better error for binder on associated type bound

compiler/rustc_ast/src/ast.rs

@@ -2483,7 +2483,7 @@ pub struct TraitRef {
 
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct PolyTraitRef {
-    /// The `'a` in `<'a> Foo<&'a T>`.
+    /// The `'a` in `for<'a> Foo<&'a T>`.
     pub bound_generic_params: Vec<GenericParam>,
 
     /// The `Foo<&'a T>` in `<'a> Foo<&'a T>`.

compiler/rustc_parse/src/parser/path.rs

@@ -518,10 +518,20 @@ fn parse_angle_arg(
         match arg {
             Some(arg) => {
                 if self.check(&token::Colon) | self.check(&token::Eq) {
-                    let (ident, gen_args) = match self.get_ident_from_generic_arg(arg) {
+                    let arg_span = arg.span();
+                    let (binder, ident, gen_args) = match self.get_ident_from_generic_arg(&arg) {
                         Ok(ident_gen_args) => ident_gen_args,
-                        Err(arg) => return Ok(Some(AngleBracketedArg::Arg(arg))),
+                        Err(()) => return Ok(Some(AngleBracketedArg::Arg(arg))),
                     };
+                    if binder.is_some() {
+                        // FIXME(compiler-errors): this could be improved by suggesting lifting
+                        // this up to the trait, at least before this becomes real syntax.
+                        // e.g. `Trait<for<'a> Assoc = Ty>` -> `for<'a> Trait<Assoc = Ty>`
+                        return Err(self.struct_span_err(
+                            arg_span,
+                            "`for<...>` is not allowed on associated type bounds",
+                        ));
+                    }
                     let kind = if self.eat(&token::Colon) {
                         // Parse associated type constraint bound.
 
@@ -700,18 +710,32 @@ pub(super) fn parse_generic_arg(
         Ok(Some(arg))
     }
 
+    /// Given a arg inside of generics, we try to destructure it as if it were the LHS in
+    /// `LHS = ...`, i.e. an associated type binding.
+    /// This returns (optionally, if they are present) any `for<'a, 'b>` binder args, the
+    /// identifier, and any GAT arguments.
     fn get_ident_from_generic_arg(
         &self,
-        gen_arg: GenericArg,
-    ) -> Result<(Ident, Option<GenericArgs>), GenericArg> {
-        if let GenericArg::Type(ty) = &gen_arg
-            && let ast::TyKind::Path(qself, path) = &ty.kind
-            && qself.is_none()
-            && path.segments.len() == 1
-        {
-            let seg = &path.segments[0];
-            return Ok((seg.ident, seg.args.as_deref().cloned()));
+        gen_arg: &GenericArg,
+    ) -> Result<(Option<Vec<ast::GenericParam>>, Ident, Option<GenericArgs>), ()> {
+        if let GenericArg::Type(ty) = gen_arg {
+            if let ast::TyKind::Path(qself, path) = &ty.kind
+                && qself.is_none()
+                && let [seg] = path.segments.as_slice()
+            {
+                return Ok((None, seg.ident, seg.args.as_deref().cloned()));
+            } else if let ast::TyKind::TraitObject(bounds, ast::TraitObjectSyntax::None) = &ty.kind
+                && let [ast::GenericBound::Trait(trait_ref, ast::TraitBoundModifier::None)] =
+                    bounds.as_slice()
+                && let [seg] = trait_ref.trait_ref.path.segments.as_slice()
+            {
+                return Ok((
+                    Some(trait_ref.bound_generic_params.clone()),
+                    seg.ident,
+                    seg.args.as_deref().cloned(),
+                ));
+            }
         }
-        Err(gen_arg)
+        Err(())
     }
 }

src/test/ui/associated-type-bounds/binder-on-bound.rs

@@ -0,0 +1,11 @@
+#![feature(generic_associated_types)]
+
+trait Trait {
+    type Bound<'a>;
+}
+
+fn foo() where Trait<for<'a> Bound<'a> = &'a ()> {
+    //~^ ERROR `for<...>` is not allowed on associated type bounds
+}
+
+fn main() {}

src/test/ui/associated-type-bounds/binder-on-bound.stderr

@@ -0,0 +1,8 @@
+error: `for<...>` is not allowed on associated type bounds
+  --> $DIR/binder-on-bound.rs:7:22
+   |
+LL | fn foo() where Trait<for<'a> Bound<'a> = &'a ()> {
+   |                      ^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+