Tweak impl signature mismatch errors involving RegionKind::ReVar lifetimes

src/librustc_errors/diagnostic.rs

@@ -193,9 +193,18 @@ impl Diagnostic {
         expected_extra: &dyn fmt::Display,
         found_extra: &dyn fmt::Display,
     ) -> &mut Self {
-        let expected_label = format!("expected {}", expected_label);
-
-        let found_label = format!("found {}", found_label);
+        let expected_label = expected_label.to_string();
+        let expected_label = if expected_label.is_empty() {
+            "expected".to_string()
+        } else {
+            format!("expected {}", expected_label)
+        };
+        let found_label = found_label.to_string();
+        let found_label = if found_label.is_empty() {
+            "found".to_string()
+        } else {
+            format!("found {}", found_label)
+        };
         let (found_padding, expected_padding) = if expected_label.len() > found_label.len() {
             (expected_label.len() - found_label.len(), 0)
         } else {

src/librustc_infer/infer/error_reporting/mod.rs

@@ -987,12 +987,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
 
         fn push_ty_ref<'tcx>(
-            r: &ty::Region<'tcx>,
+            region: &ty::Region<'tcx>,
             ty: Ty<'tcx>,
             mutbl: hir::Mutability,
             s: &mut DiagnosticStyledString,
         ) {
-            let mut r = r.to_string();
+            let mut r = region.to_string();
             if r == "'_" {
                 r.clear();
             } else {

src/librustc_infer/infer/error_reporting/nice_region_error/trait_impl_difference.rs

@@ -2,11 +2,16 @@
 
 use crate::infer::error_reporting::nice_region_error::NiceRegionError;
 use crate::infer::lexical_region_resolve::RegionResolutionError;
-use crate::infer::{Subtype, ValuePairs};
+use crate::infer::{Subtype, TyCtxtInferExt, ValuePairs};
 use crate::traits::ObligationCauseCode::CompareImplMethodObligation;
 use rustc_errors::ErrorReported;
-use rustc_middle::ty::Ty;
-use rustc_span::Span;
+use rustc_hir as hir;
+use rustc_hir::def::Res;
+use rustc_hir::def_id::DefId;
+use rustc_hir::intravisit::Visitor;
+use rustc_middle::ty::error::ExpectedFound;
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_span::{MultiSpan, Span};
 
 impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
     /// Print the error message for lifetime errors when the `impl` doesn't conform to the `trait`.
@@ -36,7 +41,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
                                 var_origin.span(),
                                 sub_expected_found.expected,
                                 sub_expected_found.found,
-                                self.tcx().def_span(*trait_item_def_id),
+                                *trait_item_def_id,
                             );
                             return Some(ErrorReported);
                         }
@@ -47,14 +52,100 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         None
     }
 
-    fn emit_err(&self, sp: Span, expected: Ty<'tcx>, found: Ty<'tcx>, impl_sp: Span) {
+    fn emit_err(&self, sp: Span, expected: Ty<'tcx>, found: Ty<'tcx>, trait_def_id: DefId) {
+        let tcx = self.tcx();
+        let trait_sp = self.tcx().def_span(trait_def_id);
         let mut err = self
             .tcx()
             .sess
             .struct_span_err(sp, "`impl` item signature doesn't match `trait` item signature");
-        err.note(&format!("expected `{:?}`\n   found `{:?}`", expected, found));
-        err.span_label(sp, &format!("found {:?}", found));
-        err.span_label(impl_sp, &format!("expected {:?}", expected));
+        err.span_label(sp, &format!("found `{:?}`", found));
+        err.span_label(trait_sp, &format!("expected `{:?}`", expected));
+
+        // Get the span of all the used type parameters in the method.
+        let assoc_item = self.tcx().associated_item(trait_def_id);
+        let mut visitor = TypeParamSpanVisitor { tcx: self.tcx(), types: vec![] };
+        match assoc_item.kind {
+            ty::AssocKind::Fn => {
+                let hir = self.tcx().hir();
+                if let Some(hir_id) = assoc_item.def_id.as_local().map(|id| hir.as_local_hir_id(id))
+                {
+                    if let Some(decl) = hir.fn_decl_by_hir_id(hir_id) {
+                        visitor.visit_fn_decl(decl);
+                    }
+                }
+            }
+            _ => {}
+        }
+        let mut type_param_span: MultiSpan =
+            visitor.types.iter().cloned().collect::<Vec<_>>().into();
+        for &span in &visitor.types {
+            type_param_span.push_span_label(
+                span,
+                "consider borrowing this type parameter in the trait".to_string(),
+            );
+        }
+
+        if let Some((expected, found)) = tcx
+            .infer_ctxt()
+            .enter(|infcx| infcx.expected_found_str_ty(&ExpectedFound { expected, found }))
+        {
+            // Highlighted the differences when showing the "expected/found" note.
+            err.note_expected_found(&"", expected, &"", found);
+        } else {
+            // This fallback shouldn't be necessary, but let's keep it in just in case.
+            err.note(&format!("expected `{:?}`\n   found `{:?}`", expected, found));
+        }
+        err.span_help(
+            type_param_span,
+            "the lifetime requirements from the `impl` do not correspond to the requirements in \
+             the `trait`",
+        );
+        if visitor.types.is_empty() {
+            err.help(
+                "verify the lifetime relationships in the `trait` and `impl` between the `self` \
+                 argument, the other inputs and its output",
+            );
+        }
         err.emit();
     }
 }
+
+struct TypeParamSpanVisitor<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    types: Vec<Span>,
+}
+
+impl Visitor<'tcx> for TypeParamSpanVisitor<'tcx> {
+    type Map = rustc_middle::hir::map::Map<'tcx>;
+
+    fn nested_visit_map(&mut self) -> hir::intravisit::NestedVisitorMap<Self::Map> {
+        hir::intravisit::NestedVisitorMap::OnlyBodies(self.tcx.hir())
+    }
+
+    fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx>) {
+        match arg.kind {
+            hir::TyKind::Rptr(_, ref mut_ty) => {
+                // We don't want to suggest looking into borrowing `&T` or `&Self`.
+                hir::intravisit::walk_ty(self, mut_ty.ty);
+                return;
+            }
+            hir::TyKind::Path(hir::QPath::Resolved(None, path)) => match &path.segments {
+                [segment]
+                    if segment
+                        .res
+                        .map(|res| match res {
+                            Res::SelfTy(_, _) | Res::Def(hir::def::DefKind::TyParam, _) => true,
+                            _ => false,
+                        })
+                        .unwrap_or(false) =>
+                {
+                    self.types.push(path.span);
+                }
+                _ => {}
+            },
+            _ => {}
+        }
+        hir::intravisit::walk_ty(self, arg);
+    }
+}

src/test/ui/associated-types/cache/project-fn-ret-invariant.transmute.stderr

@@ -14,16 +14,16 @@ note: ...so that the expression is assignable
    |
 LL |    bar(foo, x)
    |             ^
-   = note: expected  `Type<'_>`
-              found  `Type<'a>`
+   = note: expected `Type<'_>`
+              found `Type<'a>`
    = note: but, the lifetime must be valid for the static lifetime...
 note: ...so that the expression is assignable
   --> $DIR/project-fn-ret-invariant.rs:48:4
    |
 LL |    bar(foo, x)
    |    ^^^^^^^^^^^
-   = note: expected  `Type<'static>`
-              found  `Type<'_>`
+   = note: expected `Type<'static>`
+              found `Type<'_>`
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0490.stderr

@@ -58,8 +58,8 @@ note: ...so that the expression is assignable
    |
 LL |     let x: &'a _ = &y;
    |                    ^^
-   = note: expected  `&'a &()`
-              found  `&'a &'b ()`
+   = note: expected `&'a &()`
+              found `&'a &'b ()`
 note: but, the lifetime must be valid for the lifetime `'a` as defined on the function body at 1:6...
   --> $DIR/E0490.rs:1:6
    |

src/test/ui/impl-header-lifetime-elision/dyn-trait.stderr

@@ -14,16 +14,16 @@ note: ...so that the expression is assignable
    |
 LL |     static_val(x);
    |                ^
-   = note: expected  `std::boxed::Box<dyn std::fmt::Debug>`
-              found  `std::boxed::Box<(dyn std::fmt::Debug + 'a)>`
+   = note: expected `std::boxed::Box<dyn std::fmt::Debug>`
+              found `std::boxed::Box<(dyn std::fmt::Debug + 'a)>`
    = note: but, the lifetime must be valid for the static lifetime...
 note: ...so that the types are compatible
   --> $DIR/dyn-trait.rs:20:5
    |
 LL |     static_val(x);
    |     ^^^^^^^^^^
-   = note: expected  `StaticTrait`
-              found  `StaticTrait`
+   = note: expected `StaticTrait`
+              found `StaticTrait`
 
 error: aborting due to previous error
 

src/test/ui/in-band-lifetimes/mismatched_trait_impl-2.stderr

@@ -2,15 +2,17 @@ error: `impl` item signature doesn't match `trait` item signature
   --> $DIR/mismatched_trait_impl-2.rs:8:5
    |
 LL |     fn deref(&self) -> &dyn Trait {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found fn(&Struct) -> &dyn Trait
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&Struct) -> &dyn Trait`
    | 
   ::: $SRC_DIR/libcore/ops/deref.rs:LL:COL
    |
 LL |     fn deref(&self) -> &Self::Target;
-   |     --------------------------------- expected fn(&Struct) -> &(dyn Trait + 'static)
+   |     --------------------------------- expected `fn(&Struct) -> &(dyn Trait + 'static)`
    |
    = note: expected `fn(&Struct) -> &(dyn Trait + 'static)`
               found `fn(&Struct) -> &dyn Trait`
+   = help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
+   = help: verify the lifetime relationships in the `trait` and `impl` between the `self` argument, the other inputs and its output
 
 error: aborting due to previous error
 

src/test/ui/in-band-lifetimes/mismatched_trait_impl.nll.stderr

@@ -2,13 +2,15 @@ error: `impl` item signature doesn't match `trait` item signature
   --> $DIR/mismatched_trait_impl.rs:9:5
    |
 LL |     fn foo(&self, x: &'a u32, y: &u32) -> &'a u32;
-   |     ---------------------------------------------- expected fn(&i32, &'a u32, &u32) -> &'a u32
+   |     ---------------------------------------------- expected `fn(&i32, &'a u32, &u32) -> &'a u32`
 ...
 LL |     fn foo(&self, x: &u32, y: &'a u32) -> &'a u32 {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found fn(&i32, &u32, &u32) -> &u32
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&i32, &u32, &u32) -> &u32`
    |
    = note: expected `fn(&i32, &'a u32, &u32) -> &'a u32`
               found `fn(&i32, &u32, &u32) -> &u32`
+   = help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
+   = help: verify the lifetime relationships in the `trait` and `impl` between the `self` argument, the other inputs and its output
 
 error: aborting due to previous error
 

src/test/ui/in-band-lifetimes/mismatched_trait_impl.stderr

@@ -2,13 +2,15 @@ error: `impl` item signature doesn't match `trait` item signature
   --> $DIR/mismatched_trait_impl.rs:9:5
    |
 LL |     fn foo(&self, x: &'a u32, y: &u32) -> &'a u32;
-   |     ---------------------------------------------- expected fn(&i32, &'a u32, &u32) -> &'a u32
+   |     ---------------------------------------------- expected `fn(&i32, &'a u32, &u32) -> &'a u32`
 ...
 LL |     fn foo(&self, x: &u32, y: &'a u32) -> &'a u32 {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found fn(&i32, &u32, &u32) -> &u32
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&i32, &u32, &u32) -> &u32`
    |
    = note: expected `fn(&i32, &'a u32, &u32) -> &'a u32`
               found `fn(&i32, &u32, &u32) -> &u32`
+   = help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
+   = help: verify the lifetime relationships in the `trait` and `impl` between the `self` argument, the other inputs and its output
 
 error[E0623]: lifetime mismatch
   --> $DIR/mismatched_trait_impl.rs:10:9

src/test/ui/issues/issue-16683.stderr

@@ -26,8 +26,8 @@ note: ...so that the types are compatible
    |
 LL |         self.a();
    |              ^
-   = note: expected  `&'a Self`
-              found  `&Self`
+   = note: expected `&'a Self`
+              found `&Self`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-17758.stderr

@@ -27,8 +27,8 @@ note: ...so that the types are compatible
    |
 LL |         self.foo();
    |              ^^^
-   = note: expected  `&'a Self`
-              found  `&Self`
+   = note: expected `&'a Self`
+              found `&Self`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-20831-debruijn.stderr

@@ -87,8 +87,8 @@ note: ...so that the types are compatible
    |
 LL |     fn subscribe(&mut self, t : Box<dyn Subscriber<Input=<Self as Publisher>::Output> + 'a>) {
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: expected  `Publisher<'_>`
-              found  `Publisher<'_>`
+   = note: expected `Publisher<'_>`
+              found `Publisher<'_>`
 
 error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
   --> $DIR/issue-20831-debruijn.rs:28:33
@@ -117,8 +117,8 @@ note: ...so that the types are compatible
    |
 LL |     fn subscribe(&mut self, t : Box<dyn Subscriber<Input=<Self as Publisher>::Output> + 'a>) {
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: expected  `Publisher<'_>`
-              found  `Publisher<'_>`
+   = note: expected `Publisher<'_>`
+              found `Publisher<'_>`
 
 error: aborting due to 4 previous errors
 

src/test/ui/issues/issue-52213.stderr

@@ -14,8 +14,8 @@ note: ...so that the types are compatible
    |
 LL |     match (&t,) {
    |           ^^^^^
-   = note: expected  `(&&(T,),)`
-              found  `(&&'a (T,),)`
+   = note: expected `(&&(T,),)`
+              found `(&&'a (T,),)`
 note: but, the lifetime must be valid for the lifetime `'b` as defined on the function body at 1:27...
   --> $DIR/issue-52213.rs:1:27
    |

src/test/ui/issues/issue-55796.stderr

@@ -20,8 +20,8 @@ note: ...so that the expression is assignable
    |
 LL |         Box::new(self.out_edges(u).map(|e| e.target()))
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: expected  `std::boxed::Box<(dyn std::iter::Iterator<Item = <Self as Graph<'a>>::Node> + 'static)>`
-              found  `std::boxed::Box<dyn std::iter::Iterator<Item = <Self as Graph<'a>>::Node>>`
+   = note: expected `std::boxed::Box<(dyn std::iter::Iterator<Item = <Self as Graph<'a>>::Node> + 'static)>`
+              found `std::boxed::Box<dyn std::iter::Iterator<Item = <Self as Graph<'a>>::Node>>`
 
 error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
   --> $DIR/issue-55796.rs:21:9
@@ -45,8 +45,8 @@ note: ...so that the expression is assignable
    |
 LL |         Box::new(self.in_edges(u).map(|e| e.target()))
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: expected  `std::boxed::Box<(dyn std::iter::Iterator<Item = <Self as Graph<'a>>::Node> + 'static)>`
-              found  `std::boxed::Box<dyn std::iter::Iterator<Item = <Self as Graph<'a>>::Node>>`
+   = note: expected `std::boxed::Box<(dyn std::iter::Iterator<Item = <Self as Graph<'a>>::Node> + 'static)>`
+              found `std::boxed::Box<dyn std::iter::Iterator<Item = <Self as Graph<'a>>::Node>>`
 
 error: aborting due to 2 previous errors
 

src/test/ui/lifetimes/lifetime-mismatch-between-trait-and-impl.stderr

@@ -2,13 +2,15 @@ error: `impl` item signature doesn't match `trait` item signature
   --> $DIR/lifetime-mismatch-between-trait-and-impl.rs:6:5
    |
 LL |     fn foo<'a>(x: &i32, y: &'a i32) -> &'a i32;
-   |     ------------------------------------------- expected fn(&i32, &'a i32) -> &'a i32
+   |     ------------------------------------------- expected `fn(&i32, &'a i32) -> &'a i32`
 ...
 LL |     fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found fn(&i32, &i32) -> &i32
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&i32, &i32) -> &i32`
    |
    = note: expected `fn(&i32, &'a i32) -> &'a i32`
               found `fn(&i32, &i32) -> &i32`
+   = help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
+   = help: verify the lifetime relationships in the `trait` and `impl` between the `self` argument, the other inputs and its output
 
 error: aborting due to previous error
 

src/test/ui/nll/issue-55394.stderr

@@ -26,8 +26,8 @@ note: ...so that the expression is assignable
    |
 LL |         Foo { bar }
    |         ^^^^^^^^^^^
-   = note: expected  `Foo<'_>`
-              found  `Foo<'_>`
+   = note: expected `Foo<'_>`
+              found `Foo<'_>`
 
 error: aborting due to previous error
 

src/test/ui/nll/normalization-bounds-error.stderr

@@ -19,8 +19,8 @@ note: ...so that the types are compatible
    |
 LL | fn visit_seq<'d, 'a: 'd>() -> <&'a () as Visitor<'d>>::Value {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: expected  `Visitor<'d>`
-              found  `Visitor<'_>`
+   = note: expected `Visitor<'d>`
+              found `Visitor<'_>`
 
 error: aborting due to previous error