[gcc r17-3125] gccrs: Fix duplicate unused assignment removals

https://gcc.gnu.org/g:3b338a1ce871cbf36731d56f02d1d458243c1c23

commit r17-3125-g3b338a1ce871cbf36731d56f02d1d458243c1c23
Author: Aiman Najjar <[email protected]>
Date:   Fri Jul 31 22:32:03 2026 -0400

    gccrs: Fix duplicate unused assignment removals
    
    Fixes Rust-GCC/gccrs#4674
    
    gcc/rust/ChangeLog:
    
            * checks/lints/unused/rust-unused-context.cc
            (UnusedContext::remove_assign): Fix issue with duplicate
            removal of unused-assignment for the same id.
    
    gcc/testsuite/ChangeLog:
    
            * rust/compile/issue-4674.rs: New test.
    
    Signed-off-by: Aiman Najjar <[email protected]>

Diff:
---
 gcc/rust/checks/lints/unused/rust-unused-context.cc |  7 ++++++-
 gcc/testsuite/rust/compile/issue-4674.rs            | 15 +++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/gcc/rust/checks/lints/unused/rust-unused-context.cc b/gcc/rust/checks/lints/unused/rust-unused-context.cc
index 9b26e01bccee..8a68a601acc3 100644
--- a/gcc/rust/checks/lints/unused/rust-unused-context.cc
+++ b/gcc/rust/checks/lints/unused/rust-unused-context.cc
@@ -44,7 +44,12 @@ void
 UnusedContext::remove_assign (HirId id_def)
 {
   if (assigned_vars.find (id_def) != assigned_vars.end ())
-    assigned_vars[id_def].pop_back ();
+    {
+      assigned_vars[id_def].pop_back ();
+
+      if (assigned_vars[id_def].empty ())
+	assigned_vars.erase (id_def);
+    }
 }
 
 bool
diff --git a/gcc/testsuite/rust/compile/issue-4674.rs b/gcc/testsuite/rust/compile/issue-4674.rs
new file mode 100644
index 000000000000..f787c5c4bcfd
--- /dev/null
+++ b/gcc/testsuite/rust/compile/issue-4674.rs
@@ -0,0 +1,15 @@
+// { dg-additional-options "-frust-unused-check-2.0" }
+#![feature(no_core)]
+#![no_core]
+
+fn foo(mut n: i32) {
+    // { dg-warning "function is never used: .foo." "" { target *-*-* } .-1 }
+    if false {
+        n = 0i32;
+    }
+
+    if n > 0i32 {
+        let _ = 1i32 / n;
+    }
+}
+