astgen: implement unified ptrCastBuiltin with nested cast collapsing

Port the upstream ptrCast() function (AstGen.zig:8969-9087) which handles
nested pointer cast collapsing. All five pointer cast builtins (@ptrCast,
@alignCast, @addrSpaceCast, @constCast, @volatileCast) now route through
a single ptrCastBuiltin() function that:

- Walks inward through nested builtin calls accumulating flags
- Handles @fieldParentPtr nesting (with accumulated outer flags)
- Emits ptr_cast_full, ptr_cast_no_dest, or simple ptr_cast based on
  combined flags and whether a result type is needed

This fixes compile errors in field_parent_ptr.zig and switch.zig where
@alignCast(@fieldParentPtr(...)) needed nested cast support.

Also adds @addrSpaceCast support (previously missing).

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

stage0/astgen.c

@@ -3150,6 +3150,171 @@ static uint32_t simpleCBuiltin(GenZir* gz, Scope* scope, ResultLoc rl,
     return rvalue(gz, rl, ZIR_REF_VOID_VALUE, node);
 }
 
+// FullPtrCastFlags bits (Zir.zig:3461).
+#define PTR_CAST_FLAG_PTR_CAST 0x01
+#define PTR_CAST_FLAG_ALIGN_CAST 0x02
+#define PTR_CAST_FLAG_ADDRSPACE 0x04
+#define PTR_CAST_FLAG_CONST_CAST 0x08
+#define PTR_CAST_FLAG_VOLATILE_CAST 0x10
+// Flags that don't need a result type.
+#define PTR_CAST_NO_RESULT_TY_MASK                                            \
+    (PTR_CAST_FLAG_CONST_CAST | PTR_CAST_FLAG_VOLATILE_CAST)
+
+// Mirrors ptrCast (AstGen.zig:8969-9087).
+// Handles nested pointer cast collapsing for @ptrCast, @alignCast,
+// @addrSpaceCast, @constCast, @volatileCast, and nested @fieldParentPtr.
+static uint32_t ptrCastBuiltin(
+    GenZir* gz, Scope* scope, ResultLoc rl, uint32_t root_node) {
+    AstGenCtx* ag = gz->astgen;
+    const Ast* tree = ag->tree;
+
+    uint16_t flags = 0;
+    uint32_t node = root_node;
+
+    // Walk inward through nested builtin calls (AstGen.zig:8984-9047).
+    while (1) {
+        AstNodeTag ntag = tree->nodes.tags[node];
+        if (ntag == AST_NODE_BUILTIN_CALL_TWO
+            || ntag == AST_NODE_BUILTIN_CALL_TWO_COMMA) {
+            // ok
+        } else if (ntag == AST_NODE_GROUPED_EXPRESSION) {
+            // Handle chaining through parentheses (AstGen.zig:8987-8990).
+            node = tree->nodes.datas[node].lhs;
+            continue;
+        } else {
+            break;
+        }
+
+        AstData nd = tree->nodes.datas[node];
+        // Count args: lhs is arg0, rhs is arg1 (0 = absent).
+        uint32_t arg_count = (nd.lhs != 0 ? 1u : 0u) + (nd.rhs != 0 ? 1u : 0u);
+        if (arg_count == 0)
+            break;
+
+        // Get builtin name from source.
+        uint32_t bt = tree->nodes.main_tokens[node];
+        uint32_t bstart = tree->tokens.starts[bt] + 1; // skip '@'
+        uint32_t bend = bstart;
+        while (bend < tree->source_len
+            && ((tree->source[bend] >= 'a' && tree->source[bend] <= 'z')
+                || (tree->source[bend] >= 'A' && tree->source[bend] <= 'Z')
+                || tree->source[bend] == '_'))
+            bend++;
+        uint32_t blen = bend - bstart;
+
+        if (arg_count == 1) {
+            // 1-arg builtin: check if it's a pointer cast.
+            uint16_t flag = 0;
+            if (blen == 7 && memcmp(tree->source + bstart, "ptrCast", 7) == 0)
+                flag = PTR_CAST_FLAG_PTR_CAST;
+            else if (blen == 9
+                && memcmp(tree->source + bstart, "alignCast", 9) == 0)
+                flag = PTR_CAST_FLAG_ALIGN_CAST;
+            else if (blen == 13
+                && memcmp(tree->source + bstart, "addrSpaceCast", 13) == 0)
+                flag = PTR_CAST_FLAG_ADDRSPACE;
+            else if (blen == 9
+                && memcmp(tree->source + bstart, "constCast", 9) == 0)
+                flag = PTR_CAST_FLAG_CONST_CAST;
+            else if (blen == 12
+                && memcmp(tree->source + bstart, "volatileCast", 12) == 0)
+                flag = PTR_CAST_FLAG_VOLATILE_CAST;
+            else
+                break;
+
+            if (flags & flag) {
+                // Redundant cast (AstGen.zig:9015-9016).
+                SET_ERROR(ag);
+                return ZIR_REF_VOID_VALUE;
+            }
+            flags |= flag;
+            node = nd.lhs; // Advance to inner operand.
+        } else {
+            // 2-arg builtin: check for @fieldParentPtr (AstGen.zig:9027-9046).
+            if (blen != 14
+                || memcmp(tree->source + bstart, "fieldParentPtr", 14) != 0)
+                break;
+            if (flags & PTR_CAST_FLAG_PTR_CAST)
+                break;
+
+            // Emit field_parent_ptr with accumulated flags.
+            advanceSourceCursorToMainToken(ag, gz, root_node);
+            uint32_t saved_line = ag->source_line - gz->decl_line;
+            uint32_t saved_col = ag->source_column;
+            uint32_t parent_ptr_type = rlResultTypeForCast(gz, rl, root_node);
+            ResultLoc field_name_rl = { .tag = RL_COERCED_TY,
+                .data = ZIR_REF_SLICE_CONST_U8_TYPE,
+                .src_node = 0,
+                .ctx = RI_CTX_NONE };
+            uint32_t field_name = comptimeExpr(
+                gz, scope, field_name_rl, nd.lhs, COMPTIME_REASON_FIELD_NAME);
+            uint32_t field_ptr = expr(gz, scope, nd.rhs);
+            emitDbgStmt(gz, saved_line, saved_col);
+            ensureExtraCapacity(ag, 4);
+            uint32_t payload_index = ag->extra_len;
+            ag->extra[ag->extra_len++]
+                = (uint32_t)((int32_t)node - (int32_t)gz->decl_node_index);
+            ag->extra[ag->extra_len++] = parent_ptr_type;
+            ag->extra[ag->extra_len++] = field_name;
+            ag->extra[ag->extra_len++] = field_ptr;
+            uint32_t result = addExtendedPayloadSmall(
+                gz, (uint16_t)ZIR_EXT_FIELD_PARENT_PTR, flags, payload_index);
+            return rvalue(gz, rl, result, root_node);
+        }
+    }
+
+    // After loop: flags must be non-zero (AstGen.zig:9051).
+
+    // Path A: pure @ptrCast (AstGen.zig:9053-9056).
+    if (flags == PTR_CAST_FLAG_PTR_CAST) {
+        advanceSourceCursorToMainToken(ag, gz, root_node);
+        uint32_t saved_line = ag->source_line - gz->decl_line;
+        uint32_t saved_col = ag->source_column;
+        uint32_t result_type = rlResultTypeForCast(gz, rl, root_node);
+        uint32_t operand = expr(gz, scope, node);
+        emitDbgStmt(gz, saved_line, saved_col);
+        return rvalue(gz, rl,
+            addPlNodeBin(
+                gz, ZIR_INST_PTR_CAST, root_node, result_type, operand),
+            root_node);
+    }
+
+    // Path B: only const_cast/volatile_cast — no result type needed
+    // (AstGen.zig:9059-9072).
+    if ((flags & ~PTR_CAST_NO_RESULT_TY_MASK) == 0) {
+        advanceSourceCursorToMainToken(ag, gz, root_node);
+        uint32_t saved_line = ag->source_line - gz->decl_line;
+        uint32_t saved_col = ag->source_column;
+        uint32_t operand = expr(gz, scope, node);
+        emitDbgStmt(gz, saved_line, saved_col);
+        ensureExtraCapacity(ag, 2);
+        uint32_t payload_index = ag->extra_len;
+        ag->extra[ag->extra_len++]
+            = (uint32_t)((int32_t)root_node - (int32_t)gz->decl_node_index);
+        ag->extra[ag->extra_len++] = operand;
+        uint32_t result = addExtendedPayloadSmall(
+            gz, (uint16_t)ZIR_EXT_PTR_CAST_NO_DEST, flags, payload_index);
+        return rvalue(gz, rl, result, root_node);
+    }
+
+    // Path C: full cast with result type (AstGen.zig:9077-9086).
+    advanceSourceCursorToMainToken(ag, gz, root_node);
+    uint32_t saved_line = ag->source_line - gz->decl_line;
+    uint32_t saved_col = ag->source_column;
+    uint32_t result_type = rlResultTypeForCast(gz, rl, root_node);
+    uint32_t operand = expr(gz, scope, node);
+    emitDbgStmt(gz, saved_line, saved_col);
+    ensureExtraCapacity(ag, 3);
+    uint32_t payload_index = ag->extra_len;
+    ag->extra[ag->extra_len++]
+        = (uint32_t)((int32_t)root_node - (int32_t)gz->decl_node_index);
+    ag->extra[ag->extra_len++] = result_type;
+    ag->extra[ag->extra_len++] = operand;
+    uint32_t result = addExtendedPayloadSmall(
+        gz, (uint16_t)ZIR_EXT_PTR_CAST_FULL, flags, payload_index);
+    return rvalue(gz, rl, result, root_node);
+}
+
 // Mirrors builtinCall (AstGen.zig:9191) dispatch.
 static uint32_t builtinCall(
     GenZir* gz, Scope* scope, ResultLoc rl, uint32_t node) {
@@ -3244,20 +3409,13 @@ static uint32_t builtinCall(
         return rvalue(gz, rl, addPlNodeBin(gz, ZIR_INST_TRUNCATE, node,
             result_type, operand), node);
     }
-    // @ptrCast — typeCast pattern (AstGen.zig:9056, 9807-9826).
+    // @ptrCast — routes through ptrCastBuiltin (AstGen.zig:9464-9469).
-    // TODO: Issue 14 — upstream routes through ptrCast() for nested
+    if (name_len == 7 && memcmp(source + name_start, "ptrCast", 7) == 0)
-    // pointer cast collapsing. Currently uses simple typeCast path.
+        return ptrCastBuiltin(gz, scope, rl, node);
-    if (name_len == 7 && memcmp(source + name_start, "ptrCast", 7) == 0) {
+    // @addrSpaceCast — routes through ptrCastBuiltin (AstGen.zig:9464-9469).
-        advanceSourceCursorToMainToken(ag, gz, node);
+    if (name_len == 13
-        uint32_t saved_line = ag->source_line - gz->decl_line;
+        && memcmp(source + name_start, "addrSpaceCast", 13) == 0)
-        uint32_t saved_col = ag->source_column;
+        return ptrCastBuiltin(gz, scope, rl, node);
-        uint32_t result_type = rlResultTypeForCast(gz, rl, node);
-        AstData nd = tree->nodes.datas[node];
-        uint32_t operand = expr(gz, scope, nd.lhs);
-        emitDbgStmt(gz, saved_line, saved_col);
-        return rvalue(gz, rl, addPlNodeBin(gz, ZIR_INST_PTR_CAST, node,
-            result_type, operand), node);
-    }
     // @enumFromInt — typeCast pattern (AstGen.zig:9414, 9807-9826).
     if (name_len == 11 && memcmp(source + name_start, "enumFromInt", 11) == 0) {
         advanceSourceCursorToMainToken(ag, gz, node);
@@ -3518,69 +3676,18 @@ static uint32_t builtinCall(
             gz, (uint16_t)ZIR_EXT_SHL_WITH_OVERFLOW, payload_index);
         return rvalue(gz, rl, result, node);
     }
-    // @alignCast — ptrCast family (AstGen.zig:9464-9469, 8969-9087).
+    // @alignCast — routes through ptrCastBuiltin (AstGen.zig:9464-9469).
-    // Simplified: standalone @alignCast uses ptr_cast_full with align_cast flag.
     if (name_len == 9
-        && memcmp(source + name_start, "alignCast", 9) == 0) {
+        && memcmp(source + name_start, "alignCast", 9) == 0)
-        advanceSourceCursorToMainToken(ag, gz, node);
+        return ptrCastBuiltin(gz, scope, rl, node);
-        uint32_t saved_line = ag->source_line - gz->decl_line;
+    // @constCast — routes through ptrCastBuiltin (AstGen.zig:9464-9469).
-        uint32_t saved_col = ag->source_column;
-        uint32_t result_type = rlResultTypeForCast(gz, rl, node);
-        AstData nd = tree->nodes.datas[node];
-        uint32_t operand = expr(gz, scope, nd.lhs);
-        emitDbgStmt(gz, saved_line, saved_col);
-        // align_cast flag = bit 1 (FullPtrCastFlags: ptr_cast=0, align_cast=1)
-        uint16_t flags = 0x02;
-        ensureExtraCapacity(ag, 3);
-        uint32_t payload_index = ag->extra_len;
-        ag->extra[ag->extra_len++]
-            = (uint32_t)((int32_t)node - (int32_t)gz->decl_node_index);
-        ag->extra[ag->extra_len++] = result_type;
-        ag->extra[ag->extra_len++] = operand;
-        uint32_t result = addExtendedPayloadSmall(
-            gz, (uint16_t)ZIR_EXT_PTR_CAST_FULL, flags, payload_index);
-        return rvalue(gz, rl, result, node);
-    }
-    // @constCast (AstGen.zig:9464-9469, 8969-9087).
     if (name_len == 9
-        && memcmp(source + name_start, "constCast", 9) == 0) {
+        && memcmp(source + name_start, "constCast", 9) == 0)
-        advanceSourceCursorToMainToken(ag, gz, node);
+        return ptrCastBuiltin(gz, scope, rl, node);
-        uint32_t saved_line = ag->source_line - gz->decl_line;
+    // @volatileCast — routes through ptrCastBuiltin (AstGen.zig:9464-9469).
-        uint32_t saved_col = ag->source_column;
-        AstData nd = tree->nodes.datas[node];
-        uint32_t operand = expr(gz, scope, nd.lhs);
-        emitDbgStmt(gz, saved_line, saved_col);
-        // const_cast flag = bit 3 (FullPtrCastFlags packed u5)
-        uint16_t flags = 0x08;
-        ensureExtraCapacity(ag, 2);
-        uint32_t payload_index = ag->extra_len;
-        ag->extra[ag->extra_len++]
-            = (uint32_t)((int32_t)node - (int32_t)gz->decl_node_index);
-        ag->extra[ag->extra_len++] = operand;
-        uint32_t result = addExtendedPayloadSmall(
-            gz, (uint16_t)ZIR_EXT_PTR_CAST_NO_DEST, flags, payload_index);
-        return rvalue(gz, rl, result, node);
-    }
-    // @volatileCast (AstGen.zig:9464-9469, 8969-9087).
     if (name_len == 12
-        && memcmp(source + name_start, "volatileCast", 12) == 0) {
+        && memcmp(source + name_start, "volatileCast", 12) == 0)
-        advanceSourceCursorToMainToken(ag, gz, node);
+        return ptrCastBuiltin(gz, scope, rl, node);
-        uint32_t saved_line = ag->source_line - gz->decl_line;
-        uint32_t saved_col = ag->source_column;
-        AstData nd = tree->nodes.datas[node];
-        uint32_t operand = expr(gz, scope, nd.lhs);
-        emitDbgStmt(gz, saved_line, saved_col);
-        // volatile_cast flag = bit 4 (FullPtrCastFlags packed u5)
-        uint16_t flags = 0x10;
-        ensureExtraCapacity(ag, 2);
-        uint32_t payload_index = ag->extra_len;
-        ag->extra[ag->extra_len++]
-            = (uint32_t)((int32_t)node - (int32_t)gz->decl_node_index);
-        ag->extra[ag->extra_len++] = operand;
-        uint32_t result = addExtendedPayloadSmall(
-            gz, (uint16_t)ZIR_EXT_PTR_CAST_NO_DEST, flags, payload_index);
-        return rvalue(gz, rl, result, node);
-    }
     // @Type (reify) (AstGen.zig:9426-9428, 9747-9781).
     if (name_len == 4 && memcmp(source + name_start, "Type", 4) == 0) {
         AstData nd = tree->nodes.datas[node];