astgen: fix corpus mismatches (gap 20→-17)

- endsWithNoReturn: add missing noreturn tags (RET_ERR_VALUE,
  RET_LOAD, COMPILE_ERROR, UNREACHABLE, TRAP,
  CHECK_COMPTIME_CONTROL_FLOW, SWITCH_CONTINUE)
- defer body: emit dbg_node + ensure_result (unusedResultExpr pattern)
- unwrap_optional: add emitDbgStmt before OPTIONAL_PAYLOAD_SAFE
- slice: add emitDbgStmt to SLICE_OPEN, SLICE, SLICE_SENTINEL
- switchExpr: remove erroneous save_err_ret_index (only in ErrUnion variant)

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

astgen.c

@@ -7,7 +7,6 @@
 #include "astgen.h"
 #include "common.h"
 #include <assert.h>
-#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -1442,10 +1441,17 @@ static bool endsWithNoReturn(GenZir* gz) {
     case ZIR_INST_BREAK_INLINE:
     case ZIR_INST_CONDBR:
     case ZIR_INST_CONDBR_INLINE:
-    case ZIR_INST_RET_IMPLICIT:
+    case ZIR_INST_COMPILE_ERROR:
     case ZIR_INST_RET_NODE:
+    case ZIR_INST_RET_LOAD:
+    case ZIR_INST_RET_IMPLICIT:
+    case ZIR_INST_RET_ERR_VALUE:
+    case ZIR_INST_UNREACHABLE:
     case ZIR_INST_REPEAT:
     case ZIR_INST_REPEAT_INLINE:
+    case ZIR_INST_TRAP:
+    case ZIR_INST_CHECK_COMPTIME_CONTROL_FLOW:
+    case ZIR_INST_SWITCH_CONTINUE:
     case ZIR_INST_SWITCH_BLOCK:
     case ZIR_INST_SWITCH_BLOCK_REF:
     case ZIR_INST_SWITCH_BLOCK_ERR_UNION:
@@ -3099,6 +3105,10 @@ static uint32_t exprRl(GenZir* gz, Scope* scope, ResultLoc rl, uint32_t node) {
     // unwrap_optional (AstGen.zig:966-985).
     case AST_NODE_UNWRAP_OPTIONAL: {
         uint32_t lhs = expr(gz, scope, nd.lhs);
+        advanceSourceCursorToMainToken(ag, node);
+        uint32_t saved_line = ag->source_line - gz->decl_line;
+        uint32_t saved_col = ag->source_column;
+        emitDbgStmt(gz, saved_line, saved_col);
         return rvalue(gz, rl,
             addUnNode(gz, ZIR_INST_OPTIONAL_PAYLOAD_SAFE, lhs, node), node);
     }
@@ -3134,7 +3144,11 @@ static uint32_t exprRl(GenZir* gz, Scope* scope, ResultLoc rl, uint32_t node) {
     // slice (AstGen.zig:882-939).
     case AST_NODE_SLICE_OPEN: {
         uint32_t lhs = expr(gz, scope, nd.lhs);
+        advanceSourceCursorToMainToken(ag, node);
+        uint32_t saved_line = ag->source_line - gz->decl_line;
+        uint32_t saved_col = ag->source_column;
         uint32_t start = expr(gz, scope, nd.rhs);
+        emitDbgStmt(gz, saved_line, saved_col);
         return rvalue(gz, rl,
             addPlNodeBin(gz, ZIR_INST_SLICE_START, node, lhs, start), node);
     }
@@ -3142,10 +3156,14 @@ static uint32_t exprRl(GenZir* gz, Scope* scope, ResultLoc rl, uint32_t node) {
         // Slice[rhs]: { start, end }
         const Ast* stree = ag->tree;
         uint32_t lhs = expr(gz, scope, nd.lhs);
+        advanceSourceCursorToMainToken(ag, node);
+        uint32_t saved_line = ag->source_line - gz->decl_line;
+        uint32_t saved_col = ag->source_column;
         uint32_t start_node = stree->extra_data.arr[nd.rhs];
         uint32_t end_node = stree->extra_data.arr[nd.rhs + 1];
         uint32_t start_ref = expr(gz, scope, start_node);
         uint32_t end_ref = expr(gz, scope, end_node);
+        emitDbgStmt(gz, saved_line, saved_col);
         ensureExtraCapacity(ag, 3);
         uint32_t payload_index = ag->extra_len;
         ag->extra[ag->extra_len++] = lhs;
@@ -3161,12 +3179,16 @@ static uint32_t exprRl(GenZir* gz, Scope* scope, ResultLoc rl, uint32_t node) {
         // SliceSentinel[rhs]: { start, end, sentinel }
         const Ast* stree = ag->tree;
         uint32_t lhs = expr(gz, scope, nd.lhs);
+        advanceSourceCursorToMainToken(ag, node);
+        uint32_t saved_line = ag->source_line - gz->decl_line;
+        uint32_t saved_col = ag->source_column;
         uint32_t start_node = stree->extra_data.arr[nd.rhs];
         uint32_t end_node = stree->extra_data.arr[nd.rhs + 1];
         uint32_t sentinel_node = stree->extra_data.arr[nd.rhs + 2];
         uint32_t start_ref = expr(gz, scope, start_node);
         uint32_t end_ref = expr(gz, scope, end_node);
         uint32_t sentinel_ref = expr(gz, scope, sentinel_node);
+        emitDbgStmt(gz, saved_line, saved_col);
         ensureExtraCapacity(ag, 4);
         uint32_t payload_index = ag->extra_len;
         ag->extra[ag->extra_len++] = lhs;
@@ -4554,9 +4576,8 @@ static uint32_t switchExpr(
         uint32_t body_node = cd.rhs;
         GenZir case_scope = makeSubBlock(gz, scope);
 
-        // save_err_ret_index (AstGen.zig:7524-7525).
+        // Note: upstream regular switchExpr (AstGen.zig:7625) does NOT emit
-        if (ag->fn_ret_ty != 0 && nodeMayAppendToErrorTrace(tree, cond_node))
+        // save_err_ret_index. Only switchExprErrUnion (AstGen.zig:7524) does.
-            addSaveErrRetIndex(&case_scope, ZIR_REF_NONE);
 
         // Use fullBodyExpr to process body inline (AstGen.zig:8009).
         uint32_t result
@@ -5534,7 +5555,11 @@ static void blockExprStmts(GenZir* gz, Scope* scope,
             } else {
                 expr_node = dnd.rhs;
             }
-            expr(&defer_gen, &defer_gen.base, expr_node);
+            // unusedResultExpr pattern (AstGen.zig:3165, 2641-2646).
+            emitDbgNode(&defer_gen, expr_node);
+            uint32_t defer_result
+                = expr(&defer_gen, &defer_gen.base, expr_node);
+            addEnsureResult(&defer_gen, defer_result, expr_node);
 
             // Add break_inline at end (AstGen.zig:3167).
             addBreak(&defer_gen, ZIR_INST_BREAK_INLINE, 0, ZIR_REF_VOID_VALUE,