From a2ef63f5b636eda66905a2130add336b54b8f9b1 Mon Sep 17 00:00:00 2001
From: Christoffer Lerno <christoffer@aegik.com>
Date: Thu, 21 Aug 2025 00:33:56 +0200
Subject: [PATCH] Error if a stack allocated variable is too big (configurable
 with `--max-stack-object-size`).

---
 releasenotes.md                          |  1 +
 src/build/build.h                        |  2 ++
 src/build/build_options.c                | 10 ++++++++++
 src/build/builder.c                      |  2 ++
 src/compiler/parse_expr.c                | 11 +++++++----
 src/compiler/parse_global.c              |  3 ++-
 src/compiler/sema_casts.c                | 14 +++++---------
 src/compiler/sema_decls.c                | 10 ++++++++++
 src/compiler/sema_internal.h             |  2 +-
 src/compiler/sema_stmts.c                | 17 ++++++-----------
 src/utils/common.h                       |  2 ++
 test/test_suite/statements/big_locals.c3 |  6 ++++++
 12 files changed, 54 insertions(+), 26 deletions(-)
 create mode 100644 test/test_suite/statements/big_locals.c3

diff --git a/releasenotes.md b/releasenotes.md
index a7e15e1f4..e58c39bd7 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -17,6 +17,7 @@
 - Fix max module name to 31 chars and the entire module path to 63 characters.
 - Improve error message for missing `$endif`.
 - `foo[x][y] = b` now interpreted as `(*&foo[x])[y] = b` which allows overloads to do chained [] accesses.
+- Error if a stack allocated variable is too big (configurable with `--max-stack-object-size`).
 
 ### Fixes
 - List.remove_at would incorrectly trigger ASAN.
diff --git a/src/build/build.h b/src/build/build.h
index 89707b3ff..eefb63961 100644
--- a/src/build/build.h
+++ b/src/build/build.h
@@ -585,6 +585,7 @@ typedef struct BuildOptions_
 	MemoryEnvironment memory_environment;
 	SanitizeMode sanitize_mode;
 	uint32_t max_vector_size;
+	uint32_t max_stack_object_size;
 	bool print_keywords;
 	bool print_attributes;
 	bool print_builtins;
@@ -725,6 +726,7 @@ typedef struct
 	LinkerType linker_type;
 	uint32_t symtab_size;
 	uint32_t max_vector_size;
+	uint32_t max_stack_object_size;
 	uint32_t switchrange_max_size;
 	uint32_t switchjump_max_size;
 	const char **args;
diff --git a/src/build/build_options.c b/src/build/build_options.c
index 0e0ca32da..02314d09c 100644
--- a/src/build/build_options.c
+++ b/src/build/build_options.c
@@ -177,6 +177,7 @@ static void usage(bool full)
 		print_opt("--win64-simd=<option>", "Win64 SIMD ABI: array, full.");
 		print_opt("--win-debug=<option>", "Select debug output on Windows: codeview or dwarf (default: codeview).");
 		print_opt("--max-vector-size <number>", "Set the maximum vector bit size to allow (default: 4096).");
+		print_opt("--max-stack-object-size <number>", "Set the maximum size of a stack object in KB (default: 128).");
 		PRINTF("");
 		print_opt("--print-linking", "Print linker arguments.");
 		PRINTF("");
@@ -943,6 +944,14 @@ static void parse_option(BuildOptions *options)
 				options->riscv_float_capability = parse_opt_select(RiscvFloatCapability, argopt, riscv_capability);
 				return;
 			}
+			if (match_longopt("max-stack-object-size"))
+			{
+				int size = (at_end() || next_is_opt()) ? 0 : atoi(next_arg());
+				if (size < 1) error_exit("Expected a valid positive integer >= 1 for --max-stack-object-size.");
+				if (size > MAX_STACK_OBJECT_SIZE) error_exit("Expected a valid positive integer <= %u for --max-vector-size.", (unsigned)MAX_STACK_OBJECT_SIZE);
+				options->max_stack_object_size = size;
+				return;
+			}
 			if (match_longopt("max-vector-size"))
 			{
 				int size = (at_end() || next_is_opt()) ? 0 : atoi(next_arg());
@@ -952,6 +961,7 @@ static void parse_option(BuildOptions *options)
 				{
 					error_exit("The --max-vector-size value must be a power of 2, try using %u instead.", next_highest_power_of_2(size));
 				}
+				options->max_vector_size = size;
 				return;
 			}
 			if ((argopt = match_argopt("memory-env")))
diff --git a/src/build/builder.c b/src/build/builder.c
index 866c652d9..7950de9bc 100644
--- a/src/build/builder.c
+++ b/src/build/builder.c
@@ -431,6 +431,7 @@ static void update_build_target_from_options(BuildTarget *target, BuildOptions *
 	OVERRIDE_IF_SET(benchfn);
 	OVERRIDE_IF_SET(symtab_size);
 	OVERRIDE_IF_SET(max_vector_size);
+	OVERRIDE_IF_SET(max_stack_object_size);
 	OVERRIDE_IF_SET(win.def);
 	OVERRIDE_IF_SET(no_entry);
 	OVERRIDE_IF_SET(echo_prefix);
@@ -446,6 +447,7 @@ static void update_build_target_from_options(BuildTarget *target, BuildOptions *
 	OVERRIDE_IF_SET(android.api_version);
 
 	if (!target->max_vector_size) target->max_vector_size = DEFAULT_VECTOR_WIDTH;
+	if (!target->max_stack_object_size) target->max_stack_object_size = DEFAULT_STACK_OBJECT_SIZE;
 
 	if (target->quiet && !options->verbosity_level) options->verbosity_level = -1;
 
diff --git a/src/compiler/parse_expr.c b/src/compiler/parse_expr.c
index 271d5f0db..443d2e641 100644
--- a/src/compiler/parse_expr.c
+++ b/src/compiler/parse_expr.c
@@ -407,7 +407,7 @@ static bool parse_param_path(ParseContext *c, DesignatorElement ***path)
 	}
 }
 
-static Expr *parse_lambda(ParseContext *c, Expr *left, SourceSpan lhs_span)
+static Expr *parse_lambda(ParseContext *c, Expr *left, SourceSpan lhs_span UNUSED)
 {
 	ASSERT(!left && "Unexpected left hand side");
 	Expr *expr = EXPR_NEW_TOKEN(EXPR_LAMBDA);
@@ -622,9 +622,10 @@ Expr *parse_ct_expression_list(ParseContext *c, bool allow_decl)
  *
  * @param c the context
  * @param left must be null.
+ * @param lhs_start unused
  * @return Expr *
  */
-static Expr *parse_type_identifier(ParseContext *c, Expr *left, SourceSpan lhs_start)
+static Expr *parse_type_identifier(ParseContext *c, Expr *left, SourceSpan lhs_start UNUSED)
 {
 	ASSERT(!left && "Unexpected left hand side");
 	return parse_type_expression_with_path(c, NULL);
@@ -635,9 +636,10 @@ static Expr *parse_type_identifier(ParseContext *c, Expr *left, SourceSpan lhs_s
  *
  * @param c the context
  * @param left must be null.
+ * @param lhs_start unused
  * @return Expr *
  */
-static Expr *parse_splat(ParseContext *c, Expr *left, SourceSpan lhs_start)
+static Expr *parse_splat(ParseContext *c, Expr *left, SourceSpan lhs_start UNUSED)
 {
 	ASSERT(!left && "Unexpected left hand side");
 	Expr *expr = expr_new(EXPR_SPLAT, c->span);
@@ -671,9 +673,10 @@ static Expr *parse_type_expr(ParseContext *c, Expr *left, SourceSpan lhs_start U
  *
  * @param c the context
  * @param left must be null
+ * @param lhs_start unused
  * @return Expr *
  */
-static Expr *parse_ct_stringify(ParseContext *c, Expr *left, SourceSpan lhs_start)
+static Expr *parse_ct_stringify(ParseContext *c, Expr *left, SourceSpan lhs_start UNUSED)
 {
 	ASSERT(!left && "Unexpected left hand side");
 	SourceSpan start_span = c->span;
diff --git a/src/compiler/parse_global.c b/src/compiler/parse_global.c
index a7eabc944..e568b3edc 100644
--- a/src/compiler/parse_global.c
+++ b/src/compiler/parse_global.c
@@ -764,7 +764,8 @@ INLINE bool parse_rethrow_bracket(ParseContext *c, SourceSpan start)
 /**
  * optional_type ::= type '!'?
  * @param c
- * @return
+ * @param allow_generic should generic be allowed
+ * @return The resulting type
  */
 static inline TypeInfo *parse_optional_type_maybe_generic(ParseContext *c, bool allow_generic)
 {
diff --git a/src/compiler/sema_casts.c b/src/compiler/sema_casts.c
index 291ce0074..cf0628d92 100644
--- a/src/compiler/sema_casts.c
+++ b/src/compiler/sema_casts.c
@@ -545,7 +545,7 @@ RETRY:
 		}
 		default:
 			// Check type sizes
-			goto CHECK_SIZE;
+			break;
 	}
 CHECK_SIZE:
 	if (type_size(expr->type) > type_size(type)) return expr;
@@ -721,13 +721,9 @@ static bool report_cast_error(CastContext *cc, bool may_cast_explicit)
 			                  type_quoted_error_string(to),
 			                  type_to_error_string(type_no_optional(to)));
 		}
-		else
-		{
-			RETURN_CAST_ERROR(expr,
-			                  "It is not possible to cast %s to the inner type %s.",
-			                  type_quoted_error_string(type_no_optional(expr->type)), type_quoted_error_string(to));
-		}
-
+		RETURN_CAST_ERROR(expr,
+		                  "It is not possible to cast %s to the inner type %s.",
+		                  type_quoted_error_string(type_no_optional(expr->type)), type_quoted_error_string(to));
 	}
 	if (may_cast_explicit)
 	{
@@ -837,7 +833,7 @@ static bool rule_ptr_to_ptr(CastContext *cc, bool is_explicit, bool is_silent)
 }
 
 
-static bool rule_all_ok(CastContext *cc, bool is_explicit, bool silent)
+static bool rule_all_ok(CastContext *cc UNUSED, bool is_explicit UNUSED, bool silent UNUSED)
 {
 	return true;
 }
diff --git a/src/compiler/sema_decls.c b/src/compiler/sema_decls.c
index 0bb43e589..c2291a1fe 100755
--- a/src/compiler/sema_decls.c
+++ b/src/compiler/sema_decls.c
@@ -4789,6 +4789,16 @@ bool sema_analyse_var_decl(SemaContext *context, Decl *decl, bool local)
 	{
 		if (!sema_set_alloca_alignment(context, decl->type, &decl->alignment)) return false;
 	}
+	if (decl->var.kind == VARDECL_LOCAL && type_size(decl->type) > compiler.build.max_stack_object_size * 1024)
+	{
+		size_t size = type_size(decl->type);
+		RETURN_SEMA_ERROR(
+			decl, "The size of this local variable (%s%d Kb) exceeds the maximum allowed stack object size (%d Kb), "
+			"you can increase this limit with --max-stack-object-size, but be aware that too large objects "
+			"allocated on the stack may lead to the stack running out of memory.", size % 1024 == 0 ? "" : "over ",
+			size / 1024,
+			compiler.build.max_stack_object_size);
+	}
 	return success;
 }
 
diff --git a/src/compiler/sema_internal.h b/src/compiler/sema_internal.h
index c322bdd35..640070c2d 100644
--- a/src/compiler/sema_internal.h
+++ b/src/compiler/sema_internal.h
@@ -98,7 +98,7 @@ bool sema_analyse_expr_value(SemaContext *context, Expr *expr);
 Expr *expr_access_inline_member(Expr *parent, Decl *parent_decl);
 bool sema_analyse_ct_expr(SemaContext *context, Expr *expr);
 Decl *sema_find_typed_operator(SemaContext *context, OperatorOverload operator_overload, SourceSpan span, Expr *lhs, Expr *rhs, bool *reverse);
-OverloadMatch sema_find_typed_operator_type(SemaContext *context, OperatorOverload operator_overload, OverloadType overloat_type, Type *lhs_type, Type *rhs_type, Expr *rhs, Decl **candidate_ref, OverloadMatch last_match, Decl **ambiguous_ref);
+OverloadMatch sema_find_typed_operator_type(SemaContext *context, OperatorOverload operator_overload, OverloadType overload_type, Type *lhs_type, Type *rhs_type, Expr *rhs, Decl **candidate_ref, OverloadMatch last_match, Decl **ambiguous_ref);
 BoolErr sema_type_has_equality_overload(SemaContext *context, Type *type);
 Decl *sema_find_untyped_operator(Type *type, OperatorOverload operator_overload, Decl *skipped);
 bool sema_insert_method_call(SemaContext *context, Expr *method_call, Decl *method_decl, Expr *parent, Expr **arguments, bool reverse_overload);
diff --git a/src/compiler/sema_stmts.c b/src/compiler/sema_stmts.c
index 8ec6dc5c4..ffbdd4c7b 100644
--- a/src/compiler/sema_stmts.c
+++ b/src/compiler/sema_stmts.c
@@ -49,7 +49,7 @@ static inline bool sema_check_value_case(SemaContext *context, Type *switch_type
 static bool sema_analyse_switch_body(SemaContext *context, Ast *statement, SourceSpan expr_span, CanonicalType *switch_type, Ast **cases);
 
 static inline bool sema_analyse_statement_inner(SemaContext *context, Ast *statement);
-static bool sema_analyse_require(SemaContext *context, Ast *directive, AstId **asserts, SourceSpan source);
+static bool sema_analyse_require(SemaContext *context, Ast *directive, AstId **asserts, SourceSpan span);
 static bool sema_analyse_ensure(SemaContext *context, Ast *directive);
 static bool sema_analyse_optional_returns(SemaContext *context, Ast *directive);
 
@@ -880,11 +880,9 @@ static inline bool sema_analyse_try_unwrap(SemaContext *context, Expr *expr)
 		{
 			if (decl->var.kind == VARDECL_UNWRAPPED)
 			{
-				SEMA_ERROR(ident, "This variable is already unwrapped, so you cannot use 'try' on it again, please remove the 'try'.");
-				return false;
+				RETURN_SEMA_ERROR(ident, "This variable is already unwrapped, so you cannot use 'try' on it again, please remove the 'try'.");
 			}
-			SEMA_ERROR(ident, "Expected this variable to be an optional, otherwise it can't be used for unwrap, maybe you didn't intend to use 'try'?");
-			return false;
+			RETURN_SEMA_ERROR(ident, "Expected this variable to be an optional, otherwise it can't be used for unwrap, maybe you didn't intend to use 'try'?");
 		}
 		expr->expr_kind = EXPR_TRY;
 		expr->try_expr = (ExprTry) { .decl = decl };
@@ -929,7 +927,6 @@ static inline bool sema_analyse_try_unwrap(SemaContext *context, Expr *expr)
 	if (!IS_OPTIONAL(optional))
 	{
 		RETURN_SEMA_ERROR(optional, "Expected an optional expression to 'try' here. If it isn't an optional, remove 'try'.");
-		return false;
 	}
 
 	if (var_type)
@@ -959,9 +956,8 @@ static inline bool sema_analyse_try_unwrap(SemaContext *context, Expr *expr)
 
 static inline bool sema_analyse_try_unwrap_chain(SemaContext *context, Expr *expr, CondType cond_type, CondResult *result)
 {
-	ASSERT(cond_type == COND_TYPE_UNWRAP_BOOL);
-
-	ASSERT(expr->expr_kind == EXPR_TRY_UNWRAP_CHAIN);
+	ASSERT_SPAN(expr, cond_type == COND_TYPE_UNWRAP_BOOL);
+	ASSERT_SPAN(expr, expr->expr_kind == EXPR_TRY_UNWRAP_CHAIN);
 
 	FOREACH(Expr *, chain_element, expr->try_unwrap_chain_expr)
 	{
@@ -3239,8 +3235,7 @@ static bool sema_analyse_ensure(SemaContext *context, Ast *directive)
 	{
 		if (expr->expr_kind == EXPR_DECL)
 		{
-			SEMA_ERROR(expr, "Only expressions are allowed.");
-			return false;
+			RETURN_SEMA_ERROR(expr, "Only expressions are allowed.");
 		}
 	}
 	return true;
diff --git a/src/utils/common.h b/src/utils/common.h
index 897b3ab61..c8b465050 100644
--- a/src/utils/common.h
+++ b/src/utils/common.h
@@ -22,7 +22,9 @@
 
 #define NO_ARENA 0
 #define MAX_VECTOR_WIDTH 65536
+#define MAX_STACK_OBJECT_SIZE (256 * 1024)
 #define DEFAULT_VECTOR_WIDTH 4096
+#define DEFAULT_STACK_OBJECT_SIZE 64
 #define MAX_ARRAY_SIZE INT64_MAX
 #define MAX_SOURCE_LOCATION_LEN 255
 #define PROJECT_JSON "project.json"
diff --git a/test/test_suite/statements/big_locals.c3 b/test/test_suite/statements/big_locals.c3
new file mode 100644
index 000000000..c3ebe594d
--- /dev/null
+++ b/test/test_suite/statements/big_locals.c3
@@ -0,0 +1,6 @@
+import std;
+fn int main(String[] args)
+{
+	int[1024 * 1024] x; // #error: The size of this local variable (4096 Kb) exceeds the maximum allowed stack object size (128 Kb)
+    return 0;
+}