On type mismatch involving fn/method call, point at definition

compiler/rustc_hir_typeck/src/demand.rs

@@ -94,6 +94,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.suggest_method_call_on_range_literal(err, expr, expr_ty, expected);
         self.suggest_return_binding_for_missing_tail_expr(err, expr, expr_ty, expected);
         self.note_wrong_return_ty_due_to_generic_arg(err, expr, expr_ty);
+        self.note_fn_method_def_due_to_call(err, expr, expected);
     }
 
     /// Really hacky heuristic to remap an `assert_eq!` error to the user
@@ -1170,6 +1171,54 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => return,
         }
     }
+
+    fn note_fn_method_def_due_to_call(
+        &self,
+        err: &mut Diagnostic,
+        expr: &hir::Expr<'_>,
+        expected: Ty<'_>,
+    ) {
+        let (def_id, ident) = if let hir::ExprKind::Call(fun, _) = expr.kind
+            && let hir::ExprKind::Path(hir::QPath::Resolved(_, path)) = fun.kind
+            && let hir::def::Res::Def(_, def_id) = path.res
+        {
+            (def_id, path.segments[0].ident)
+        } else if let hir::ExprKind::MethodCall(method, ..) = expr.kind
+            && let Some(def_id) = self.typeck_results.borrow().type_dependent_def_id(expr.hir_id)
+        {
+            (def_id, method.ident)
+        } else {
+            return;
+        };
+        if !matches!(self.tcx.def_kind(def_id), hir::def::DefKind::AssocFn | hir::def::DefKind::Fn)
+        {
+            return;
+        }
+        err.span_note(
+            self.tcx.def_span(def_id),
+            format!("the {} {ident} is defined here", self.tcx.def_descr(def_id)),
+        );
+
+        if let Some(local_did) = def_id.as_local()
+            && let Some(node) = self.tcx.opt_hir_node(self.tcx.local_def_id_to_hir_id(local_did))
+            && !matches!(node, hir::Node::TraitItem(..))
+            && let Some(sig) = node.fn_sig()
+            && let ret_span = sig.decl.output.span()
+            && !ret_span.from_expansion()
+            && expected.has_concrete_skeleton()
+        {
+            let sugg = match sig.decl.output {
+                hir::FnRetTy::DefaultReturn(..) => format!("-> {expected}"),
+                hir::FnRetTy::Return(..) => format!("{expected}"),
+            };
+            err.span_suggestion(
+                ret_span,
+                format!("consider changing {ident}'s return type"),
+                sugg,
+                Applicability::MaybeIncorrect,
+            );
+        }
+    }
 }
 
 pub enum TypeMismatchSource<'tcx> {

tests/incremental/circular-dependencies.rs

@@ -14,7 +14,7 @@ pub struct Foo;
 pub fn consume_foo(_: Foo) {}
 //[cfail2]~^ NOTE function defined here
 
-pub fn produce_foo() -> Foo {
+pub fn produce_foo() -> Foo { //[cfail2]~ NOTE the function produce_foo is defined here
     Foo
 }
 
@@ -28,10 +28,11 @@ fn test() {
     //[cfail2]~| NOTE the crate `circular_dependencies` is compiled multiple times, possibly with different configurations
     //[cfail2]~| NOTE function defined here
 
-    consume_foo(aux::produce_foo());
+    consume_foo(aux::produce_foo()); //
     //[cfail2]~^ ERROR mismatched types [E0308]
     //[cfail2]~| NOTE expected `Foo`, found `circular_dependencies::Foo`
     //[cfail2]~| NOTE arguments to this function are incorrect
     //[cfail2]~| NOTE `circular_dependencies::Foo` and `Foo` have similar names, but are actually distinct types
     //[cfail2]~| NOTE the crate `circular_dependencies` is compiled multiple times, possibly with different configurations
+    //[cfail2]~| NOTE the function aux is defined here
 }

tests/ui/alloc-error/alloc-error-handler-bad-signature-1.stderr

@@ -29,14 +29,24 @@ LL |    #[alloc_error_handler]
 LL | // fn oom(
 LL | ||     info: &Layout,
 LL | || ) -> ()
-   | ||_______^ expected `!`, found `()`
+   | ||      -^
+   | ||______||
+   | |       |expected `!`, found `()`
+   | |       help: consider changing oom's return type: `!`
 LL | |  {
 LL | |      loop {}
 LL | |  }
    | |__- expected `!` because of return type
    |
    = note:   expected type `!`
            found unit type `()`
+note: the function oom is defined here
+  --> $DIR/alloc-error-handler-bad-signature-1.rs:10:1
+   |
+LL | / fn oom(
+LL | |     info: &Layout,
+LL | | ) -> ()
+   | |_______^
    = note: this error originates in the attribute macro `alloc_error_handler` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 2 previous errors

tests/ui/alloc-error/alloc-error-handler-bad-signature-2.stderr

@@ -36,13 +36,22 @@ LL |    #[alloc_error_handler]
 LL | // fn oom(
 LL | ||     info: Layout,
 LL | || ) {
-   | ||_^ expected `!`, found `()`
+   | || ^- help: consider changing oom's return type: `-> !`
+   | ||_|
+   | |  expected `!`, found `()`
 LL | |      loop {}
 LL | |  }
    | |__- expected `!` because of return type
    |
    = note:   expected type `!`
            found unit type `()`
+note: the function oom is defined here
+  --> $DIR/alloc-error-handler-bad-signature-2.rs:10:1
+   |
+LL | / fn oom(
+LL | |     info: Layout,
+LL | | ) {
+   | |_^
    = note: this error originates in the attribute macro `alloc_error_handler` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 2 previous errors

tests/ui/associated-type-bounds/do-not-look-at-parent-item-in-suggestion-for-type-param-of-current-assoc-item.stderr

@@ -10,6 +10,11 @@ LL |         let _low = self.lows.remove(low.identify()).unwrap();
            found associated type `<impl LowT as Identify>::Id`
    = help: consider constraining the associated type `<impl LowT as Identify>::Id` to `&I`
    = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+note: the method identify is defined here
+  --> $DIR/do-not-look-at-parent-item-in-suggestion-for-type-param-of-current-assoc-item.rs:8:5
+   |
+LL |     fn identify(&self) -> Self::Id;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 note: method defined here
   --> $SRC_DIR/std/src/collections/hash/map.rs:LL:COL
 

tests/ui/associated-type-bounds/elision.stderr

@@ -21,6 +21,8 @@ LL | fn f(x: &mut dyn Iterator<Item: Iterator<Item = &'_ ()>>) -> Option<&'_ ()>
    |
    = note: expected enum `Option<&()>`
               found enum `Option<impl Iterator<Item = &'_ ()>>`
+note: the method next is defined here
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
 
 error: aborting due to 2 previous errors
 

tests/ui/associated-types/associated-types-eq-3.stderr

@@ -8,6 +8,11 @@ LL |     let _: Bar = x.boo();
    |
    = note:       expected struct `Bar`
            found associated type `<I as Foo>::A`
+note: the method boo is defined here
+  --> $DIR/associated-types-eq-3.rs:6:5
+   |
+LL |     fn boo(&self) -> <Self as Foo>::A;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 help: consider constraining the associated type `<I as Foo>::A` to `Bar`
    |
 LL | fn foo2<I: Foo<A = Bar>>(x: I) {

tests/ui/associated-types/associated-types-path-2.stderr

@@ -86,10 +86,19 @@ LL |     let _: i32 = f2(2i32);
    |            |
    |            expected due to this
    |
+note: the function f2 is defined here
+  --> $DIR/associated-types-path-2.rs:14:1
+   |
+LL | pub fn f2<T: Foo>(a: T) -> T::A {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 help: you can convert a `u32` to an `i32` and panic if the converted value doesn't fit
    |
 LL |     let _: i32 = f2(2i32).try_into().unwrap();
    |                          ++++++++++++++++++++
+help: consider changing f2's return type
+   |
+LL | pub fn f2<T: Foo>(a: T) -> i32 {
+   |                            ~~~
 
 error: aborting due to 8 previous errors
 

tests/ui/async-await/coroutine-desc.stderr

@@ -25,6 +25,11 @@ LL |     fun(one(), two());
    |
    = help: consider `await`ing on both `Future`s
    = note: distinct uses of `impl Trait` result in different opaque types
+note: the function two is defined here
+  --> $DIR/coroutine-desc.rs:6:1
+   |
+LL | async fn two() {}
+   | ^^^^^^^^^^^^^^
 note: function defined here
   --> $DIR/coroutine-desc.rs:8:4
    |

tests/ui/async-await/issues/issue-102206.stderr

@@ -6,6 +6,11 @@ LL |     std::mem::size_of_val(foo());
    |     |
    |     arguments to this function are incorrect
    |
+note: the function foo is defined here
+  --> $DIR/issue-102206.rs:3:1
+   |
+LL | async fn foo() {}
+   | ^^^^^^^^^^^^^^
 note: function defined here
   --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
 help: consider borrowing here

tests/ui/async-await/suggest-missing-await.stderr

@@ -32,6 +32,11 @@ note: calling an async function returns a future
    |
 LL |     dummy()
    |     ^^^^^^^
+note: the function dummy is defined here
+  --> $DIR/suggest-missing-await.rs:18:1
+   |
+LL | async fn dummy() {}
+   | ^^^^^^^^^^^^^^^^
 help: consider `await`ing on the `Future`
    |
 LL |     dummy().await

tests/ui/block-result/unexpected-return-on-unit.stderr

@@ -4,6 +4,11 @@ error[E0308]: mismatched types
 LL |     foo()
    |     ^^^^^ expected `()`, found `usize`
    |
+note: the function foo is defined here
+  --> $DIR/unexpected-return-on-unit.rs:4:1
+   |
+LL | fn foo() -> usize {
+   | ^^^^^^^^^^^^^^^^^
 help: consider using a semicolon here
    |
 LL |     foo();
@@ -12,6 +17,10 @@ help: try adding a return type
    |
 LL | fn bar() -> usize {
    |          ++++++++
+help: consider changing foo's return type
+   |
+LL | fn foo() -> () {
+   |             ~~
 
 error: aborting due to 1 previous error
 

tests/ui/closures/add_semicolon_non_block_closure.rs

@@ -1,6 +1,6 @@
 fn foo(_f: impl Fn()) {}
 
-fn bar() -> i32 {
+fn bar() -> i32 { //~ HELP consider changing bar's return type
     1
 }
 

tests/ui/closures/add_semicolon_non_block_closure.stderr

@@ -6,10 +6,19 @@ LL | fn main() {
 LL |     foo(|| bar())
    |            ^^^^^ expected `()`, found `i32`
    |
+note: the function bar is defined here
+  --> $DIR/add_semicolon_non_block_closure.rs:3:1
+   |
+LL | fn bar() -> i32 {
+   | ^^^^^^^^^^^^^^^
 help: consider using a semicolon here
    |
 LL |     foo(|| { bar(); })
    |            +      +++
+help: consider changing bar's return type
+   |
+LL | fn bar() -> () {
+   |             ~~
 
 error: aborting due to 1 previous error
 

tests/ui/coercion/retslot-cast.stderr

@@ -6,9 +6,18 @@ LL |             -> Option<&Iterator<Item=()>> {
 ...
 LL |     inner(x)
    |     ^^^^^^^^ expected trait `Iterator<Item = ()>`, found trait `Iterator<Item = ()> + Send`
+...
+LL |              -> Option<&(Iterator<Item=()>+Send)> {
+   |                 --------------------------------- help: consider changing inner's return type: `Option<&dyn Iterator<Item = ()>>`
    |
    = note: expected enum `Option<&dyn Iterator<Item = ()>>`
               found enum `Option<&dyn Iterator<Item = ()> + Send>`
+note: the function inner is defined here
+  --> $DIR/retslot-cast.rs:16:1
+   |
+LL | / pub fn inner(x: Option<&(Iterator<Item=()>+Send)>)
+LL | |              -> Option<&(Iterator<Item=()>+Send)> {
+   | |_________________________________________________^
 
 error: aborting due to 1 previous error
 

tests/ui/const-generics/occurs-check/unify-n-nplusone.stderr

@@ -1,8 +1,17 @@
 error[E0308]: mismatched types
   --> $DIR/unify-n-nplusone.rs:14:11
    |
+LL | fn bind<const N: usize>(value: [u8; N]) -> [u8; N + 1] {
+   |                                            ----------- help: consider changing bind's return type: `[u8; _]`
+...
 LL |     arr = bind(arr);
    |           ^^^^^^^^^ encountered a self-referencing constant
+   |
+note: the function bind is defined here
+  --> $DIR/unify-n-nplusone.rs:6:1
+   |
+LL | fn bind<const N: usize>(value: [u8; N]) -> [u8; N + 1] {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 

tests/ui/const-generics/occurs-check/unused-substs-4.stderr

@@ -1,8 +1,17 @@
 error[E0308]: mismatched types
   --> $DIR/unused-substs-4.rs:10:11
    |
+LL | fn bind<const N: usize>(value: [u8; N]) -> [u8; 3 + 4] {
+   |                                            ----------- help: consider changing bind's return type: `[u8; _]`
+...
 LL |     arr = bind(arr);
    |           ^^^^^^^^^ encountered a self-referencing constant
+   |
+note: the function bind is defined here
+  --> $DIR/unused-substs-4.rs:4:1
+   |
+LL | fn bind<const N: usize>(value: [u8; N]) -> [u8; 3 + 4] {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 

tests/ui/const-generics/occurs-check/unused-substs-5.stderr

@@ -5,6 +5,12 @@ LL |     x = q::<_, N>(x);
    |         ^^^^^^^^^^^^- help: try using a conversion method: `.to_vec()`
    |         |
    |         cyclic type of infinite size
+   |
+note: the function q is defined here
+  --> $DIR/unused-substs-5.rs:5:1
+   |
+LL | fn q<T, const N: usize>(_: T) -> [u8; N + 1] {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 

tests/ui/did_you_mean/compatible-variants.rs

@@ -8,6 +8,7 @@ struct Foo {
 }
 
 fn f() {}
+//~^ HELP consider changing f's return type
 
 fn a() -> Option<()> {
     while false {