tychk: Ensure diverging else of let-else is actually diverging

Make sure the `else` expression resolves to the never type.

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-stmt.cc (TypeCheckStmt::visit): Check diverging else
	against `!`.

gcc/testsuite/ChangeLog:

	* rust/compile/let-else-invalid-type.rs: New test.

Author: CohenArthur

Diff for: gcc/rust/typecheck/rust-hir-type-check-stmt.cc

@@ -17,12 +17,14 @@
 // <http://www.gnu.org/licenses/>.
 
 #include "rust-hir-type-check-stmt.h"
+#include "rich-location.h"
 #include "rust-hir-type-check-type.h"
 #include "rust-hir-type-check-expr.h"
 #include "rust-hir-type-check-implitem.h"
 #include "rust-hir-type-check-item.h"
 #include "rust-hir-type-check-pattern.h"
 #include "rust-type-util.h"
+#include "rust-tyty.h"
 
 namespace Rust {
 namespace Resolver {
@@ -78,6 +80,7 @@ TypeCheckStmt::visit (HIR::LetStmt &stmt)
   auto &stmt_pattern = stmt.get_pattern ();
   TyTy::BaseType *init_expr_ty = nullptr;
   location_t init_expr_locus = UNKNOWN_LOCATION;
+
   if (stmt.has_init_expr ())
     {
       init_expr_locus = stmt.get_init_expr ().get_locus ();
@@ -97,6 +100,22 @@ TypeCheckStmt::visit (HIR::LetStmt &stmt)
       specified_ty_locus = stmt.get_type ().get_locus ();
     }
 
+  if (stmt.has_else_expr ())
+    {
+      auto &else_expr = stmt.get_else_expr ();
+      auto else_expr_ty = TypeCheckExpr::Resolve (else_expr);
+
+      if (else_expr_ty->get_kind () != TyTy::TypeKind::NEVER)
+	{
+	  rust_error_at (else_expr.get_locus (),
+			 "%<else%> clause of let-else does not diverge");
+	  return;
+	}
+
+      // FIXME: Is that enough? Do we need to do something like
+      // `append_reference` here as well?
+    }
+
   // let x:i32 = 123;
   if (specified_ty != nullptr && init_expr_ty != nullptr)
     {

Diff for: gcc/testsuite/rust/compile/let-else-invalid-type.rs

@@ -0,0 +1,8 @@
+enum FakeOption {
+    Some(i32),
+    None,
+}
+
+fn main() {
+    let FakeOption::Some(a) = FakeOption::None else { FakeOption::Some(15) }; // { dg-error "does not diverge" }
+}