Auto merge of #26275 - arielb1:unconstrained-projection, r=nikomatsakis

Fixes #26262

Because this rejects code that previously compiled, this is a [breaking-change]

r? @nikomatsakis

src/librustc_typeck/constrained_type_params.rs

@@ -19,10 +19,21 @@ pub enum Parameter {
     Region(ty::EarlyBoundRegion),
 }
 
+/// Returns the list of parameters that are constrained by the type `ty`
+/// - i.e. the value of each parameter in the list is uniquely determined
+/// by `ty` (see RFC 447).
 pub fn parameters_for_type<'tcx>(ty: Ty<'tcx>) -> Vec<Parameter> {
-    ty.walk()
-      .flat_map(|ty| parameters_for_type_shallow(ty))
-      .collect()
+    let mut result = vec![];
+    ty::maybe_walk_ty(ty, |t| {
+        if let ty::TyProjection(..) = t.sty {
+            false // projections are not injective.
+        } else {
+            result.append(&mut parameters_for_type_shallow(t));
+            // non-projection type constructors are injective.
+            true
+        }
+    });
+    result
 }
 
 pub fn parameters_for_trait_ref<'tcx>(trait_ref: &ty::TraitRef<'tcx>) -> Vec<Parameter> {

src/test/compile-fail/issue-26262.rs

@@ -0,0 +1,32 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Check that projections don't count as constraining type parameters.
+
+struct S<T>(T);
+
+trait Tr { type Assoc; fn test(); }
+
+impl<T: Tr> S<T::Assoc> {
+//~^ ERROR the type parameter `T` is not constrained
+    fn foo(self, _: T) {
+        T::test();
+    }
+}
+
+trait Trait1<T> { type Bar; }
+trait Trait2<'x> { type Foo; }
+
+impl<'a,T: Trait2<'a>> Trait1<<T as Trait2<'a>>::Foo> for T {
+//~^ ERROR the lifetime parameter `'a` is not constrained
+    type Bar = &'a ();
+}
+
+fn main() {}