Suggest adding super trait constraints

src/librustc_trait_selection/traits/error_reporting/suggestions.rs

@@ -15,7 +15,7 @@ use rustc_middle::ty::{
     self, suggest_constraining_type_param, AdtKind, DefIdTree, Infer, InferTy, ToPredicate, Ty,
     TyCtxt, TypeFoldable, WithConstness,
 };
-use rustc_span::symbol::{kw, sym, Symbol};
+use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{MultiSpan, Span, DUMMY_SP};
 use std::fmt;
 
@@ -156,7 +156,7 @@ fn predicate_constraint(generics: &hir::Generics<'_>, pred: String) -> (Span, St
     (
         generics.where_clause.span_for_predicates_or_empty_place().shrink_to_hi(),
         format!(
-            "{} {} ",
+            "{} {}",
             if !generics.where_clause.predicates.is_empty() { "," } else { " where" },
             pred,
         ),
@@ -173,7 +173,13 @@ fn suggest_restriction(
     fn_sig: Option<&hir::FnSig<'_>>,
     projection: Option<&ty::ProjectionTy<'_>>,
     trait_ref: ty::PolyTraitRef<'_>,
+    super_traits: Option<(&Ident, &hir::GenericBounds<'_>)>,
 ) {
+    // When we are dealing with a trait, `super_traits` will be `Some`:
+    // Given `trait T: A + B + C {}`
+    //              -  ^^^^^^^^^ GenericBounds
+    //              |
+    //              &Ident
     let span = generics.where_clause.span_for_predicates_or_empty_place();
     if span.from_expansion() || span.desugaring_kind().is_some() {
         return;
@@ -262,10 +268,28 @@ fn suggest_restriction(
         );
     } else {
         // Trivial case: `T` needs an extra bound: `T: Bound`.
-        let (sp, sugg) =
-            predicate_constraint(generics, trait_ref.without_const().to_predicate().to_string());
-        let appl = Applicability::MachineApplicable;
-        err.span_suggestion(sp, &format!("consider further restricting {}", msg), sugg, appl);
+        let (sp, suggestion) = match super_traits {
+            None => {
+                predicate_constraint(generics, trait_ref.without_const().to_predicate().to_string())
+            }
+            Some((ident, bounds)) => match bounds {
+                [.., bound] => (
+                    bound.span().shrink_to_hi(),
+                    format!(" + {}", trait_ref.print_only_trait_path().to_string()),
+                ),
+                [] => (
+                    ident.span.shrink_to_hi(),
+                    format!(": {}", trait_ref.print_only_trait_path().to_string()),
+                ),
+            },
+        };
+
+        err.span_suggestion_verbose(
+            sp,
+            &format!("consider further restricting {}", msg),
+            suggestion,
+            Applicability::MachineApplicable,
+        );
     }
 }
 
@@ -288,13 +312,35 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let mut hir_id = body_id;
         while let Some(node) = self.tcx.hir().find(hir_id) {
             match node {
+                hir::Node::Item(hir::Item {
+                    ident,
+                    kind: hir::ItemKind::Trait(_, _, generics, bounds, _),
+                    ..
+                }) if self_ty == self.tcx.types.self_param => {
+                    assert!(param_ty);
+                    // Restricting `Self` for a single method.
+                    suggest_restriction(
+                        &generics,
+                        "`Self`",
+                        err,
+                        None,
+                        projection,
+                        trait_ref,
+                        Some((ident, bounds)),
+                    );
+                    return;
+                }
+
                 hir::Node::TraitItem(hir::TraitItem {
                     generics,
                     kind: hir::TraitItemKind::Fn(..),
                     ..
-                }) if param_ty && self_ty == self.tcx.types.self_param => {
+                }) if self_ty == self.tcx.types.self_param => {
+                    assert!(param_ty);
                     // Restricting `Self` for a single method.
-                    suggest_restriction(&generics, "`Self`", err, None, projection, trait_ref);
+                    suggest_restriction(
+                        &generics, "`Self`", err, None, projection, trait_ref, None,
+                    );
                     return;
                 }
 
@@ -319,6 +365,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         Some(fn_sig),
                         projection,
                         trait_ref,
+                        None,
                     );
                     return;
                 }
@@ -336,6 +383,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         None,
                         projection,
                         trait_ref,
+                        None,
                     );
                     return;
                 }

src/test/ui/associated-type-bounds/bad-bounds-on-assoc-in-trait.stderr

@@ -16,11 +16,13 @@ error[E0277]: `<<T as Case1>::C as std::iter::Iterator>::Item` is not an iterato
   --> $DIR/bad-bounds-on-assoc-in-trait.rs:36:20
    |
 LL | fn assume_case1<T: Case1>() {
-   |                    ^^^^^   - help: consider further restricting the associated type: `where <<T as Case1>::C as std::iter::Iterator>::Item: std::iter::Iterator`
-   |                    |
-   |                    `<<T as Case1>::C as std::iter::Iterator>::Item` is not an iterator
+   |                    ^^^^^ `<<T as Case1>::C as std::iter::Iterator>::Item` is not an iterator
    |
    = help: the trait `std::iter::Iterator` is not implemented for `<<T as Case1>::C as std::iter::Iterator>::Item`
+help: consider further restricting the associated type
+   |
+LL | fn assume_case1<T: Case1>() where <<T as Case1>::C as std::iter::Iterator>::Item: std::iter::Iterator {
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: `<<T as Case1>::C as std::iter::Iterator>::Item` cannot be sent between threads safely
   --> $DIR/bad-bounds-on-assoc-in-trait.rs:36:20
@@ -32,11 +34,13 @@ LL |         Send + Iterator<Item:
    |         ---- required by this bound in `Case1`
 ...
 LL | fn assume_case1<T: Case1>() {
-   |                    ^^^^^   - help: consider further restricting the associated type: `where <<T as Case1>::C as std::iter::Iterator>::Item: std::marker::Send`
-   |                    |
-   |                    `<<T as Case1>::C as std::iter::Iterator>::Item` cannot be sent between threads safely
+   |                    ^^^^^ `<<T as Case1>::C as std::iter::Iterator>::Item` cannot be sent between threads safely
    |
    = help: the trait `std::marker::Send` is not implemented for `<<T as Case1>::C as std::iter::Iterator>::Item`
+help: consider further restricting the associated type
+   |
+LL | fn assume_case1<T: Case1>() where <<T as Case1>::C as std::iter::Iterator>::Item: std::marker::Send {
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: `<<T as Case1>::C as std::iter::Iterator>::Item` cannot be shared between threads safely
   --> $DIR/bad-bounds-on-assoc-in-trait.rs:36:20
@@ -48,11 +52,13 @@ LL |         > + Sync>;
    |             ---- required by this bound in `Case1`
 ...
 LL | fn assume_case1<T: Case1>() {
-   |                    ^^^^^   - help: consider further restricting the associated type: `where <<T as Case1>::C as std::iter::Iterator>::Item: std::marker::Sync`
-   |                    |
-   |                    `<<T as Case1>::C as std::iter::Iterator>::Item` cannot be shared between threads safely
+   |                    ^^^^^ `<<T as Case1>::C as std::iter::Iterator>::Item` cannot be shared between threads safely
    |
    = help: the trait `std::marker::Sync` is not implemented for `<<T as Case1>::C as std::iter::Iterator>::Item`
+help: consider further restricting the associated type
+   |
+LL | fn assume_case1<T: Case1>() where <<T as Case1>::C as std::iter::Iterator>::Item: std::marker::Sync {
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: `<_ as Lam<&'a u8>>::App` doesn't implement `std::fmt::Debug`
   --> $DIR/bad-bounds-on-assoc-in-trait.rs:36:20

src/test/ui/associated-types/associated-types-bound-failure.fixed

@@ -14,7 +14,7 @@ pub trait GetToInt
 }
 
 fn foo<G>(g: G) -> isize
-    where G : GetToInt, <G as GetToInt>::R: ToInt 
+    where G : GetToInt, <G as GetToInt>::R: ToInt
 {
     ToInt::to_int(&g.get()) //~ ERROR E0277
 }

src/test/ui/associated-types/associated-types-bound-failure.stderr

@@ -4,11 +4,13 @@ error[E0277]: the trait bound `<G as GetToInt>::R: ToInt` is not satisfied
 LL |     fn to_int(&self) -> isize;
    |     -------------------------- required by `ToInt::to_int`
 ...
-LL |     where G : GetToInt
-   |                       - help: consider further restricting the associated type: `, <G as GetToInt>::R: ToInt`
-LL | {
 LL |     ToInt::to_int(&g.get())
    |                   ^^^^^^^^ the trait `ToInt` is not implemented for `<G as GetToInt>::R`
+   |
+help: consider further restricting the associated type
+   |
+LL |     where G : GetToInt, <G as GetToInt>::R: ToInt
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/associated-types/associated-types-for-unimpl-trait.fixed

@@ -7,7 +7,7 @@ trait Get {
 }
 
 trait Other {
-    fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) where Self: Get  {}
+    fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) where Self: Get {}
     //~^ ERROR the trait bound `Self: Get` is not satisfied
 }
 

src/test/ui/associated-types/associated-types-for-unimpl-trait.stderr

@@ -2,10 +2,12 @@ error[E0277]: the trait bound `Self: Get` is not satisfied
   --> $DIR/associated-types-for-unimpl-trait.rs:10:5
    |
 LL |     fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-^^
-   |     |                                                       |
-   |     |                                                       help: consider further restricting `Self`: `where Self: Get`
-   |     the trait `Get` is not implemented for `Self`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
+   |
+help: consider further restricting `Self`
+   |
+LL |     fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) where Self: Get {}
+   |                                                              ^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/associated-types/associated-types-no-suitable-supertrait-2.stderr

@@ -2,10 +2,12 @@ error[E0277]: the trait bound `Self: Get` is not satisfied
   --> $DIR/associated-types-no-suitable-supertrait-2.rs:17:5
    |
 LL |     fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-^^
-   |     |                                                       |
-   |     |                                                       help: consider further restricting `Self`: `where Self: Get`
-   |     the trait `Get` is not implemented for `Self`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
+   |
+help: consider further restricting `Self`
+   |
+LL |     fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) where Self: Get {}
+   |                                                              ^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/associated-types/associated-types-no-suitable-supertrait.stderr

@@ -2,10 +2,12 @@ error[E0277]: the trait bound `Self: Get` is not satisfied
   --> $DIR/associated-types-no-suitable-supertrait.rs:17:5
    |
 LL |     fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-^^
-   |     |                                                       |
-   |     |                                                       help: consider further restricting `Self`: `where Self: Get`
-   |     the trait `Get` is not implemented for `Self`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
+   |
+help: consider further restricting `Self`
+   |
+LL |     fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) where Self: Get {}
+   |                                                              ^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `(T, U): Get` is not satisfied
   --> $DIR/associated-types-no-suitable-supertrait.rs:22:5

src/test/ui/associated-types/associated-types-projection-to-unrelated-trait-in-method-without-default.fixed

@@ -7,7 +7,7 @@ trait Get {
 }
 
 trait Other {
-    fn okay<U:Get>(&self, foo: U, bar: <Self as Get>::Value) where Self: Get ;
+    fn okay<U:Get>(&self, foo: U, bar: <Self as Get>::Value) where Self: Get;
     //~^ ERROR E0277
 }
 

src/test/ui/associated-types/associated-types-projection-to-unrelated-trait-in-method-without-default.stderr

@@ -2,10 +2,12 @@ error[E0277]: the trait bound `Self: Get` is not satisfied
   --> $DIR/associated-types-projection-to-unrelated-trait-in-method-without-default.rs:10:5
    |
 LL |     fn okay<U:Get>(&self, foo: U, bar: <Self as Get>::Value);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-
-   |     |                                                       |
-   |     |                                                       help: consider further restricting `Self`: `where Self: Get`
-   |     the trait `Get` is not implemented for `Self`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
+   |
+help: consider further restricting `Self`
+   |
+LL |     fn okay<U:Get>(&self, foo: U, bar: <Self as Get>::Value) where Self: Get;
+   |                                                              ^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/associated-types/associated-types-unsized.fixed

@@ -6,7 +6,7 @@ trait Get {
     fn get(&self) -> <Self as Get>::Value;
 }
 
-fn foo<T:Get>(t: T) where <T as Get>::Value: std::marker::Sized  {
+fn foo<T:Get>(t: T) where <T as Get>::Value: std::marker::Sized {
     let x = t.get(); //~ ERROR the size for values of type
 }
 

src/test/ui/associated-types/associated-types-unsized.stderr

@@ -1,15 +1,17 @@
 error[E0277]: the size for values of type `<T as Get>::Value` cannot be known at compilation time
   --> $DIR/associated-types-unsized.rs:10:9
    |
-LL | fn foo<T:Get>(t: T) {
-   |                    - help: consider further restricting the associated type: `where <T as Get>::Value: std::marker::Sized`
 LL |     let x = t.get();
    |         ^ doesn't have a size known at compile-time
    |
    = help: the trait `std::marker::Sized` is not implemented for `<T as Get>::Value`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: all local variables must have a statically known size
    = help: unsized locals are gated as an unstable feature
+help: consider further restricting the associated type
+   |
+LL | fn foo<T:Get>(t: T) where <T as Get>::Value: std::marker::Sized {
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/associated-types/defaults-suitability.stderr

@@ -85,25 +85,29 @@ error[E0277]: the trait bound `<Self as Foo2<T>>::Baz: std::clone::Clone` is not
   --> $DIR/defaults-suitability.rs:72:15
    |
 LL | trait Foo2<T> {
-   | -------------- help: consider further restricting the associated type: `where <Self as Foo2<T>>::Baz: std::clone::Clone`
-   | |
-   | required by `Foo2`
+   | ------------- required by `Foo2`
 LL |     type Bar: Clone = Vec<Self::Baz>;
    |               ^^^^^ the trait `std::clone::Clone` is not implemented for `<Self as Foo2<T>>::Baz`
    |
    = note: required because of the requirements on the impl of `std::clone::Clone` for `std::vec::Vec<<Self as Foo2<T>>::Baz>`
+help: consider further restricting the associated type
+   |
+LL | trait Foo2<T> where <Self as Foo2<T>>::Baz: std::clone::Clone {
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `<Self as Foo25<T>>::Baz: std::clone::Clone` is not satisfied
   --> $DIR/defaults-suitability.rs:81:15
    |
 LL | trait Foo25<T: Clone> {
-   | ---------------------- help: consider further restricting the associated type: `where <Self as Foo25<T>>::Baz: std::clone::Clone`
-   | |
-   | required by `Foo25`
+   | --------------------- required by `Foo25`
 LL |     type Bar: Clone = Vec<Self::Baz>;
    |               ^^^^^ the trait `std::clone::Clone` is not implemented for `<Self as Foo25<T>>::Baz`
    |
    = note: required because of the requirements on the impl of `std::clone::Clone` for `std::vec::Vec<<Self as Foo25<T>>::Baz>`
+help: consider further restricting the associated type
+   |
+LL | trait Foo25<T: Clone> where <Self as Foo25<T>>::Baz: std::clone::Clone {
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `T: std::clone::Clone` is not satisfied
   --> $DIR/defaults-suitability.rs:90:16

src/test/ui/associated-types/defaults-unsound-62211-1.stderr

@@ -6,6 +6,11 @@ LL | trait UncheckedCopy: Sized {
 ...
 LL |     type Output: Copy
    |                  ^^^^ the trait `std::marker::Copy` is not implemented for `Self`
+   |
+help: consider further restricting `Self`
+   |
+LL | trait UncheckedCopy: Sized + std::marker::Copy {
+   |                            ^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: cannot add-assign `&'static str` to `Self`
   --> $DIR/defaults-unsound-62211-1.rs:25:7
@@ -17,6 +22,10 @@ LL |     + AddAssign<&'static str>
    |       ^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `Self += &'static str`
    |
    = help: the trait `std::ops::AddAssign<&'static str>` is not implemented for `Self`
+help: consider further restricting `Self`
+   |
+LL | trait UncheckedCopy: Sized + std::ops::AddAssign<&'static str> {
+   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `Self: std::ops::Deref` is not satisfied
   --> $DIR/defaults-unsound-62211-1.rs:23:7
@@ -26,6 +35,11 @@ LL | trait UncheckedCopy: Sized {
 ...
 LL |     + Deref<Target = str>
    |       ^^^^^^^^^^^^^^^^^^^ the trait `std::ops::Deref` is not implemented for `Self`
+   |
+help: consider further restricting `Self`
+   |
+LL | trait UncheckedCopy: Sized + std::ops::Deref {
+   |                            ^^^^^^^^^^^^^^^^^
 
 error[E0277]: `Self` doesn't implement `std::fmt::Display`
   --> $DIR/defaults-unsound-62211-1.rs:28:7
@@ -38,6 +52,10 @@ LL |     + Display = Self;
    |
    = help: the trait `std::fmt::Display` is not implemented for `Self`
    = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
+help: consider further restricting `Self`
+   |
+LL | trait UncheckedCopy: Sized + std::fmt::Display {
+   |                            ^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: `T` doesn't implement `std::fmt::Display`
   --> $DIR/defaults-unsound-62211-1.rs:41:9