Suggest mut when possbile for temporary value dropped while borrowed

Author: chenyukang

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -3229,8 +3229,20 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                                     Applicability::MaybeIncorrect,
                                 );
                             }
+
+                            let mutability = if matches!(borrow.kind(), BorrowKind::Mut { .. }) {
+                                "mut "
+                            } else {
+                                ""
+                            };
+
                             if !is_format_arguments_item {
-                                let addition = format!("let binding = {};\n{}", s, " ".repeat(p));
+                                let addition = format!(
+                                    "let {}binding = {};\n{}",
+                                    mutability,
+                                    s,
+                                    " ".repeat(p)
+                                );
                                 err.multipart_suggestion_verbose(
                                     msg,
                                     vec![

tests/ui/coroutine/auto-trait-regions.stderr

@@ -11,7 +11,7 @@ LL |         assert_foo(a);
    |
 help: consider using a `let` binding to create a longer lived value
    |
-LL ~         let binding = true;
+LL ~         let mut binding = true;
 LL ~         let a = A(&mut binding, &mut true, No);
    |
 
@@ -28,7 +28,7 @@ LL |         assert_foo(a);
    |
 help: consider using a `let` binding to create a longer lived value
    |
-LL ~         let binding = true;
+LL ~         let mut binding = true;
 LL ~         let a = A(&mut true, &mut binding, No);
    |
 

tests/ui/nll/sugg-mut-for-binding-issue-137486.fixed

@@ -0,0 +1,23 @@
+//@ run-rustfix
+#![allow(unused_assignments)]
+
+use std::pin::Pin;
+fn main() {
+    let mut s = String::from("hello");
+    let mut ref_s = &mut s;
+
+    let mut binding = String::from("world");
+    ref_s = &mut binding; //~ ERROR temporary value dropped while borrowed [E0716]
+
+    print!("r1 = {}", ref_s);
+
+    let mut val: u8 = 5;
+    let mut s = Pin::new(&mut val);
+    let mut ref_s = &mut s;
+
+    let mut val2: u8 = 10;
+    let mut binding = Pin::new(&mut val2);
+    ref_s = &mut binding; //~ ERROR temporary value dropped while borrowed [E0716]
+
+    print!("r1 = {}", ref_s);
+}

tests/ui/nll/sugg-mut-for-binding-issue-137486.rs

@@ -0,0 +1,21 @@
+//@ run-rustfix
+#![allow(unused_assignments)]
+
+use std::pin::Pin;
+fn main() {
+    let mut s = String::from("hello");
+    let mut ref_s = &mut s;
+
+    ref_s = &mut String::from("world"); //~ ERROR temporary value dropped while borrowed [E0716]
+
+    print!("r1 = {}", ref_s);
+
+    let mut val: u8 = 5;
+    let mut s = Pin::new(&mut val);
+    let mut ref_s = &mut s;
+
+    let mut val2: u8 = 10;
+    ref_s = &mut Pin::new(&mut val2); //~ ERROR temporary value dropped while borrowed [E0716]
+
+    print!("r1 = {}", ref_s);
+}

tests/ui/nll/sugg-mut-for-binding-issue-137486.stderr

@@ -0,0 +1,37 @@
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/sugg-mut-for-binding-issue-137486.rs:9:18
+   |
+LL |     ref_s = &mut String::from("world");
+   |                  ^^^^^^^^^^^^^^^^^^^^^- temporary value is freed at the end of this statement
+   |                  |
+   |                  creates a temporary value which is freed while still in use
+LL |
+LL |     print!("r1 = {}", ref_s);
+   |                       ----- borrow later used here
+   |
+help: consider using a `let` binding to create a longer lived value
+   |
+LL ~     let mut binding = String::from("world");
+LL ~     ref_s = &mut binding;
+   |
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/sugg-mut-for-binding-issue-137486.rs:18:18
+   |
+LL |     ref_s = &mut Pin::new(&mut val2);
+   |                  ^^^^^^^^^^^^^^^^^^^- temporary value is freed at the end of this statement
+   |                  |
+   |                  creates a temporary value which is freed while still in use
+LL |
+LL |     print!("r1 = {}", ref_s);
+   |                       ----- borrow later used here
+   |
+help: consider using a `let` binding to create a longer lived value
+   |
+LL ~     let mut binding = Pin::new(&mut val2);
+LL ~     ref_s = &mut binding;
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0716`.