From a38a627a1d37998a56ef1f34006d3345f0307f90 Mon Sep 17 00:00:00 2001
From: Christoffer Lerno <christoffer@aegik.com>
Date: Sat, 25 Oct 2025 20:33:47 +0200
Subject: [PATCH] Allow `(Foo)0` bitstruct casts even if type sizes do not
 match.

---
 releasenotes.md                               |  1 +
 resources/testproject/src/{ => hello}/bar.c3  |  0
 .../src/{ => hello}/hello_world.c3            |  0
 src/compiler/sema_casts.c                     | 30 +++++++++++++++++--
 .../bitstruct/bistruct_cast_wrong_size.c3     | 22 ++++++++------
 .../bitstruct/bitstruct_cast_literal.c3t      | 27 +++++++++++++++++
 6 files changed, 69 insertions(+), 11 deletions(-)
 rename resources/testproject/src/{ => hello}/bar.c3 (100%)
 rename resources/testproject/src/{ => hello}/hello_world.c3 (100%)
 create mode 100644 test/test_suite/bitstruct/bitstruct_cast_literal.c3t

diff --git a/releasenotes.md b/releasenotes.md
index 14a371858..21bb22b5c 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -15,6 +15,7 @@
 - ABI change for vectors: store and pass them as arrays #2542.
 - Add @simd and @align attributes to typedef #2543.
 - Rename `@extern` to `@cname`, deprecating the old name #2493.
+- Allow `(Foo)0` bitstruct casts even if type sizes do not match.
 
 ### Fixes
 - Bug in `io::write_using_write_byte`.
diff --git a/resources/testproject/src/bar.c3 b/resources/testproject/src/hello/bar.c3
similarity index 100%
rename from resources/testproject/src/bar.c3
rename to resources/testproject/src/hello/bar.c3
diff --git a/resources/testproject/src/hello_world.c3 b/resources/testproject/src/hello/hello_world.c3
similarity index 100%
rename from resources/testproject/src/hello_world.c3
rename to resources/testproject/src/hello/hello_world.c3
diff --git a/src/compiler/sema_casts.c b/src/compiler/sema_casts.c
index 549128c8d..d2a3a4174 100644
--- a/src/compiler/sema_casts.c
+++ b/src/compiler/sema_casts.c
@@ -1523,7 +1523,24 @@ static bool rule_int_to_bits(CastContext *cc, bool is_explicit, bool is_silent)
 {
 	Type *base_type = cc->to->decl->strukt.container_type->type;
 	Type *from_type = cc->from;
-	bool success = type_is_integer(base_type) && type_size(from_type) == type_size(base_type);
+	Expr *expr = cc->expr;
+	bool success = false;
+	do
+	{
+		if (!type_is_integer(base_type)) break;
+		if (type_size(base_type) == type_size(from_type))
+		{
+			success = true;
+			break;
+		}
+		if (!sema_cast_const(expr) || !expr_is_const_int(expr)) break;
+		if (!int_fits(cc->expr->const_expr.ixx, base_type->canonical->type_kind))
+		{
+			if (is_silent) return false;
+			RETURN_CAST_ERROR(expr, "The value '%s' does not fit in the container type of the bitstruct %s, which is %s.", int_to_str(cc->expr->const_expr.ixx, 10, false), type_quoted_error_string(cc->to), type_quoted_error_string(base_type));
+		}
+		success = true;
+	} while (0);
 	if (!is_explicit || !success) return sema_cast_error(cc, success, is_silent);
 	return true;
 }
@@ -1857,7 +1874,16 @@ static void cast_expand_to_vec(Expr *expr, Type *type)
 }
 
 static void cast_bitstruct_to_int_arr(Expr *expr, Type *type) { expr_rewrite_recast(expr, type); }
-static void cast_int_arr_to_bitstruct(Expr *expr, Type *type) { expr_rewrite_recast(expr, type); }
+static void cast_int_arr_to_bitstruct(Expr *expr, Type *type)
+{
+	if (expr_is_const_int(expr))
+	{
+		Type *widening_type = type_flatten(type)->decl->strukt.container_type->type->canonical;
+		expr->const_expr.ixx.type = widening_type->type_kind;
+		expr->type = widening_type;
+	}
+	expr_rewrite_recast(expr, type);
+}
 
 static void cast_bitstruct_to_bool(Expr *expr, Type *type)
 {
diff --git a/test/test_suite/bitstruct/bistruct_cast_wrong_size.c3 b/test/test_suite/bitstruct/bistruct_cast_wrong_size.c3
index 27b1e34c7..48d415eed 100644
--- a/test/test_suite/bitstruct/bistruct_cast_wrong_size.c3
+++ b/test/test_suite/bitstruct/bistruct_cast_wrong_size.c3
@@ -1,14 +1,18 @@
 module bug_repro;
 
-bitstruct Edge_Flags : char {
-  bool left;
-  bool top;
-  bool right;
-  bool bottom;
+bitstruct Edge_Flags : char
+{
+	bool left;
+	bool top;
+	bool right;
+	bool bottom;
 }
 
-fn int main() {
-  Edge_Flags edges;
-  Edge_Flags a = edges & (Edge_Flags)(1 << 0); // #error: 'int'
-  return 0;
+fn int main()
+{
+	Edge_Flags edges;
+	Edge_Flags a = edges & (Edge_Flags)(1 << 8); // #error: The value '256' does not fit in the container type of the bitstruct
+	int x = 0;
+	Edge_Flags ab = edges & (Edge_Flags)x; // #error: 'int'
+	return 0;
 }
\ No newline at end of file
diff --git a/test/test_suite/bitstruct/bitstruct_cast_literal.c3t b/test/test_suite/bitstruct/bitstruct_cast_literal.c3t
new file mode 100644
index 000000000..8cd75b015
--- /dev/null
+++ b/test/test_suite/bitstruct/bitstruct_cast_literal.c3t
@@ -0,0 +1,27 @@
+// #target: macos-x64
+module test;
+bitstruct Foo : char
+{
+	bool baz;
+}
+
+fn int main()
+{
+	char ch = 0;
+	Foo foo1 = (Foo)ch;
+	Foo foo2 = (Foo)0;
+	return 0;
+}
+
+/* #expect: test.ll
+
+entry:
+  %ch = alloca i8, align 1
+  %foo1 = alloca i8, align 1
+  %foo2 = alloca i8, align 1
+  store i8 0, ptr %ch, align 1
+  %0 = load i8, ptr %ch, align 1
+  store i8 %0, ptr %foo1, align 1
+  store i8 0, ptr %foo2, align 1
+  ret i32 0
+}