astgen: add reachableExprComptime and comptime_token handling in varDecl

Port two missing features from upstream AstGen.zig varDecl:

1. Add reachableExprComptime (AstGen.zig:418-438) which wraps init
   expressions in comptimeExpr when force_comptime is set, and checks
   for noreturn results. Replace plain exprRl calls in all three varDecl
   paths (const rvalue, const alloc, var) with reachableExprComptime.

2. Extract comptime_token by scanning backwards from mut_token (matching
   Ast.zig fullVarDeclComponents). For const path, set force_comptime to
   wrap init in comptime block. For var path, use comptime_token to set
   is_comptime which selects alloc_comptime_mut/alloc_inferred_comptime_mut
   tags and sets maybe_comptime on the scope.

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

astgen.c

@@ -2314,6 +2314,27 @@ static uint32_t reachableExpr(GenZir* gz, Scope* scope, ResultLoc rl,
     return result;
 }
 
+// Forward declaration needed by reachableExprComptime.
+static uint32_t comptimeExpr(
+    GenZir* gz, Scope* scope, ResultLoc rl, uint32_t node, uint32_t reason);
+
+// Mirrors reachableExprComptime (AstGen.zig:418-438).
+// Like reachableExpr but optionally wraps in comptimeExpr when
+// comptime_reason is non-zero (i.e. force_comptime is set).
+static uint32_t reachableExprComptime(GenZir* gz, Scope* scope, ResultLoc rl,
+    uint32_t node, uint32_t reachable_node, uint32_t comptime_reason) {
+    uint32_t result;
+    if (comptime_reason != 0)
+        result = comptimeExpr(gz, scope, rl, node, comptime_reason);
+    else
+        result = exprRl(gz, scope, rl, node);
+    if (refIsNoReturn(gz, result)) {
+        (void)reachable_node;
+        SET_ERROR(gz->astgen);
+    }
+    return result;
+}
+
 static uint32_t tryResolvePrimitiveIdent(GenZir* gz, uint32_t node);
 
 // SimpleComptimeReason (std.zig:727) — values used in block_comptime payload.
@@ -6586,6 +6607,14 @@ static void varDecl(GenZir* gz, Scope* scope, uint32_t node,
 
     uint32_t ident_name = identAsString(ag, name_token);
 
+    // Extract comptime_token by scanning backwards from mut_token
+    // (Ast.zig:2012-2023, fullVarDeclComponents).
+    bool has_comptime_token = false;
+    if (mut_token > 0
+        && tree->tokens.tags[mut_token - 1] == TOKEN_KEYWORD_COMPTIME) {
+        has_comptime_token = true;
+    }
+
     // Extract type_node and init_node based on variant.
     uint32_t type_node = 0;
     uint32_t init_node = 0;
@@ -6619,6 +6648,12 @@ static void varDecl(GenZir* gz, Scope* scope, uint32_t node,
 
     if (is_const) {
         // --- CONST path (AstGen.zig:3232-3340) ---
+
+        // `comptime const` is a non-fatal error; treat it like the init was
+        // marked `comptime` (AstGen.zig:3234-3239).
+        uint32_t force_comptime
+            = has_comptime_token ? COMPTIME_REASON_COMPTIME_KEYWORD : 0;
+
         if (!nodesNeedRlContains(ag, node)) {
             // Rvalue path (AstGen.zig:3246-3271).
             // Evaluate type annotation and build result_info
@@ -6638,7 +6673,8 @@ static void varDecl(GenZir* gz, Scope* scope, uint32_t node,
             }
 
             // Evaluate init expression (AstGen.zig:3251-3252).
-            uint32_t init_ref = exprRl(gz, scope, result_info, init_node);
+            uint32_t init_ref = reachableExprComptime(
+                gz, scope, result_info, init_node, node, force_comptime);
 
             if (ag->has_compile_errors)
                 return;
@@ -6699,7 +6735,8 @@ static void varDecl(GenZir* gz, Scope* scope, uint32_t node,
                 init_rl.src_node = 0;
             }
             init_rl.ctx = RI_CTX_CONST_INIT;
-            uint32_t init_ref = exprRl(gz, scope, init_rl, init_node);
+            uint32_t init_ref = reachableExprComptime(
+                gz, scope, init_rl, init_node, node, force_comptime);
 
             if (ag->has_compile_errors)
                 return;
@@ -6733,20 +6770,22 @@ static void varDecl(GenZir* gz, Scope* scope, uint32_t node,
     } else {
         // --- VAR path (AstGen.zig:3342-3416) ---
 
+        // comptime_token handling (AstGen.zig:3343-3345).
+        bool is_comptime = has_comptime_token || gz->is_comptime;
+
         uint32_t alloc_ref;
         bool resolve_inferred = false;
 
         if (type_node != 0) {
             // Typed var: alloc_mut (AstGen.zig:3361-3375).
             uint32_t type_ref = typeExpr(gz, scope, type_node);
-            ZirInstTag alloc_tag = gz->is_comptime
+            ZirInstTag alloc_tag = is_comptime ? ZIR_INST_ALLOC_COMPTIME_MUT
-                ? ZIR_INST_ALLOC_COMPTIME_MUT
+                                               : ZIR_INST_ALLOC_MUT;
-                : ZIR_INST_ALLOC_MUT;
             alloc_ref = addUnNode(gz, alloc_tag, type_ref, node);
         } else {
             // Inferred type var: alloc_inferred_mut
             // (AstGen.zig:3384-3392).
-            ZirInstTag alloc_tag = gz->is_comptime
+            ZirInstTag alloc_tag = is_comptime
                 ? ZIR_INST_ALLOC_INFERRED_COMPTIME_MUT
                 : ZIR_INST_ALLOC_INFERRED_MUT;
             ZirInstData adata;
@@ -6768,7 +6807,10 @@ static void varDecl(GenZir* gz, Scope* scope, uint32_t node,
             var_init_rl.src_node = 0;
         }
         var_init_rl.ctx = RI_CTX_NONE;
-        uint32_t init_ref = exprRl(gz, scope, var_init_rl, init_node);
+        uint32_t comptime_reason
+            = has_comptime_token ? COMPTIME_REASON_COMPTIME_KEYWORD : 0;
+        uint32_t init_ref = reachableExprComptime(
+            gz, scope, var_init_rl, init_node, node, comptime_reason);
         (void)init_ref;
 
         if (ag->has_compile_errors)
@@ -6791,7 +6833,7 @@ static void varDecl(GenZir* gz, Scope* scope, uint32_t node,
         ptr_out->ptr = final_ptr;
         ptr_out->token_src = name_token;
         ptr_out->name = ident_name;
-        ptr_out->maybe_comptime = gz->is_comptime;
+        ptr_out->maybe_comptime = is_comptime;
         *scope_out = &ptr_out->base;
     }
 }