[gcc r16-6827] gccrs: Fix rogue macro error during lowering on expansion failure

[Arthur Cohen]

https://gcc.gnu.org/g:1cfa7d0771790c1b02f78b855f22e203cd38055a

commit r16-6827-g1cfa7d0771790c1b02f78b855f22e203cd38055a
Author: Harishankar <harishankarpp7@gmail.com>
Date:   Thu Jan 1 10:47:23 2026 +0530

    gccrs: Fix rogue macro error during lowering on expansion failure
    
    When a macro expansion fails (e.g. due to a parsing error like invalid
    syntax in the macro body), the expander previously returned an error
    fragment but did not update the AST. This left the original macro
    invocation in place, which subsequently caused an ICE (rogue macro
    detected) during the lowering phase.
    
    This patch updates `expand_invoc` to replace the macro invocation with
    an empty fragment if expansion fails, ensuring the compiler can proceed
    (or exit gracefully) without crashing.
    
    Fixes Rust-GCC/gccrs#4213
    
    gcc/rust/ChangeLog:
    
            * expand/rust-macro-expand.cc (MacroExpander::expand_invoc): Handle
            error fragments by replacing them with empty fragments.
    
    gcc/testsuite/ChangeLog:
    
            * rust/compile/issue-4213.rs: New test.
    
    Signed-off-by: Harishankar <harishankarpp7@gmail.com>

Diff:
---
 gcc/rust/expand/rust-macro-expand.cc     |  9 ++++++++-
 gcc/testsuite/rust/compile/issue-4213.rs | 34 ++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/gcc/rust/expand/rust-macro-expand.cc b/gcc/rust/expand/rust-macro-expand.cc
index cc044395e3e6..4d658fcd55ae 100644
--- a/gcc/rust/expand/rust-macro-expand.cc
+++ b/gcc/rust/expand/rust-macro-expand.cc
@@ -327,7 +327,14 @@ MacroExpander::expand_invoc (AST::MacroInvocation &invoc,
   else
     fragment
       = expand_decl_macro (invoc.get_locus (), invoc_data, *rdef, semicolon);
-
+  // fix: if the expansion is failing, we must replace the marco with an empty
+  // error or node
+  // makes sure that it doesn't panic on Rouge macro (it -> Lowering Phase)
+  // added the parsing errors in gcc/testsuite/rust/compile/issue-4213.rs
+  if (fragment.is_error ())
+    {
+      fragment = AST::Fragment::create_empty ();
+    }
   set_expanded_fragment (std::move (fragment));
 }
 
diff --git a/gcc/testsuite/rust/compile/issue-4213.rs b/gcc/testsuite/rust/compile/issue-4213.rs
new file mode 100644
index 000000000000..762787b2193a
--- /dev/null
+++ b/gcc/testsuite/rust/compile/issue-4213.rs
@@ -0,0 +1,34 @@
+// Test for issue #4213 - rogue macro detected during Lowering.
+
+macro_rules! inner {
+    () => {
+        $crate::    // { dg-error "could not parse path expression segment" }
+    };
+}
+
+macro_rules! multi_arm {
+    (a) => { $crate:: };     // { dg-error "could not parse path expression segment" }
+    (b) => { () };
+}
+
+macro_rules! another_macro {
+    () => { $crate:: }      // { dg-error "could not parse path expression segment" }
+}
+macro_rules! generic_macro {
+    ($t:ty) => {
+        $crate::            // { dg-error "could not parse path expression segment" }
+    };
+}
+
+macro_rules! empty_case {
+    () => {};
+}
+
+pub fn main() {
+    let _ = inner!();
+    let _ = multi_arm!(a);
+    let _ = multi_arm!(b);
+    let _ = another_macro!();
+    let _ = generic_macro!(i32);
+    empty_case!();
+}
\ No newline at end of file