Tweak "borrow closure argument" suggestion

Fix #45727.

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -1350,6 +1350,7 @@ fn report_selection_error(
                         expected_trait_ref,
                         obligation.cause.code(),
                         found_node,
+                        obligation.param_env,
                     )
                 } else {
                     let (closure_span, closure_arg_span, found) = found_did

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -283,6 +283,7 @@ fn report_closure_arg_mismatch(
         expected: ty::PolyTraitRef<'tcx>,
         cause: &ObligationCauseCode<'tcx>,
         found_node: Option<Node<'_>>,
+        param_env: ty::ParamEnv<'tcx>,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
 
     fn note_conflicting_closure_bounds(
@@ -1978,6 +1979,7 @@ fn report_closure_arg_mismatch(
         expected: ty::PolyTraitRef<'tcx>,
         cause: &ObligationCauseCode<'tcx>,
         found_node: Option<Node<'_>>,
+        param_env: ty::ParamEnv<'tcx>,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         pub(crate) fn build_fn_sig_ty<'tcx>(
             infcx: &InferCtxt<'tcx>,
@@ -2040,7 +2042,7 @@ pub(crate) fn build_fn_sig_ty<'tcx>(
         self.note_conflicting_closure_bounds(cause, &mut err);
 
         if let Some(found_node) = found_node {
-            hint_missing_borrow(span, found, expected, found_node, &mut err);
+            hint_missing_borrow(self, param_env, span, found, expected, found_node, &mut err);
         }
 
         err
@@ -3747,6 +3749,8 @@ fn probe_assoc_types_at_expr(
 
 /// Add a hint to add a missing borrow or remove an unnecessary one.
 fn hint_missing_borrow<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
     span: Span,
     found: Ty<'tcx>,
     expected: Ty<'tcx>,
@@ -3769,7 +3773,7 @@ fn hint_missing_borrow<'tcx>(
     // This could be a variant constructor, for example.
     let Some(fn_decl) = found_node.fn_decl() else { return; };
 
-    let arg_spans = fn_decl.inputs.iter().map(|ty| ty.span);
+    let args = fn_decl.inputs.iter().map(|ty| ty);
 
     fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, usize) {
         let mut refs = 0;
@@ -3785,21 +3789,34 @@ fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, usize) {
     let mut to_borrow = Vec::new();
     let mut remove_borrow = Vec::new();
 
-    for ((found_arg, expected_arg), arg_span) in found_args.zip(expected_args).zip(arg_spans) {
+    for ((found_arg, expected_arg), arg) in found_args.zip(expected_args).zip(args) {
         let (found_ty, found_refs) = get_deref_type_and_refs(*found_arg);
         let (expected_ty, expected_refs) = get_deref_type_and_refs(*expected_arg);
 
-        if found_ty == expected_ty {
+        if infcx.can_eq(param_env, found_ty, expected_ty).is_ok() {
             if found_refs < expected_refs {
-                to_borrow.push((arg_span, expected_arg.to_string()));
+                to_borrow.push((arg.span.shrink_to_lo(), "&".repeat(expected_refs - found_refs)));
             } else if found_refs > expected_refs {
-                remove_borrow.push((arg_span, expected_arg.to_string()));
+                let mut span = arg.span.shrink_to_lo();
+                let mut left = found_refs - expected_refs;
+                let mut ty = arg;
+                while let hir::TyKind::Ref(_, mut_ty) = &ty.kind && left > 0 {
+                    span = span.with_hi(mut_ty.ty.span.lo());
+                    ty = mut_ty.ty;
+                    left -= 1;
+                }
+                let sugg = if left == 0 {
+                    (span, String::new())
+                } else {
+                    (arg.span, expected_arg.to_string())
+                };
+                remove_borrow.push(sugg);
             }
         }
     }
 
     if !to_borrow.is_empty() {
-        err.multipart_suggestion(
+        err.multipart_suggestion_verbose(
             "consider borrowing the argument",
             to_borrow,
             Applicability::MaybeIncorrect,
@@ -3807,7 +3824,7 @@ fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, usize) {
     }
 
     if !remove_borrow.is_empty() {
-        err.multipart_suggestion(
+        err.multipart_suggestion_verbose(
             "do not borrow the argument",
             remove_borrow,
             Applicability::MaybeIncorrect,

tests/ui/anonymous-higher-ranked-lifetime.stderr

@@ -16,7 +16,7 @@ LL | fn f1<F>(_: F) where F: Fn(&(), &()) {}
 help: consider borrowing the argument
    |
 LL |     f1(|_: &(), _: &()| {});
-   |            ~~~     ~~~
+   |            +       +
 
 error[E0631]: type mismatch in closure arguments
   --> $DIR/anonymous-higher-ranked-lifetime.rs:3:5
@@ -35,8 +35,8 @@ LL | fn f2<F>(_: F) where F: for<'a> Fn(&'a (), &()) {}
    |                         ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `f2`
 help: consider borrowing the argument
    |
-LL |     f2(|_: &'a (), _: &()| {});
+LL |     f2(|_: &(), _: &()| {});
-   |            ~~~~~~     ~~~
+   |            +       +
 
 error[E0631]: type mismatch in closure arguments
   --> $DIR/anonymous-higher-ranked-lifetime.rs:4:5
@@ -56,7 +56,7 @@ LL | fn f3<'a, F>(_: F) where F: Fn(&'a (), &()) {}
 help: consider borrowing the argument
    |
 LL |     f3(|_: &(), _: &()| {});
-   |            ~~~     ~~~
+   |            +       +
 
 error[E0631]: type mismatch in closure arguments
   --> $DIR/anonymous-higher-ranked-lifetime.rs:5:5
@@ -75,8 +75,8 @@ LL | fn f4<F>(_: F) where F: for<'r> Fn(&(), &'r ()) {}
    |                         ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `f4`
 help: consider borrowing the argument
    |
-LL |     f4(|_: &(), _: &'r ()| {});
+LL |     f4(|_: &(), _: &()| {});
-   |            ~~~     ~~~~~~
+   |            +       +
 
 error[E0631]: type mismatch in closure arguments
   --> $DIR/anonymous-higher-ranked-lifetime.rs:6:5
@@ -95,17 +95,15 @@ LL | fn f5<F>(_: F) where F: for<'r> Fn(&'r (), &'r ()) {}
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `f5`
 help: consider borrowing the argument
    |
-LL |     f5(|_: &'r (), _: &'r ()| {});
+LL |     f5(|_: &(), _: &()| {});
-   |            ~~~~~~     ~~~~~~
+   |            +       +
 
 error[E0631]: type mismatch in closure arguments
   --> $DIR/anonymous-higher-ranked-lifetime.rs:7:5
    |
 LL |     g1(|_: (), _: ()| {});
-   |     ^^ --------------
+   |     ^^ -------------- found signature defined here
-   |     |  |   |
+   |     |
-   |     |  |   help: consider borrowing the argument: `&()`
-   |     |  found signature defined here
    |     expected due to this
    |
    = note: expected closure signature `for<'a> fn(&'a (), Box<(dyn for<'a> Fn(&'a ()) + 'static)>) -> _`
@@ -115,15 +113,17 @@ note: required by a bound in `g1`
    |
 LL | fn g1<F>(_: F) where F: Fn(&(), Box<dyn Fn(&())>) {}
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `g1`
+help: consider borrowing the argument
+   |
+LL |     g1(|_: &(), _: ()| {});
+   |            +
 
 error[E0631]: type mismatch in closure arguments
   --> $DIR/anonymous-higher-ranked-lifetime.rs:8:5
    |
 LL |     g2(|_: (), _: ()| {});
-   |     ^^ --------------
+   |     ^^ -------------- found signature defined here
-   |     |  |   |
+   |     |
-   |     |  |   help: consider borrowing the argument: `&()`
-   |     |  found signature defined here
    |     expected due to this
    |
    = note: expected closure signature `for<'a> fn(&'a (), for<'a> fn(&'a ())) -> _`
@@ -133,15 +133,17 @@ note: required by a bound in `g2`
    |
 LL | fn g2<F>(_: F) where F: Fn(&(), fn(&())) {}
    |                         ^^^^^^^^^^^^^^^^ required by this bound in `g2`
+help: consider borrowing the argument
+   |
+LL |     g2(|_: &(), _: ()| {});
+   |            +
 
 error[E0631]: type mismatch in closure arguments
   --> $DIR/anonymous-higher-ranked-lifetime.rs:9:5
    |
 LL |     g3(|_: (), _: ()| {});
-   |     ^^ --------------
+   |     ^^ -------------- found signature defined here
-   |     |  |   |
+   |     |
-   |     |  |   help: consider borrowing the argument: `&'s ()`
-   |     |  found signature defined here
    |     expected due to this
    |
    = note: expected closure signature `for<'s> fn(&'s (), Box<(dyn for<'a> Fn(&'a ()) + 'static)>) -> _`
@@ -151,15 +153,17 @@ note: required by a bound in `g3`
    |
 LL | fn g3<F>(_: F) where F: for<'s> Fn(&'s (), Box<dyn Fn(&())>) {}
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `g3`
+help: consider borrowing the argument
+   |
+LL |     g3(|_: &(), _: ()| {});
+   |            +
 
 error[E0631]: type mismatch in closure arguments
   --> $DIR/anonymous-higher-ranked-lifetime.rs:10:5
    |
 LL |     g4(|_: (), _: ()| {});
-   |     ^^ --------------
+   |     ^^ -------------- found signature defined here
-   |     |  |   |
+   |     |
-   |     |  |   help: consider borrowing the argument: `&()`
-   |     |  found signature defined here
    |     expected due to this
    |
    = note: expected closure signature `for<'a> fn(&'a (), for<'r> fn(&'r ())) -> _`
@@ -169,6 +173,10 @@ note: required by a bound in `g4`
    |
 LL | fn g4<F>(_: F) where F: Fn(&(), for<'r> fn(&'r ())) {}
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `g4`
+help: consider borrowing the argument
+   |
+LL |     g4(|_: &(), _: ()| {});
+   |            +
 
 error[E0631]: type mismatch in closure arguments
   --> $DIR/anonymous-higher-ranked-lifetime.rs:11:5
@@ -188,7 +196,7 @@ LL | fn h1<F>(_: F) where F: Fn(&(), Box<dyn Fn(&())>, &(), fn(&(), &())) {}
 help: consider borrowing the argument
    |
 LL |     h1(|_: &(), _: (), _: &(), _: ()| {});
-   |            ~~~            ~~~
+   |            +              +
 
 error[E0631]: type mismatch in closure arguments
   --> $DIR/anonymous-higher-ranked-lifetime.rs:12:5
@@ -207,8 +215,8 @@ LL | fn h2<F>(_: F) where F: for<'t0> Fn(&(), Box<dyn Fn(&())>, &'t0 (), fn(&(),
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `h2`
 help: consider borrowing the argument
    |
-LL |     h2(|_: &(), _: (), _: &'t0 (), _: ()| {});
+LL |     h2(|_: &(), _: (), _: &(), _: ()| {});
-   |            ~~~            ~~~~~~~
+   |            +              +
 
 error: aborting due to 11 previous errors
 

tests/ui/closures/multiple-fn-bounds.stderr

@@ -2,10 +2,8 @@ error[E0631]: type mismatch in closure arguments
   --> $DIR/multiple-fn-bounds.rs:10:5
    |
 LL |     foo(move |x| v);
-   |     ^^^ --------
+   |     ^^^ -------- found signature defined here
-   |     |   |     |
+   |     |
-   |     |   |     help: do not borrow the argument: `char`
-   |     |   found signature defined here
    |     expected due to this
    |
    = note: expected closure signature `fn(char) -> _`
@@ -20,6 +18,10 @@ note: required by a bound in `foo`
    |
 LL | fn foo<F: Fn(&char) -> bool + Fn(char) -> bool>(f: F) {
    |                               ^^^^^^^^^^^^^^^^ required by this bound in `foo`
+help: do not borrow the argument
+   |
+LL |     foo(move |char| v);
+   |               ~~~~
 
 error: aborting due to previous error
 

tests/ui/mismatched_types/closure-arg-type-mismatch-issue-45727.fixed

@@ -0,0 +1,5 @@
+// run-rustfix
+fn main() {
+    let _ = (-10..=10).find(|x: &i32| x.signum() == 0); //~ ERROR type mismatch in closure arguments
+    let _ = (-10..=10).find(|x: &i32| x.signum() == 0); //~ ERROR type mismatch in closure arguments
+}

tests/ui/mismatched_types/closure-arg-type-mismatch-issue-45727.rs

@@ -0,0 +1,5 @@
+// run-rustfix
+fn main() {
+    let _ = (-10..=10).find(|x: i32| x.signum() == 0); //~ ERROR type mismatch in closure arguments
+    let _ = (-10..=10).find(|x: &&&i32| x.signum() == 0); //~ ERROR type mismatch in closure arguments
+}

tests/ui/mismatched_types/closure-arg-type-mismatch-issue-45727.stderr

@@ -0,0 +1,38 @@
+error[E0631]: type mismatch in closure arguments
+  --> $DIR/closure-arg-type-mismatch-issue-45727.rs:3:24
+   |
+LL |     let _ = (-10..=10).find(|x: i32| x.signum() == 0);
+   |                        ^^^^ -------- found signature defined here
+   |                        |
+   |                        expected due to this
+   |
+   = note: expected closure signature `for<'a> fn(&'a {integer}) -> _`
+              found closure signature `fn(i32) -> _`
+note: required by a bound in `find`
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+help: consider borrowing the argument
+   |
+LL |     let _ = (-10..=10).find(|x: &i32| x.signum() == 0);
+   |                                 +
+
+error[E0631]: type mismatch in closure arguments
+  --> $DIR/closure-arg-type-mismatch-issue-45727.rs:4:24
+   |
+LL |     let _ = (-10..=10).find(|x: &&&i32| x.signum() == 0);
+   |                        ^^^^ ----------- found signature defined here
+   |                        |
+   |                        expected due to this
+   |
+   = note: expected closure signature `for<'a> fn(&'a {integer}) -> _`
+              found closure signature `for<'a, 'b, 'c> fn(&'a &'b &'c i32) -> _`
+note: required by a bound in `find`
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+help: do not borrow the argument
+   |
+LL -     let _ = (-10..=10).find(|x: &&&i32| x.signum() == 0);
+LL +     let _ = (-10..=10).find(|x: &i32| x.signum() == 0);
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0631`.

tests/ui/mismatched_types/closure-arg-type-mismatch.stderr

@@ -2,16 +2,18 @@ error[E0631]: type mismatch in closure arguments
   --> $DIR/closure-arg-type-mismatch.rs:3:14
    |
 LL |     a.iter().map(|_: (u32, u32)| 45);
-   |              ^^^ ---------------
+   |              ^^^ --------------- found signature defined here
-   |              |   |   |
+   |              |
-   |              |   |   help: consider borrowing the argument: `&(u32, u32)`
-   |              |   found signature defined here
    |              expected due to this
    |
    = note: expected closure signature `fn(&(u32, u32)) -> _`
               found closure signature `fn((u32, u32)) -> _`
 note: required by a bound in `map`
   --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+help: consider borrowing the argument
+   |
+LL |     a.iter().map(|_: &(u32, u32)| 45);
+   |                      +
 
 error[E0631]: type mismatch in closure arguments
   --> $DIR/closure-arg-type-mismatch.rs:4:14

tests/ui/mismatched_types/issue-36053-2.stderr

@@ -2,16 +2,18 @@ error[E0631]: type mismatch in closure arguments
   --> $DIR/issue-36053-2.rs:7:32
    |
 LL |     once::<&str>("str").fuse().filter(|a: &str| true).count();
-   |                                ^^^^^^ ---------
+   |                                ^^^^^^ --------- found signature defined here
-   |                                |      |   |
+   |                                |
-   |                                |      |   help: consider borrowing the argument: `&&str`
-   |                                |      found signature defined here
    |                                expected due to this
    |
    = note: expected closure signature `for<'a> fn(&'a &str) -> _`
               found closure signature `for<'a> fn(&'a str) -> _`
 note: required by a bound in `filter`
   --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+help: consider borrowing the argument
+   |
+LL |     once::<&str>("str").fuse().filter(|a: &&str| true).count();
+   |                                           +
 
 error[E0599]: the method `count` exists for struct `Filter<Fuse<Once<&str>>, [closure@issue-36053-2.rs:7:39]>`, but its trait bounds were not satisfied
   --> $DIR/issue-36053-2.rs:7:55