Backport only: avoid ICE on bad placeholder type

This change avoids the ICE by actually emitting an appropriate error. The
output will be duplicated in some cases, but that's better than the
avoidable ICE.

Author: estebank

src/librustc_typeck/collect.rs

@@ -316,7 +316,7 @@ impl AstConv<'tcx> for ItemCtxt<'tcx> {
     }
 
     fn ty_infer(&self, _: Option<&ty::GenericParamDef>, span: Span) -> Ty<'tcx> {
-        self.tcx().sess.delay_span_bug(span, "bad placeholder type");
+        placeholder_type_error(self.tcx(), span, &[], vec![span], false);
         self.tcx().types.err
     }
 

src/test/ui/did_you_mean/bad-assoc-ty.rs

@@ -17,6 +17,7 @@ type D = (u8, u8)::AssocTy;
 type E = _::AssocTy;
 //~^ ERROR missing angle brackets in associated item path
 //~| ERROR the type placeholder `_` is not allowed within types on item signatures
+//~| ERROR the type placeholder `_` is not allowed within types on item signatures
 
 type F = &'static (u8)::AssocTy;
 //~^ ERROR missing angle brackets in associated item path

src/test/ui/did_you_mean/bad-assoc-ty.stderr

@@ -29,25 +29,25 @@ LL | type E = _::AssocTy;
    |          ^^^^^^^^^^ help: try: `<_>::AssocTy`
 
 error: missing angle brackets in associated item path
-  --> $DIR/bad-assoc-ty.rs:21:19
+  --> $DIR/bad-assoc-ty.rs:22:19
    |
 LL | type F = &'static (u8)::AssocTy;
    |                   ^^^^^^^^^^^^^ help: try: `<(u8)>::AssocTy`
 
 error: missing angle brackets in associated item path
-  --> $DIR/bad-assoc-ty.rs:27:10
+  --> $DIR/bad-assoc-ty.rs:28:10
    |
 LL | type G = dyn 'static + (Send)::AssocTy;
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `<dyn 'static + (Send)>::AssocTy`
 
 error: missing angle brackets in associated item path
-  --> $DIR/bad-assoc-ty.rs:44:10
+  --> $DIR/bad-assoc-ty.rs:45:10
    |
 LL | type I = ty!()::AssocTy;
    |          ^^^^^^^^^^^^^^ help: try: `<ty!()>::AssocTy`
 
 error: missing angle brackets in associated item path
-  --> $DIR/bad-assoc-ty.rs:37:19
+  --> $DIR/bad-assoc-ty.rs:38:19
    |
 LL |     ($ty: ty) => ($ty::AssocTy);
    |                   ^^^^^^^^^^^^ help: try: `<$ty>::AssocTy`
@@ -85,26 +85,32 @@ error[E0121]: the type placeholder `_` is not allowed within types on item signa
 LL | type E = _::AssocTy;
    |          ^ not allowed in type signatures
 
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/bad-assoc-ty.rs:17:10
+   |
+LL | type E = _::AssocTy;
+   |          ^ not allowed in type signatures
+
 error[E0223]: ambiguous associated type
-  --> $DIR/bad-assoc-ty.rs:21:19
+  --> $DIR/bad-assoc-ty.rs:22:19
    |
 LL | type F = &'static (u8)::AssocTy;
    |                   ^^^^^^^^^^^^^ help: use fully-qualified syntax: `<u8 as Trait>::AssocTy`
 
 error[E0223]: ambiguous associated type
-  --> $DIR/bad-assoc-ty.rs:27:10
+  --> $DIR/bad-assoc-ty.rs:28:10
    |
 LL | type G = dyn 'static + (Send)::AssocTy;
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<(dyn std::marker::Send + 'static) as Trait>::AssocTy`
 
 error[E0223]: ambiguous associated type
-  --> $DIR/bad-assoc-ty.rs:33:10
+  --> $DIR/bad-assoc-ty.rs:34:10
    |
 LL | type H = Fn(u8) -> (u8)::Output;
    |          ^^^^^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<(dyn std::ops::Fn(u8) -> u8 + 'static) as Trait>::Output`
 
 error[E0223]: ambiguous associated type
-  --> $DIR/bad-assoc-ty.rs:37:19
+  --> $DIR/bad-assoc-ty.rs:38:19
    |
 LL |     ($ty: ty) => ($ty::AssocTy);
    |                   ^^^^^^^^^^^^ help: use fully-qualified syntax: `<u8 as Trait>::AssocTy`
@@ -113,12 +119,12 @@ LL | type J = ty!(u8);
    |          ------- in this macro invocation
 
 error[E0223]: ambiguous associated type
-  --> $DIR/bad-assoc-ty.rs:44:10
+  --> $DIR/bad-assoc-ty.rs:45:10
    |
 LL | type I = ty!()::AssocTy;
    |          ^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<u8 as Trait>::AssocTy`
 
-error: aborting due to 19 previous errors
+error: aborting due to 20 previous errors
 
 Some errors have detailed explanations: E0121, E0223.
 For more information about an error, try `rustc --explain E0121`.

src/test/ui/self/self-infer.rs

@@ -2,7 +2,9 @@ struct S;
 
 impl S {
     fn f(self: _) {} //~ERROR the type placeholder `_` is not allowed within types on item sig
+    //~^ ERROR the type placeholder `_` is not allowed within types on item sig
     fn g(self: &_) {} //~ERROR the type placeholder `_` is not allowed within types on item sig
+    //~^ ERROR the type placeholder `_` is not allowed within types on item sig
 }
 
 fn main() {}

src/test/ui/self/self-infer.stderr

@@ -1,3 +1,9 @@
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/self-infer.rs:4:16
+   |
+LL |     fn f(self: _) {}
+   |                ^ not allowed in type signatures
+
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
   --> $DIR/self-infer.rs:4:16
    |
@@ -10,7 +16,13 @@ LL |     fn f<T>(self: T) {}
    |         ^^^       ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/self-infer.rs:5:17
+  --> $DIR/self-infer.rs:6:17
+   |
+LL |     fn g(self: &_) {}
+   |                 ^ not allowed in type signatures
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/self-infer.rs:6:17
    |
 LL |     fn g(self: &_) {}
    |                 ^ not allowed in type signatures
@@ -20,6 +32,6 @@ help: use type parameters instead
 LL |     fn g<T>(self: &T) {}
    |         ^^^        ^
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0121`.