Fix ICE with continue/break/return in while condition

Fixes #3977

The predicate expression must be evaluated before type checking
to ensure side effects occur even when the predicate has never type.
This prevents skipping function calls, panics, or other side effects
in diverging predicates.

Proof of fix using -fdump-tree-gimple:

__attribute__((cdecl))
struct () test::main ()
{
  struct () D.107;

  <D.101>:
  <D.103>:
  {
    <D.104>:
    {
      <D.102>:
      goto <D.102>; // Side-effect correctly preserved
      if (0 != 0) goto <D.105>; else goto <D.106>;
      <D.106>:
      {

      }
    }
    goto <D.104>;
    <D.105>:
  }
  goto <D.103>;
  return D.107;
}

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::visit): Always
	evaluate predicate expression before checking for never type
	to preserve side effects in while loop conditions.
	* typecheck/rust-hir-type-check-expr.cc: Update handling of break/continue.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3977.rs: New test.

Signed-off-by: Harishankar <[email protected]>

Author: Harishankar14

gcc/rust/backend/rust-compile-expr.cc

@@ -803,7 +803,18 @@ CompileExpr::visit (HIR::WhileLoopExpr &expr)
   ctx->add_statement (loop_begin_label_decl);
   ctx->push_loop_begin_label (loop_begin_label);
 
-  tree condition = CompileExpr::Compile (expr.get_predicate_expr (), ctx);
+  HIR::Expr &predicate = expr.get_predicate_expr ();
+  TyTy::BaseType *predicate_type = nullptr;
+  bool ok
+    = ctx->get_tyctx ()->lookup_type (predicate.get_mappings ().get_hirid (),
+				      &predicate_type);
+  rust_assert (ok && predicate_type != nullptr);
+  tree condition = CompileExpr::Compile (predicate, ctx);
+  if (predicate_type->get_kind () == TyTy::TypeKind::NEVER)
+    {
+      ctx->add_statement (condition);
+      condition = boolean_true_node;
+    }
   tree exit_condition = fold_build1_loc (expr.get_locus (), TRUTH_NOT_EXPR,
 					 boolean_type_node, condition);
   tree exit_expr = Backend::exit_expression (exit_condition, expr.get_locus ());

gcc/rust/typecheck/rust-hir-type-check-expr.cc

@@ -1550,18 +1550,25 @@ void
 TypeCheckExpr::visit (HIR::WhileLoopExpr &expr)
 {
   context->push_new_while_loop_context (expr.get_mappings ().get_hirid ());
-
-  TypeCheckExpr::Resolve (expr.get_predicate_expr ());
+  TyTy::BaseType *predicate_type
+    = TypeCheckExpr::Resolve (expr.get_predicate_expr ());
+  if (predicate_type->get_kind () != TyTy::TypeKind::BOOL
+      && predicate_type->get_kind () != TyTy::TypeKind::NEVER)
+    {
+      rust_error_at (expr.get_predicate_expr ().get_locus (),
+		     "expected boolean expression in %<while%> condition");
+      context->pop_loop_context ();
+      return;
+    }
   TyTy::BaseType *block_expr = TypeCheckExpr::Resolve (expr.get_loop_block ());
-
   if (!block_expr->is_unit ())
     {
       rust_error_at (expr.get_loop_block ().get_locus (),
 		     "expected %<()%> got %s",
 		     block_expr->as_string ().c_str ());
+      context->pop_loop_context ();
       return;
     }
-
   context->pop_loop_context ();
   infered = TyTy::TupleType::get_unit_type ();
 }
@@ -1611,7 +1618,6 @@ TypeCheckExpr::visit (HIR::ContinueExpr &expr)
 		     "%<continue%> outside of a loop");
       return;
     }
-
   infered = new TyTy::NeverType (expr.get_mappings ().get_hirid ());
 }
 

gcc/testsuite/rust/compile/issue-3977.rs

@@ -0,0 +1,65 @@
+// Test for issue #3977 - ICE with continue/break/return in while condition
+
+fn diverge() -> ! {
+    loop {}
+}
+
+fn test_continue() {
+    loop {
+        while continue {}
+    }
+}
+
+fn test_break() {      
+    loop {
+        while break {}
+    }
+}
+
+fn test_return() {
+    loop {
+        while return {}
+    }
+}
+
+fn test_labeled_break() {
+    'outer: loop {
+        loop {
+            while break 'outer {}
+        }
+    }
+}
+
+fn test_labeled_continue() {
+    'outer: loop {
+        loop {
+            while continue 'outer {}
+        }
+    }
+}
+
+fn test_complex_if_else() {
+    loop {
+        while if true { continue } else { break } {}
+    }
+}
+
+fn foo() {
+    while diverge() {
+        break
+    }
+    let _x = 5;
+}
+
+fn main() {
+    // Just reference them so they're "used"
+    if false {
+        test_continue();
+        test_break();
+        test_return();
+        test_labeled_break();
+        test_labeled_continue();
+        test_complex_if_else();
+        foo();
+    }
+}