On Fn arg mismatch for a fn path, suggest a closure

When encountering a fn call that has a path to another fn being passed
in, where an `Fn` impl is expected, and the arguments differ, suggest
wrapping the argument with a closure with the appropriate arguments.

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

@@ -222,6 +222,15 @@ pub trait TypeErrCtxtExt<'tcx> {
         param_env: ty::ParamEnv<'tcx>,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
 
+    fn note_conflicting_fn_args(
+        &self,
+        err: &mut Diagnostic,
+        cause: &ObligationCauseCode<'tcx>,
+        expected: Ty<'tcx>,
+        found: Ty<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    );
+
     fn note_conflicting_closure_bounds(
         &self,
         cause: &ObligationCauseCode<'tcx>,
@@ -2005,6 +2014,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         let signature_kind = format!("{argument_kind} signature");
         err.note_expected_found(&signature_kind, expected_str, &signature_kind, found_str);
 
+        self.note_conflicting_fn_args(&mut err, cause, expected, found, param_env);
         self.note_conflicting_closure_bounds(cause, &mut err);
 
         if let Some(found_node) = found_node {
@@ -2014,6 +2024,152 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         err
     }
 
+    fn note_conflicting_fn_args(
+        &self,
+        err: &mut Diagnostic,
+        cause: &ObligationCauseCode<'tcx>,
+        expected: Ty<'tcx>,
+        found: Ty<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) {
+        let ObligationCauseCode::FunctionArgumentObligation { arg_hir_id, .. } = cause else {
+            return;
+        };
+        let ty::FnPtr(expected) = expected.kind() else {
+            return;
+        };
+        let ty::FnPtr(found) = found.kind() else {
+            return;
+        };
+        let Some(Node::Expr(arg)) = self.tcx.hir().find(*arg_hir_id) else {
+            return;
+        };
+        let hir::ExprKind::Path(path) = arg.kind else {
+            return;
+        };
+        let expected_inputs = self.tcx.erase_late_bound_regions(*expected).inputs();
+        let found_inputs = self.tcx.erase_late_bound_regions(*found).inputs();
+        let both_tys = expected_inputs.iter().cloned().zip(found_inputs.iter().cloned());
+
+        let arg_expr = |infcx: &InferCtxt<'tcx>, name, expected: Ty<'tcx>, found: Ty<'tcx>| {
+            let (expected_ty, expected_refs) = get_deref_type_and_refs(expected);
+            let (found_ty, found_refs) = get_deref_type_and_refs(found);
+
+            if infcx.can_eq(param_env, found_ty, expected_ty) {
+                if found_refs.len() == expected_refs.len()
+                    && found_refs.iter().zip(expected_refs.iter()).all(|(e, f)| e == f)
+                {
+                    name
+                } else if found_refs.len() > expected_refs.len() {
+                    if found_refs[..found_refs.len() - expected_refs.len()]
+                        .iter()
+                        .zip(expected_refs.iter())
+                        .any(|(e, f)| e != f)
+                    {
+                        // The refs have different mutability.
+                        format!(
+                            "{}*{name}",
+                            found_refs[..found_refs.len() - expected_refs.len()]
+                                .iter()
+                                .map(|mutbl| format!("&{}", mutbl.prefix_str()))
+                                .collect::<Vec<_>>()
+                                .join(""),
+                        )
+                    } else {
+                        format!(
+                            "{}{name}",
+                            found_refs[..found_refs.len() - expected_refs.len()]
+                                .iter()
+                                .map(|mutbl| format!("&{}", mutbl.prefix_str()))
+                                .collect::<Vec<_>>()
+                                .join(""),
+                        )
+                    }
+                } else if expected_refs.len() > found_refs.len() {
+                    format!(
+                        "{}{name}",
+                        (0..(expected_refs.len() - found_refs.len()))
+                            .map(|_| "*")
+                            .collect::<Vec<_>>()
+                            .join(""),
+                    )
+                } else {
+                    format!(
+                        "{}{name}",
+                        found_refs
+                            .iter()
+                            .map(|mutbl| format!("&{}", mutbl.prefix_str()))
+                            .chain(found_refs.iter().map(|_| "*".to_string()))
+                            .collect::<Vec<_>>()
+                            .join(""),
+                    )
+                }
+            } else {
+                format!("/* {found} */")
+            }
+        };
+        let (closure_names, call_names): (Vec<_>, Vec<_>) =
+            if both_tys.clone().all(|(expected, found)| {
+                let (expected_ty, _) = get_deref_type_and_refs(expected);
+                let (found_ty, _) = get_deref_type_and_refs(found);
+                self.can_eq(param_env, found_ty, expected_ty)
+            }) && !expected_inputs.is_empty()
+                && expected_inputs.len() == found_inputs.len()
+                && let hir::QPath::Resolved(_, path) = path
+                && let hir::def::Res::Def(_, fn_def_id) = path.res
+                && let Some(node) = self.tcx.hir().get_if_local(fn_def_id)
+                && let Some(body_id) = node.body_id()
+            {
+                let closure = self
+                    .tcx
+                    .hir()
+                    .body_param_names(body_id)
+                    .map(|name| format!("{name}"))
+                    .collect();
+                let args = self
+                    .tcx
+                    .hir()
+                    .body_param_names(body_id)
+                    .zip(both_tys)
+                    .map(|(name, (expected, found))| {
+                        arg_expr(self.infcx, format!("{name}"), expected, found)
+                    })
+                    .collect();
+                (closure, args)
+            } else {
+                let closure_args = expected_inputs
+                    .iter()
+                    .enumerate()
+                    .map(|(i, _)| format!("arg{i}"))
+                    .collect::<Vec<_>>();
+                let call_args = both_tys
+                    .enumerate()
+                    .map(|(i, (expected, found))| {
+                        arg_expr(self.infcx, format!("arg{i}"), expected, found)
+                    })
+                    .collect::<Vec<_>>();
+                (closure_args, call_args)
+            };
+        let closure_names: Vec<_> = closure_names
+            .into_iter()
+            .zip(expected_inputs.iter())
+            .map(|(name, ty)| {
+                format!(
+                    "{name}{}",
+                    if ty.has_infer_types() { String::new() } else { format!(": {ty}") }
+                )
+            })
+            .collect();
+        err.multipart_suggestion(
+            format!("consider wrapping the function in a closure"),
+            vec![
+                (arg.span.shrink_to_lo(), format!("|{}| ", closure_names.join(", "))),
+                (arg.span.shrink_to_hi(), format!("({})", call_names.join(", "))),
+            ],
+            Applicability::MaybeIncorrect,
+        );
+    }
+
     // Add a note if there are two `Fn`-family bounds that have conflicting argument
     // requirements, which will always cause a closure to have a type error.
     fn note_conflicting_closure_bounds(
@@ -4349,17 +4505,6 @@ fn hint_missing_borrow<'tcx>(
 
     let args = fn_decl.inputs.iter();
 
-    fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, Vec<hir::Mutability>) {
-        let mut refs = vec![];
-
-        while let ty::Ref(_, new_ty, mutbl) = ty.kind() {
-            ty = *new_ty;
-            refs.push(*mutbl);
-        }
-
-        (ty, refs)
-    }
-
     let mut to_borrow = Vec::new();
     let mut remove_borrow = Vec::new();
 
@@ -4652,3 +4797,14 @@ pub fn suggest_desugaring_async_fn_to_impl_future_in_trait<'tcx>(
 
     Some(sugg)
 }
+
+fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, Vec<hir::Mutability>) {
+    let mut refs = vec![];
+
+    while let ty::Ref(_, new_ty, mutbl) = ty.kind() {
+        ty = *new_ty;
+        refs.push(*mutbl);
+    }
+
+    (ty, refs)
+}

tests/ui/generic-associated-types/bugs/issue-88382.stderr

@@ -16,6 +16,10 @@ note: required by a bound in `do_something`
    |
 LL | fn do_something<I: Iterable>(i: I, mut f: impl for<'a> Fn(&mut I::Iterator<'a>)) {
    |                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `do_something`
+help: consider wrapping the function in a closure
+   |
+LL |     do_something(SomeImplementation(), |arg0: &mut std::iter::Empty<usize>| test(/* &mut <_ as Iterable>::Iterator<'_> */));
+   |                                        ++++++++++++++++++++++++++++++++++++     ++++++++++++++++++++++++++++++++++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/intrinsics/const-eval-select-bad.stderr

@@ -86,6 +86,10 @@ LL |     const_eval_select((true,), foo, baz);
               found function signature `fn(i32) -> _`
 note: required by a bound in `const_eval_select`
   --> $SRC_DIR/core/src/intrinsics.rs:LL:COL
+help: consider wrapping the function in a closure
+   |
+LL |     const_eval_select((true,), |arg0: bool| foo(/* i32 */), baz);
+   |                                ++++++++++++    +++++++++++
 
 error: this argument must be a `const fn`
   --> $DIR/const-eval-select-bad.rs:42:29

tests/ui/mismatched_types/E0631.stderr

@@ -48,6 +48,10 @@ note: required by a bound in `foo`
    |
 LL | fn foo<F: Fn(usize)>(_: F) {}
    |           ^^^^^^^^^ required by this bound in `foo`
+help: consider wrapping the function in a closure
+   |
+LL |     foo(|arg0: usize| f(/* u64 */));
+   |         +++++++++++++  +++++++++++
 
 error[E0631]: type mismatch in function arguments
   --> $DIR/E0631.rs:10:9
@@ -67,6 +71,10 @@ note: required by a bound in `bar`
    |
 LL | fn bar<F: Fn<(usize,)>>(_: F) {}
    |           ^^^^^^^^^^^^ required by this bound in `bar`
+help: consider wrapping the function in a closure
+   |
+LL |     bar(|arg0: usize| f(/* u64 */));
+   |         +++++++++++++  +++++++++++
 
 error: aborting due to 4 previous errors
 

tests/ui/mismatched_types/closure-ref-114180.stderr

@@ -12,6 +12,10 @@ LL |     v.sort_by(compare);
               found closure signature `fn((_,), (_,)) -> _`
 note: required by a bound in `slice::<impl [T]>::sort_by`
   --> $SRC_DIR/alloc/src/slice.rs:LL:COL
+help: consider wrapping the function in a closure
+   |
+LL |     v.sort_by(|arg0, arg1| compare(*arg0, *arg1));
+   |               ++++++++++++        ++++++++++++++
 help: consider adjusting the signature so it borrows its arguments
    |
 LL |     let compare = |&(a,), &(e,)| todo!();

tests/ui/mismatched_types/fn-variance-1.stderr

@@ -16,6 +16,10 @@ note: required by a bound in `apply`
    |
 LL | fn apply<T, F>(t: T, f: F) where F: FnOnce(T) {
    |                                     ^^^^^^^^^ required by this bound in `apply`
+help: consider wrapping the function in a closure
+   |
+LL |     apply(&3, |x| takes_mut(&mut *x));
+   |               +++          +++++++++
 
 error[E0631]: type mismatch in function arguments
   --> $DIR/fn-variance-1.rs:15:19
@@ -35,6 +39,10 @@ note: required by a bound in `apply`
    |
 LL | fn apply<T, F>(t: T, f: F) where F: FnOnce(T) {
    |                                     ^^^^^^^^^ required by this bound in `apply`
+help: consider wrapping the function in a closure
+   |
+LL |     apply(&mut 3, |x| takes_imm(&*x));
+   |                   +++          +++++
 
 error: aborting due to 2 previous errors
 

tests/ui/mismatched_types/suggest-option-asderef-inference-var.stderr

@@ -13,6 +13,10 @@ LL |     let _has_inference_vars: Option<i32> = Some(0).map(deref_int);
               found function signature `for<'a> fn(&'a i32) -> _`
 note: required by a bound in `Option::<T>::map`
   --> $SRC_DIR/core/src/option.rs:LL:COL
+help: consider wrapping the function in a closure
+   |
+LL |     let _has_inference_vars: Option<i32> = Some(0).map(|a| deref_int(&a));
+   |                                                        +++          ++++
 help: consider adjusting the signature so it does not borrow its argument
    |
 LL - fn deref_int(a: &i32) -> i32 {

tests/ui/mismatched_types/suggest-option-asderef-unfixable.stderr

@@ -13,6 +13,10 @@ LL |     let _ = produces_string().and_then(takes_str_but_too_many_refs);
               found function signature `for<'a, 'b> fn(&'a &'b str) -> _`
 note: required by a bound in `Option::<T>::and_then`
   --> $SRC_DIR/core/src/option.rs:LL:COL
+help: consider wrapping the function in a closure
+   |
+LL |     let _ = produces_string().and_then(|arg0: String| takes_str_but_too_many_refs(/* &&str */));
+   |                                        ++++++++++++++                            +++++++++++++
 
 error[E0277]: expected a `FnOnce(String)` closure, found `for<'a> extern "C" fn(&'a str) -> Option<()> {takes_str_but_wrong_abi}`
   --> $DIR/suggest-option-asderef-unfixable.rs:26:40
@@ -68,6 +72,10 @@ LL |     let _ = Some(TypeWithoutDeref).and_then(takes_str_but_too_many_refs);
               found function signature `for<'a, 'b> fn(&'a &'b str) -> _`
 note: required by a bound in `Option::<T>::and_then`
   --> $SRC_DIR/core/src/option.rs:LL:COL
+help: consider wrapping the function in a closure
+   |
+LL |     let _ = Some(TypeWithoutDeref).and_then(|arg0: TypeWithoutDeref| takes_str_but_too_many_refs(/* &&str */));
+   |                                             ++++++++++++++++++++++++                            +++++++++++++
 
 error: aborting due to 5 previous errors
 

tests/ui/mismatched_types/suggest-option-asderef.fixed

@@ -1,39 +0,0 @@
-// run-rustfix
-
-fn produces_string() -> Option<String> {
-    Some("my cool string".to_owned())
-}
-
-fn takes_str(_: &str) -> Option<()> {
-    Some(())
-}
-
-fn takes_str_mut(_: &mut str) -> Option<()> {
-    Some(())
-}
-
-fn generic<T>(_: T) -> Option<()> {
-    Some(())
-}
-
-fn generic_ref<T>(_: T) -> Option<()> {
-    //~^ HELP consider adjusting the signature so it does not borrow its argument
-    Some(())
-}
-
-fn main() {
-    let _: Option<()> = produces_string().as_deref().and_then(takes_str);
-    //~^ ERROR type mismatch in function arguments
-    //~| HELP call `Option::as_deref()` first
-    let _: Option<Option<()>> = produces_string().as_deref().map(takes_str);
-    //~^ ERROR type mismatch in function arguments
-    //~| HELP call `Option::as_deref()` first
-    let _: Option<Option<()>> = produces_string().as_deref_mut().map(takes_str_mut);
-    //~^ ERROR type mismatch in function arguments
-    //~| HELP call `Option::as_deref_mut()` first
-    let _ = produces_string().and_then(generic);
-
-    let _ = produces_string().as_deref().and_then(generic_ref);
-    //~^ ERROR type mismatch in function arguments
-    //~| HELP call `Option::as_deref()` first
-}

tests/ui/mismatched_types/suggest-option-asderef.rs

@@ -1,4 +1,4 @@
-// run-rustfix
+// this isn't auto-fixable now because we produce two similar suggestions
 
 fn produces_string() -> Option<String> {
     Some("my cool string".to_owned())
@@ -25,15 +25,19 @@ fn main() {
     let _: Option<()> = produces_string().and_then(takes_str);
     //~^ ERROR type mismatch in function arguments
     //~| HELP call `Option::as_deref()` first
+    //~| HELP consider wrapping the function in a closure
     let _: Option<Option<()>> = produces_string().map(takes_str);
     //~^ ERROR type mismatch in function arguments
     //~| HELP call `Option::as_deref()` first
+    //~| HELP consider wrapping the function in a closure
     let _: Option<Option<()>> = produces_string().map(takes_str_mut);
     //~^ ERROR type mismatch in function arguments
     //~| HELP call `Option::as_deref_mut()` first
+    //~| HELP consider wrapping the function in a closure
     let _ = produces_string().and_then(generic);
 
     let _ = produces_string().and_then(generic_ref);
     //~^ ERROR type mismatch in function arguments
     //~| HELP call `Option::as_deref()` first
+    //~| HELP consider wrapping the function in a closure
 }

tests/ui/mismatched_types/suggest-option-asderef.stderr

@@ -13,13 +13,17 @@ LL |     let _: Option<()> = produces_string().and_then(takes_str);
               found function signature `for<'a> fn(&'a str) -> _`
 note: required by a bound in `Option::<T>::and_then`
   --> $SRC_DIR/core/src/option.rs:LL:COL
+help: consider wrapping the function in a closure
+   |
+LL |     let _: Option<()> = produces_string().and_then(|arg0: String| takes_str(/* &str */));
+   |                                                    ++++++++++++++          ++++++++++++
 help: call `Option::as_deref()` first
    |
 LL |     let _: Option<()> = produces_string().as_deref().and_then(takes_str);
    |                                          +++++++++++
 
 error[E0631]: type mismatch in function arguments
-  --> $DIR/suggest-option-asderef.rs:28:55
+  --> $DIR/suggest-option-asderef.rs:29:55
    |
 LL | fn takes_str(_: &str) -> Option<()> {
    | ----------------------------------- found signature defined here
@@ -33,13 +37,17 @@ LL |     let _: Option<Option<()>> = produces_string().map(takes_str);
               found function signature `for<'a> fn(&'a str) -> _`
 note: required by a bound in `Option::<T>::map`
   --> $SRC_DIR/core/src/option.rs:LL:COL
+help: consider wrapping the function in a closure
+   |
+LL |     let _: Option<Option<()>> = produces_string().map(|arg0: String| takes_str(/* &str */));
+   |                                                       ++++++++++++++          ++++++++++++
 help: call `Option::as_deref()` first
    |
 LL |     let _: Option<Option<()>> = produces_string().as_deref().map(takes_str);
    |                                                  +++++++++++
 
 error[E0631]: type mismatch in function arguments
-  --> $DIR/suggest-option-asderef.rs:31:55
+  --> $DIR/suggest-option-asderef.rs:33:55
    |
 LL | fn takes_str_mut(_: &mut str) -> Option<()> {
    | ------------------------------------------- found signature defined here
@@ -53,13 +61,17 @@ LL |     let _: Option<Option<()>> = produces_string().map(takes_str_mut);
               found function signature `for<'a> fn(&'a mut str) -> _`
 note: required by a bound in `Option::<T>::map`
   --> $SRC_DIR/core/src/option.rs:LL:COL
+help: consider wrapping the function in a closure
+   |
+LL |     let _: Option<Option<()>> = produces_string().map(|arg0: String| takes_str_mut(/* &mut str */));
+   |                                                       ++++++++++++++              ++++++++++++++++
 help: call `Option::as_deref_mut()` first
    |
 LL |     let _: Option<Option<()>> = produces_string().as_deref_mut().map(takes_str_mut);
    |                                                  +++++++++++++++
 
 error[E0631]: type mismatch in function arguments
-  --> $DIR/suggest-option-asderef.rs:36:40
+  --> $DIR/suggest-option-asderef.rs:39:40
    |
 LL | fn generic_ref<T>(_: &T) -> Option<()> {
    | -------------------------------------- found signature defined here
@@ -73,6 +85,10 @@ LL |     let _ = produces_string().and_then(generic_ref);
               found function signature `for<'a> fn(&'a _) -> _`
 note: required by a bound in `Option::<T>::and_then`
   --> $SRC_DIR/core/src/option.rs:LL:COL
+help: consider wrapping the function in a closure
+   |
+LL |     let _ = produces_string().and_then(|: String| generic_ref(&));
+   |                                        ++++++++++            +++
 help: consider adjusting the signature so it does not borrow its argument
    |
 LL - fn generic_ref<T>(_: &T) -> Option<()> {

tests/ui/mismatched_types/unboxed-closures-vtable-mismatch.stderr

@@ -16,6 +16,10 @@ note: required by a bound in `call_it`
    |
 LL | fn call_it<F: FnMut(isize, isize) -> isize>(y: isize, mut f: F) -> isize {
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `call_it`
+help: consider wrapping the function in a closure
+   |
+LL |     let z = call_it(3, |arg0: isize, arg1: isize| f(/* usize */, arg1));
+   |                        ++++++++++++++++++++++++++  +++++++++++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/suggestions/enum-variant-arg-mismatch.stderr

@@ -16,6 +16,10 @@ note: required by a bound in `map`
    |
 LL | fn map<'a, F: Fn(String) -> Sexpr<'a>>(f: F) {}
    |               ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `map`
+help: consider wrapping the function in a closure
+   |
+LL |     map(|arg0: String| Sexpr::Ident(/* &str */));
+   |         ++++++++++++++             ++++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/suggestions/late-bound-in-borrow-closure-sugg.stderr

@@ -16,6 +16,10 @@ note: required by a bound in `Trader::<'a>::set_closure`
    |
 LL |     pub fn set_closure(&mut self, function: impl Fn(&mut Trader) + 'a) {
    |                                                  ^^^^^^^^^^^^^^^ required by this bound in `Trader::<'a>::set_closure`
+help: consider wrapping the function in a closure
+   |
+LL |     trader.set_closure(|arg0: &mut Trader<'_>| closure(*arg0));
+   |                        +++++++++++++++++++++++        +++++++
 help: consider adjusting the signature so it borrows its argument
    |
 LL |     let closure = |trader : &mut Trader| {

tests/ui/typeck/mismatched-map-under-self.stderr

@@ -30,6 +30,10 @@ LL |         self.map(Insertable::values).unwrap_or_default()
               found function signature `for<'a> fn(&'a _) -> _`
 note: required by a bound in `Option::<T>::map`
   --> $SRC_DIR/core/src/option.rs:LL:COL
+help: consider wrapping the function in a closure
+   |
+LL |         self.map(|arg0: T| Insertable::values(&arg0)).unwrap_or_default()
+   |                  +++++++++                   +++++++
 
 error: aborting due to 2 previous errors