Allocate a new diagnostic for defaulted type parameters cannot use Self

(Without this commit, you still get an error (a very similar one, at
that), but it complains about use of forward declaration, which is
confusing since people do not necessarily think of `Self` as being
declared at all.)

Update: incorporate review feedback.

src/librustc_resolve/diagnostics.rs

@@ -354,6 +354,17 @@ impl<'a> Resolver<'a> {
                     span, "defaulted type parameters cannot be forward declared".to_string());
                 err
             }
+            ResolutionError::SelfInTyParamDefault => {
+                let mut err = struct_span_err!(
+                    self.session,
+                    span,
+                    E0735,
+                    "type parameters cannot use `Self` in their defaults"
+                );
+                err.span_label(
+                    span, "`Self` in type parameter default".to_string());
+                err
+            }
             ResolutionError::ConstParamDependentOnTypeParam => {
                 let mut err = struct_span_err!(
                     self.session,

src/librustc_resolve/error_codes.rs

@@ -8,9 +8,9 @@
 Erroneous code example:
 
 ```compile_fail,E0128
-struct Foo<T=U, U=()> {
+struct Foo<T = U, U = ()> {
     field1: T,
-    filed2: U,
+    field2: U,
 }
 // error: type parameters with a default cannot use forward declared
 // identifiers
@@ -20,9 +20,9 @@ struct Foo<T=U, U=()> {
 by doing:
 
 ```
-struct Foo<U=(), T=U> {
+struct Foo<U = (), T = U> {
     field1: T,
-    filed2: U,
+    field2: U,
 }
 ```
 
@@ -1705,6 +1705,21 @@ fn const_id<T, const N: T>() -> T { // error: const parameter
 }
 ```
 "##,
+
+E0735: r##"
+Type parameter defaults cannot use `Self` on structs, enums, or unions.
+
+Erroneous code example:
+
+```compile_fail,E0735
+struct Foo<X = Box<Self>> {
+    field1: Option<X>,
+    field2: Option<X>,
+}
+// error: type parameters cannot use `Self` in their defaults.
+```
+"##,
+
 ;
 //  E0153, unused error code
 //  E0157, unused error code

src/librustc_resolve/lib.rs

@@ -214,6 +214,8 @@ enum ResolutionError<'a> {
     BindingShadowsSomethingUnacceptable(&'a str, Name, &'a NameBinding<'a>),
     /// Error E0128: type parameters with a default cannot use forward-declared identifiers.
     ForwardDeclaredTyParam, // FIXME(const_generics:defaults)
+    /// Error E0735: type parameters with a default cannot use `Self`
+    SelfInTyParamDefault,
     /// Error E0671: const parameter cannot depend on type parameter.
     ConstParamDependentOnTypeParam,
 }
@@ -1536,7 +1538,7 @@ fn resolve_ident_in_lexical_scope(&mut self,
             if let Some(res) = ribs[i].bindings.get(&rib_ident).cloned() {
                 // The ident resolves to a type parameter or local variable.
                 return Some(LexicalScopeBinding::Res(
-                    self.validate_res_from_ribs(i, res, record_used, path_span, ribs),
+                    self.validate_res_from_ribs(i, rib_ident, res, record_used, path_span, ribs),
                 ));
             }
 
@@ -2122,6 +2124,7 @@ fn lint_if_path_starts_with_module(
     fn validate_res_from_ribs(
         &mut self,
         rib_index: usize,
+        rib_ident: Ident,
         res: Res,
         record_used: bool,
         span: Span,
@@ -2133,7 +2136,12 @@ fn validate_res_from_ribs(
         // An invalid forward use of a type parameter from a previous default.
         if let ForwardTyParamBanRibKind = all_ribs[rib_index].kind {
             if record_used {
-                self.report_error(span, ResolutionError::ForwardDeclaredTyParam);
+                let res_error = if rib_ident.name == kw::SelfUpper {
+                    ResolutionError::SelfInTyParamDefault
+                } else {
+                    ResolutionError::ForwardDeclaredTyParam
+                };
+                self.report_error(span, res_error);
             }
             assert_eq!(res, Res::Err);
             return Res::Err;