From aa58439f87f58aa9c7b1ccfb5d52ae5d3d0f1106 Mon Sep 17 00:00:00 2001
From: Santiago Pastorino <spastorino@gmail.com>
Date: Fri, 14 Feb 2025 18:42:06 -0300
Subject: [PATCH] Fail gracefully if mutating on a use closure and the closure
 it not declared mut

---
 .../src/diagnostics/mutability_errors.rs        |  2 +-
 tests/ui/ergonomic-clones/closure/mutation2.rs  | 10 ++++++++++
 .../ergonomic-clones/closure/mutation2.stderr   | 17 +++++++++++++++++
 3 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/ergonomic-clones/closure/mutation2.rs
 create mode 100644 tests/ui/ergonomic-clones/closure/mutation2.stderr

diff --git a/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs b/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
index be4a7736b1c2..145137f92369 100644
--- a/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
@@ -823,7 +823,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                             ) => {
                                 capture_reason = format!("mutable borrow of `{upvar}`");
                             }
-                            ty::UpvarCapture::ByValue => {
+                            ty::UpvarCapture::ByValue | ty::UpvarCapture::ByUse => {
                                 capture_reason = format!("possible mutation of `{upvar}`");
                             }
                             _ => bug!("upvar `{upvar}` borrowed, but not mutably"),
diff --git a/tests/ui/ergonomic-clones/closure/mutation2.rs b/tests/ui/ergonomic-clones/closure/mutation2.rs
new file mode 100644
index 000000000000..8a43f9232bf4
--- /dev/null
+++ b/tests/ui/ergonomic-clones/closure/mutation2.rs
@@ -0,0 +1,10 @@
+#![feature(ergonomic_clones)]
+
+fn main() {
+    let mut my_var = false;
+    let callback = use || {
+        my_var = true;
+    };
+    callback();
+    //~^ ERROR cannot borrow `callback` as mutable, as it is not declared as mutable [E0596]
+}
diff --git a/tests/ui/ergonomic-clones/closure/mutation2.stderr b/tests/ui/ergonomic-clones/closure/mutation2.stderr
new file mode 100644
index 000000000000..3ee8f80ce223
--- /dev/null
+++ b/tests/ui/ergonomic-clones/closure/mutation2.stderr
@@ -0,0 +1,17 @@
+error[E0596]: cannot borrow `callback` as mutable, as it is not declared as mutable
+  --> $DIR/mutation2.rs:8:5
+   |
+LL |         my_var = true;
+   |         ------ calling `callback` requires mutable binding due to possible mutation of `my_var`
+LL |     };
+LL |     callback();
+   |     ^^^^^^^^ cannot borrow as mutable
+   |
+help: consider changing this to be mutable
+   |
+LL |     let mut callback = use || {
+   |         +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0596`.