Improve error messages for bad type constraints

Co-authored-by: Esteban Kuber <esteban@kuber.com.ar>

Author: notriddle

compiler/rustc_parse/src/parser/path.rs

@@ -495,25 +495,28 @@ impl<'a> Parser<'a> {
             None => {
                 let after_eq = eq.shrink_to_hi();
                 let before_next = self.token.span.shrink_to_lo();
-                let the_type_placeholder = if matches!(self.token.kind, token::Comma | token::Gt) {
-                    " TheType"
-                } else {
-                    " TheType "
-                };
-                self.struct_span_err(after_eq.to(before_next), "missing type to the right of `=`")
-                    .span_suggestion(
+                let mut err = self
+                    .struct_span_err(after_eq.to(before_next), "missing type to the right of `=`");
+                if matches!(self.token.kind, token::Comma | token::Gt) {
+                    err.span_suggestion(
                         self.sess.source_map().next_point(eq).to(before_next),
                         "to constrain the associated type, add a type after `=`",
-                        the_type_placeholder.to_string(),
+                        " TheType".to_string(),
                         Applicability::HasPlaceholders,
-                    )
-                    .span_suggestion(
+                    );
+                    err.span_suggestion(
                         eq.to(before_next),
                         &format!("remove the `=` if `{}` is a type", ident),
                         String::new(),
                         Applicability::MaybeIncorrect,
                     )
-                    .emit();
+                } else {
+                    err.span_label(
+                        self.token.span,
+                        &format!("expected type, found {}", super::token_descr(&self.token)),
+                    )
+                };
+                return Err(err);
             }
         }
         Ok(self.mk_ty(span, ast::TyKind::Err))
@@ -584,6 +587,12 @@ impl<'a> Parser<'a> {
                 "expected lifetime, type, or constant, found keyword `const`",
             );
             if self.check_const_arg() {
+                err.span_suggestion_verbose(
+                    start.until(self.token.span),
+                    "the `const` keyword is only needed in the definition of the type",
+                    String::new(),
+                    Applicability::MaybeIncorrect,
+                );
                 err.emit();
                 GenericArg::Const(self.parse_const_arg()?)
             } else {

src/test/ui/const-generics/parser-error-recovery/issue-89013-no-assoc.stderr

@@ -3,6 +3,12 @@ error: expected lifetime, type, or constant, found keyword `const`
    |
 LL | impl Foo<const 3> for Bar {
    |          ^^^^^
+   |
+help: the `const` keyword is only needed in the definition of the type
+   |
+LL - impl Foo<const 3> for Bar {
+LL + impl Foo<3> for Bar {
+   | 
 
 error: aborting due to previous error
 

src/test/ui/const-generics/parser-error-recovery/issue-89013-type.rs

@@ -8,7 +8,6 @@ const T: usize = 42;
 
 impl Foo<N = type 3> for Bar {
 //~^ERROR missing type to the right of `=`
-//~^^ERROR found keyword `type`
     fn do_x(&self) -> [u8; 3] {
         [0u8; 3]
     }

src/test/ui/const-generics/parser-error-recovery/issue-89013-type.stderr

@@ -2,23 +2,7 @@ error: missing type to the right of `=`
   --> $DIR/issue-89013-type.rs:9:13
    |
 LL | impl Foo<N = type 3> for Bar {
-   |             ^
-   |
-help: to constrain the associated type, add a type after `=`
-   |
-LL | impl Foo<N = TheType type 3> for Bar {
-   |              +++++++
-help: remove the `=` if `N` is a type
-   |
-LL - impl Foo<N = type 3> for Bar {
-LL + impl Foo<N type 3> for Bar {
-   | 
-
-error: expected one of `,`, `>`, a const expression, lifetime, or type, found keyword `type`
-  --> $DIR/issue-89013-type.rs:9:14
-   |
-LL | impl Foo<N = type 3> for Bar {
-   |              ^^^^ expected one of `,`, `>`, a const expression, lifetime, or type
+   |             ^---- expected type, found keyword `type`
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 

src/test/ui/const-generics/parser-error-recovery/issue-89013.stderr

@@ -3,6 +3,12 @@ error: expected lifetime, type, or constant, found keyword `const`
    |
 LL | impl Foo<N = const 3> for Bar {
    |              ^^^^^
+   |
+help: the `const` keyword is only needed in the definition of the type
+   |
+LL - impl Foo<N = const 3> for Bar {
+LL + impl Foo<N = 3> for Bar {
+   | 
 
 error: cannot constrain an associated constant to a value
   --> $DIR/issue-89013.rs:9:10