From c949bd3108178e2188a19aa204000ed2fd937127 Mon Sep 17 00:00:00 2001
From: Christoffer Lerno <christoffer@aegik.com>
Date: Mon, 29 Dec 2025 22:24:41 +0100
Subject: [PATCH] Assert when struct size would exceed 4 GB.

---
 releasenotes.md                            |  1 +
 src/compiler/sema_decls.c                  |  2 ++
 src/utils/common.h                         |  1 +
 test/test_suite/struct/struct_too_large.c3 | 12 ++++++++++++
 4 files changed, 16 insertions(+)
 create mode 100644 test/test_suite/struct/struct_too_large.c3

diff --git a/releasenotes.md b/releasenotes.md
index cc43652b3..a46fc33d1 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -38,6 +38,7 @@
 - Converting between simd/non-simd bool vector would hit a compiler assert. #2691
 - `i<n>` suffixes were not caught when n < 8, causing an assert.
 - Parse error in `$defined` was not handled correctly, leading to an assertion.
+- Assert when struct size would exceed 4 GB.
 
 ### Stdlib changes
 - Add `ThreadPool` join function to wait for all threads to finish in the pool without destroying the threads.
diff --git a/src/compiler/sema_decls.c b/src/compiler/sema_decls.c
index a3a992856..6fcad4f7b 100644
--- a/src/compiler/sema_decls.c
+++ b/src/compiler/sema_decls.c
@@ -637,7 +637,9 @@ static bool sema_analyse_struct_members(SemaContext *context, Decl *decl)
 
 		offset = align_offset;
 		member->offset = offset;
+		AlignSize sz = offset;
 		offset += type_size(member->type);
+		if (offset < sz || offset > MAX_STRUCT_SIZE) RETURN_SEMA_ERROR(member, "Struct member '%s' would cause the struct to become too large (exceeding 2 GB).", member->name);
 	}
 
 	// Set the alignment:
diff --git a/src/utils/common.h b/src/utils/common.h
index 899fe152e..6b86cc982 100644
--- a/src/utils/common.h
+++ b/src/utils/common.h
@@ -29,6 +29,7 @@
 #define DEFAULT_STACK_OBJECT_SIZE 64
 #define MAX_ARRAY_SIZE INT64_MAX
 #define MAX_SOURCE_LOCATION_LEN 255
+#define MAX_STRUCT_SIZE (2U * 1024U * 1024U * 1024U)
 #define PROJECT_JSON "project.json"
 #define PROJECT_JSON5 "project.json5"
 
diff --git a/test/test_suite/struct/struct_too_large.c3 b/test/test_suite/struct/struct_too_large.c3
new file mode 100644
index 000000000..7b80726e1
--- /dev/null
+++ b/test/test_suite/struct/struct_too_large.c3
@@ -0,0 +1,12 @@
+import std;
+struct Foo
+{
+    int a;
+    int[int.max] b; // #error: "Struct member 'b' would cause the struct to become too large
+}
+
+fn int main()
+{
+    Foo c = (Foo){};
+    return 0;
+}