Use structured suggestion for restricting bounds

src/librustc/hir/mod.rs

@@ -669,6 +669,12 @@ impl WhereClause {
             Some(self.span)
         }
     }
+
+    /// The `WhereClause` under normal circumstances points at either the predicates or the empty
+    /// space where the `where` clause should be. Only of use for diagnostic suggestions.
+    pub fn span_for_predicates_or_empty_place(&self) -> Span {
+        self.span
+    }
 }
 
 /// A single predicate in a where-clause.

src/librustc/traits/error_reporting.rs

@@ -715,8 +715,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             // these notes will often be of the form
                             //     "the type `T` can't be frobnicated"
                             // which is somewhat confusing.
-                            err.help(&format!("consider adding a `where {}` bound",
-                                              trait_ref.to_predicate()));
+                            self.suggest_restricting_param_bound(
+                                &mut err,
+                                &trait_ref,
+                                obligation.cause.body_id,
+                            );
                         } else {
                             if !have_alt_message {
                                 // Can't show anything else useful, try to find similar impls.
@@ -960,6 +963,175 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         err.emit();
     }
 
+    fn suggest_restricting_param_bound(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        trait_ref: &ty::PolyTraitRef<'_>,
+        body_id: hir::HirId,
+    ) {
+        let self_ty = trait_ref.self_ty();
+        let (param_ty, projection) = match &self_ty.kind {
+            ty::Param(_) => (true, None),
+            ty::Projection(projection) => (false, Some(projection)),
+            _ => return,
+        };
+
+        let mut suggest_restriction = |generics: &hir::Generics, msg| {
+            let span = generics.where_clause.span_for_predicates_or_empty_place();
+            if !span.from_expansion() && span.desugaring_kind().is_none() {
+                err.span_suggestion(
+                    generics.where_clause.span_for_predicates_or_empty_place().shrink_to_hi(),
+                    &format!("consider further restricting {}", msg),
+                    format!(
+                        "{} {} ",
+                        if !generics.where_clause.predicates.is_empty() {
+                            ","
+                        } else {
+                            " where"
+                        },
+                        trait_ref.to_predicate(),
+                    ),
+                    Applicability::MachineApplicable,
+                );
+            }
+        };
+
+        // FIXME: Add check for trait bound that is already present, particularly `?Sized` so we
+        //        don't suggest `T: Sized + ?Sized`.
+        let mut hir_id = body_id;
+        while let Some(node) = self.tcx.hir().find(hir_id) {
+            match node {
+                hir::Node::TraitItem(hir::TraitItem {
+                    generics,
+                    kind: hir::TraitItemKind::Method(..), ..
+                }) if param_ty && self_ty == self.tcx.types.self_param => {
+                    // Restricting `Self` for a single method.
+                    suggest_restriction(&generics, "`Self`");
+                    return;
+                }
+
+                hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::Fn(_, _, generics, _), ..
+                }) |
+                hir::Node::TraitItem(hir::TraitItem {
+                    generics,
+                    kind: hir::TraitItemKind::Method(..), ..
+                }) |
+                hir::Node::ImplItem(hir::ImplItem {
+                    generics,
+                    kind: hir::ImplItemKind::Method(..), ..
+                }) |
+                hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::Trait(_, _, generics, _, _), ..
+                }) |
+                hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::Impl(_, _, _, generics, ..), ..
+                }) if projection.is_some() => {
+                    // Missing associated type bound.
+                    suggest_restriction(&generics, "the associated type");
+                    return;
+                }
+
+                hir::Node::Item(hir::Item { kind: hir::ItemKind::Struct(_, generics), span, .. }) |
+                hir::Node::Item(hir::Item { kind: hir::ItemKind::Enum(_, generics), span, .. }) |
+                hir::Node::Item(hir::Item { kind: hir::ItemKind::Union(_, generics), span, .. }) |
+                hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::Trait(_, _, generics, ..), span, ..
+                }) |
+                hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::Impl(_, _, _, generics, ..), span, ..
+                }) |
+                hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::Fn(_, _, generics, _), span, ..
+                }) |
+                hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::TyAlias(_, generics), span, ..
+                }) |
+                hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::TraitAlias(generics, _), span, ..
+                }) |
+                hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::OpaqueTy(hir::OpaqueTy { generics, .. }), span, ..
+                }) |
+                hir::Node::TraitItem(hir::TraitItem { generics, span, .. }) |
+                hir::Node::ImplItem(hir::ImplItem { generics, span, .. })
+                if param_ty => {
+                    // Missing generic type parameter bound.
+                    let restrict_msg = "consider further restricting this bound";
+                    let param_name = self_ty.to_string();
+                    for param in generics.params.iter().filter(|p| {
+                        &param_name == std::convert::AsRef::<str>::as_ref(&p.name.ident().as_str())
+                    }) {
+                        if param_name.starts_with("impl ") {
+                            // `impl Trait` in argument:
+                            // `fn foo(x: impl Trait) {}` → `fn foo(t: impl Trait + Trait2) {}`
+                            err.span_suggestion(
+                                param.span,
+                                restrict_msg,
+                                // `impl CurrentTrait + MissingTrait`
+                                format!("{} + {}", param.name.ident(), trait_ref),
+                                Applicability::MachineApplicable,
+                            );
+                        } else if generics.where_clause.predicates.is_empty() &&
+                                param.bounds.is_empty()
+                        {
+                            // If there are no bounds whatsoever, suggest adding a constraint
+                            // to the type parameter:
+                            // `fn foo<T>(t: T) {}` → `fn foo<T: Trait>(t: T) {}`
+                            err.span_suggestion(
+                                param.span,
+                                "consider restricting this bound",
+                                format!("{}", trait_ref.to_predicate()),
+                                Applicability::MachineApplicable,
+                            );
+                        } else if !generics.where_clause.predicates.is_empty() {
+                            // There is a `where` clause, so suggest expanding it:
+                            // `fn foo<T>(t: T) where T: Debug {}` →
+                            // `fn foo<T>(t: T) where T: Debug, T: Trait {}`
+                            err.span_suggestion(
+                                generics.where_clause.span().unwrap().shrink_to_hi(),
+                                &format!(
+                                    "consider further restricting type parameter `{}`",
+                                    param_name,
+                                ),
+                                format!(", {}", trait_ref.to_predicate()),
+                                Applicability::MachineApplicable,
+                            );
+                        } else {
+                            // If there is no `where` clause lean towards constraining to the
+                            // type parameter:
+                            // `fn foo<X: Bar, T>(t: T, x: X) {}` → `fn foo<T: Trait>(t: T) {}`
+                            // `fn foo<T: Bar>(t: T) {}` → `fn foo<T: Bar + Trait>(t: T) {}`
+                            let sp = param.span.with_hi(span.hi());
+                            let span = self.tcx.sess.source_map()
+                                .span_through_char(sp, ':');
+                            if sp != param.span && sp != span {
+                                // Only suggest if we have high certainty that the span
+                                // covers the colon in `foo<T: Trait>`.
+                                err.span_suggestion(span, restrict_msg, format!(
+                                    "{} + ",
+                                    trait_ref.to_predicate(),
+                                ), Applicability::MachineApplicable);
+                            } else {
+                                err.span_label(param.span, &format!(
+                                    "consider adding a `where {}` bound",
+                                    trait_ref.to_predicate(),
+                                ));
+                            }
+                        }
+                        return;
+                    }
+                }
+
+                hir::Node::Crate => return,
+
+                _ => {}
+            }
+
+            hir_id = self.tcx.hir().get_parent_item(hir_id);
+        }
+    }
+
     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
     /// suggestion to borrow the initializer in order to use have a slice instead.
     fn suggest_borrow_on_unsized_slice(

src/librustc_typeck/check/demand.rs

@@ -350,11 +350,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // If the span is from a macro, then it's hard to extract the text
         // and make a good suggestion, so don't bother.
-        let is_desugaring = match sp.desugaring_kind() {
-            Some(k) => sp.is_desugaring(k),
-            None => false
-        };
-        let is_macro = sp.from_expansion() && !is_desugaring;
+        let is_macro = sp.from_expansion() && sp.desugaring_kind().is_none();
 
         // `ExprKind::DropTemps` is semantically irrelevant for these suggestions.
         let expr = expr.peel_drop_temps();

src/test/ui/associated-const/associated-const-type-parameter-arrays-2.stderr

@@ -4,10 +4,10 @@ error[E0277]: the trait bound `A: Foo` is not satisfied
 LL |     const Y: usize;
    |     --------------- required by `Foo::Y`
 ...
+LL | pub fn test<A: Foo, B: Foo>() {
+   |             -- help: consider further restricting this bound: `A: Foo +`
 LL |     let _array = [4; <A as Foo>::Y];
    |                      ^^^^^^^^^^^^^ the trait `Foo` is not implemented for `A`
-   |
-   = help: consider adding a `where A: Foo` bound
 
 error: aborting due to previous error
 

src/test/ui/associated-const/associated-const-type-parameter-arrays.stderr

@@ -4,10 +4,10 @@ error[E0277]: the trait bound `A: Foo` is not satisfied
 LL |     const Y: usize;
    |     --------------- required by `Foo::Y`
 ...
+LL | pub fn test<A: Foo, B: Foo>() {
+   |             -- help: consider further restricting this bound: `A: Foo +`
 LL |     let _array: [u32; <A as Foo>::Y];
    |                       ^^^^^^^^^^^^^ the trait `Foo` is not implemented for `A`
-   |
-   = help: consider adding a `where A: Foo` bound
 
 error: aborting due to previous error
 

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

@@ -9,7 +9,10 @@ LL | impl Case1 for S1 {
 error[E0277]: `<<T as Case1>::C as std::iter::Iterator>::Item` is not an iterator
   --> $DIR/bad-bounds-on-assoc-in-trait.rs:37:1
    |
-LL | / fn assume_case1<T: 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::iter::Iterator`
+   |  _|
+   | |
 LL | |
 LL | |
 LL | |
@@ -19,15 +22,17 @@ LL | | }
    | |_^ `<<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 adding a `where <<T as Case1>::C as std::iter::Iterator>::Item: std::iter::Iterator` bound
 
 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:37:1
    |
 LL |   trait Case1 {
    |   ----------- required by `Case1`
 ...
-LL | / fn assume_case1<T: 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`
+   |  _|
+   | |
 LL | |
 LL | |
 LL | |
@@ -37,15 +42,17 @@ LL | | }
    | |_^ `<<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 adding a `where <<T as Case1>::C as std::iter::Iterator>::Item: std::marker::Send` bound
 
 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:37:1
    |
 LL |   trait Case1 {
    |   ----------- required by `Case1`
 ...
-LL | / fn assume_case1<T: 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`
+   |  _|
+   | |
 LL | |
 LL | |
 LL | |
@@ -55,7 +62,6 @@ LL | | }
    | |_^ `<<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 adding a `where <<T as Case1>::C as std::iter::Iterator>::Item: std::marker::Sync` bound
 
 error[E0277]: `<_ as Lam<&'a u8>>::App` doesn't implement `std::fmt::Debug`
   --> $DIR/bad-bounds-on-assoc-in-trait.rs:37:1

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

@@ -0,0 +1,29 @@
+// run-rustfix
+// Test equality constraints on associated types in a where clause.
+#![allow(dead_code)]
+
+pub trait ToInt {
+    fn to_int(&self) -> isize;
+}
+
+pub trait GetToInt
+{
+    type R;
+
+    fn get(&self) -> <Self as GetToInt>::R;
+}
+
+fn foo<G>(g: G) -> isize
+    where G : GetToInt, <G as GetToInt>::R: ToInt 
+{
+    ToInt::to_int(&g.get()) //~ ERROR E0277
+}
+
+fn bar<G : GetToInt>(g: G) -> isize
+    where G::R : ToInt
+{
+    ToInt::to_int(&g.get()) // OK
+}
+
+pub fn main() {
+}

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

@@ -1,4 +1,6 @@
+// run-rustfix
 // Test equality constraints on associated types in a where clause.
+#![allow(dead_code)]
 
 pub trait ToInt {
     fn to_int(&self) -> isize;

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

@@ -1,13 +1,14 @@
 error[E0277]: the trait bound `<G as GetToInt>::R: ToInt` is not satisfied
-  --> $DIR/associated-types-bound-failure.rs:17:19
+  --> $DIR/associated-types-bound-failure.rs:19:19
    |
 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 adding a `where <G as GetToInt>::R: ToInt` bound
 
 error: aborting due to previous error
 

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

@@ -0,0 +1,15 @@
+// run-rustfix
+#![allow(unused_variables)]
+
+trait Get {
+    type Value;
+    fn get(&self) -> <Self as Get>::Value;
+}
+
+trait Other {
+    fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) where Self: Get  {}
+    //~^ ERROR the trait bound `Self: Get` is not satisfied
+}
+
+fn main() {
+}

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

@@ -1,3 +1,6 @@
+// run-rustfix
+#![allow(unused_variables)]
+
 trait Get {
     type Value;
     fn get(&self) -> <Self as Get>::Value;

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

@@ -1,10 +1,11 @@
 error[E0277]: the trait bound `Self: Get` is not satisfied
-  --> $DIR/associated-types-for-unimpl-trait.rs:7:5
+  --> $DIR/associated-types-for-unimpl-trait.rs:10:5
    |
 LL |     fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
-   |
-   = help: consider adding a `where Self: Get` bound
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-^^
+   |     |                                                       |
+   |     |                                                       help: consider further restricting `Self`: `where Self: Get`
+   |     the trait `Get` is not implemented for `Self`
 
 error: aborting due to previous error