Suggest removal of auto trait super traits and type params

Author: estebank

src/librustc_ast_passes/ast_validation.rs

@@ -910,6 +910,12 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                             item.ident.span,
                             "auto trait cannot have generic parameters",
                         );
+                        err.span_suggestion_verbose(
+                            generics.span,
+                            "remove the parameters for the auto trait to be valid",
+                            String::new(),
+                            Applicability::MachineApplicable,
+                        );
                         err.emit();
                     }
                     if !bounds.is_empty() {
@@ -922,6 +928,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                             E0568,
                             "auto traits cannot have super traits"
                         );
+                        err.span_label(item.ident.span, "auto trait cannot have super traits");
                         if let Some(span) = last {
                             err.span_label(
                                 span,
@@ -931,8 +938,13 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                                     s = pluralize!(len)
                                 ),
                             );
+                            err.span_suggestion_verbose(
+                                generics.span.shrink_to_hi().to(span),
+                                "remove the super traits for the auto trait to be valid",
+                                String::new(),
+                                Applicability::MachineApplicable,
+                            );
                         }
-                        err.span_label(item.ident.span, "auto trait cannot have super traits");
                         err.emit();
                     }
                     if !trait_items.is_empty() {

src/test/ui/auto-trait-validation.stderr

@@ -5,6 +5,11 @@ LL | auto trait Generic<T> {}
    |            ------- ^ cannot have this generic parameter
    |            |
    |            auto trait cannot have generic parameters
+   |
+help: remove the parameters for the auto trait to be valid
+   |
+LL | auto trait Generic {}
+   |                  --
 
 error[E0568]: auto traits cannot have super traits
   --> $DIR/auto-trait-validation.rs:5:20
@@ -13,6 +18,11 @@ LL | auto trait Bound : Copy {}
    |            -----   ^^^^ cannot have this super trait
    |            |
    |            auto trait cannot have super traits
+   |
+help: remove the super traits for the auto trait to be valid
+   |
+LL | auto trait Bound {}
+   |                --
 
 error[E0380]: auto traits cannot have methods or associated items
   --> $DIR/auto-trait-validation.rs:7:25

src/test/ui/traits/traits-inductive-overflow-supertrait-oibit.stderr

@@ -5,6 +5,11 @@ LL | auto trait Magic: Copy {}
    |            -----  ^^^^ cannot have this super trait
    |            |
    |            auto trait cannot have super traits
+   |
+help: remove the super traits for the auto trait to be valid
+   |
+LL | auto trait Magic {}
+   |                --
 
 error[E0277]: the trait bound `NoClone: std::marker::Copy` is not satisfied
   --> $DIR/traits-inductive-overflow-supertrait-oibit.rs:15:23

src/test/ui/typeck/typeck-auto-trait-no-supertraits-2.stderr

@@ -5,6 +5,11 @@ LL | auto trait Magic : Sized where Option<Self> : Magic {}
    |            -----   ^^^^^ cannot have this super trait
    |            |
    |            auto trait cannot have super traits
+   |
+help: remove the super traits for the auto trait to be valid
+   |
+LL | auto trait Magic where Option<Self> : Magic {}
+   |                --
 
 error: aborting due to previous error
 

src/test/ui/typeck/typeck-auto-trait-no-supertraits.stderr

@@ -5,6 +5,11 @@ LL | auto trait Magic: Copy {}
    |            -----  ^^^^ cannot have this super trait
    |            |
    |            auto trait cannot have super traits
+   |
+help: remove the super traits for the auto trait to be valid
+   |
+LL | auto trait Magic {}
+   |                --
 
 error: aborting due to previous error