Rollup merge of rust-lang#36114 - zjhmale:fix-E0393, r=jonathandturner

Update E0393 to new error format

Fixes rust-lang#35632.
Part of rust-lang#35233.

r? @jonathandturner

and a wired thing is that if i add another label

```rust
.span_label(span, &format!("missing reference to `{}`", def.name))
.span_label(span, &format!("because of the default `Self` reference, type parameters must be specified on object types"))
```

and add a new note in the test case like

```rust
trait A<T=Self> {}

fn together_we_will_rule_the_galaxy(son: &A) {}
//~^ ERROR E0393
//~| NOTE missing reference to `T`
//~| NOTE because of the default `Self` reference, type parameters must be specified on object types
```

it will complain that

```
running 1 test
test [compile-fail] compile-fail/E0393.rs ... FAILED

failures:

---- [compile-fail] compile-fail/E0393.rs stdout ----

error: /Users/zjh/Documents/rustspace/rust/src/test/compile-fail/E0393.rs:13: unexpected "error": '13:43: 13:44: the type parameter `T` must be explicitly specified [E0393]'

unexpected errors (from JSON output): [
    Error {
        line_num: 13,
        kind: Some(
            Error
        ),
        msg: "13:43: 13:44: the type parameter `T` must be explicitly specified [E0393]"
    }
]
```

it is a little bit confusing and through the blog post we can use `//~^` and `//~|` to support multiple notes, @jonathandturner am i missing something here?

src/librustc_typeck/astconv.rs

@@ -515,12 +515,13 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
                 // defaults. This will lead to an ICE if we are not
                 // careful!
                 if default_needs_object_self(def) {
-                    span_err!(tcx.sess, span, E0393,
-                              "the type parameter `{}` must be explicitly specified \
-                               in an object type because its default value `{}` references \
-                               the type `Self`",
-                              def.name,
-                              default);
+                    struct_span_err!(tcx.sess, span, E0393,
+                                     "the type parameter `{}` must be explicitly specified",
+                                     def.name)
+                        .span_label(span, &format!("missing reference to `{}`", def.name))
+                        .note(&format!("because of the default `Self` reference, \
+                                        type parameters must be specified on object types"))
+                        .emit();
                     tcx.types.err
                 } else {
                     // This is a default type parameter.

src/test/compile-fail/E0393.rs

@@ -10,7 +10,10 @@
 
 trait A<T=Self> {}
 
-fn together_we_will_rule_the_galaxy(son: &A) {} //~ ERROR E0393
+fn together_we_will_rule_the_galaxy(son: &A) {}
+//~^ ERROR E0393
+//~| NOTE missing reference to `T`
+//~| NOTE because of the default `Self` reference, type parameters must be specified on object types
 
 fn main() {
 }