ -135,8 +135,8 @@ pub fn zigSema(gpa: Allocator, src_path: []const u8) !ZigSemaResult {
     return .{
         .comp = comp,
         .dirs = dirs,
-        .tmp_dir = tmp_dir,
         .arena_state = arena_state,
         .thread_pool = thread_pool,
+        .gpa = gpa,
     };
 }