gccrs: improve mutability checks

This ensures that we handle var decls readonly checks much better

Addresses: Rust-GCC#807 Rust-GCC#3287

gcc/rust/ChangeLog:

	* checks/errors/rust-readonly-check.cc (check_decl): improve mut check
	(emit_error): helper
	(check_modify_expr): likewise
	(readonly_walk_fn): reuse helper
	(ReadonlyCheck::Lint): cleanup context each run

gcc/testsuite/ChangeLog:

	* rust/execute/torture/builtin_macro_include_bytes.rs: needs mut
	* rust/compile/mutability_checks1.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/gcc/testsuite/rust/compile/mutability_checks1.rs b/gcc/testsuite/rust/compile/mutability_checks1.rs
new file mode 100644
index 0000000..4affae0
--- /dev/null
+++ b/gcc/testsuite/rust/compile/mutability_checks1.rs
@@ -0,0 +1,15 @@
+pub fn test() {
+    let a;
+    a = 1;
+    a = 2 + 1;
+    // { dg-error "assignment of read-only variable" "" { target *-*-* } .-1 }
+
+    struct Foo(i32);
+    let a = Foo(1);
+    a.0 = 2;
+    // { dg-error "assignment of read-only variable" "" { target *-*-* } .-1 }
+
+    let a = [1, 2, 3, 4];
+    a[0] = 1 + 2;
+    // { dg-error "assignment of read-only variable" "" { target *-*-* } .-1 }
+}