gccrs: check for invalid const calls during code-gen

Closure calls are not const so this is invalid. This patch fixes two bugs

  1. Make the look at the parent context optional for generics
  2. Ensure we look for non const calls during call expr code-gen

Fixes Rust-GCC#3551

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::visit): add const call check
	* backend/rust-compile-item.cc (CompileItem::visit): ensure we upfront compile types where
	possible
	* backend/rust-compile-item.h: update header
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit): make parent ctx optional

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3551.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/issue-3551.rs b/gcc/testsuite/rust/compile/issue-3551.rs
new file mode 100644
index 0000000..6d6a812
--- /dev/null
+++ b/gcc/testsuite/rust/compile/issue-3551.rs
@@ -0,0 +1,15 @@
+#[lang = "sized"]
+pub trait Sized {}
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
+struct Bug {
+    a: [(); (|| 0)()],
+    // { dg-error "calls in constants are limited to constant functions, tuple structs and tuple variants" "" { target *-*-* } .-1 }
+}