Probe for candidates on based on the actual receiver type

Impl blocks Self type is a TypeNoBouns which means it can be for types
such as: impl<T> &T {}.

I think we might need to change the probe algorithm for method calls to be
fully based on the autoderef rather than trying to filter based on the Self
type. More investigation is needed for the probe phase here.

Fixes #808

gcc/rust/typecheck/rust-hir-type-check-expr.h

@@ -265,8 +265,10 @@ class TypeCheckExpr : public TypeCheckBase
     bool probe_impls = !receiver_is_generic;
     bool ignore_mandatory_trait_items = !receiver_is_generic;
 
+    auto probe_type = probe_impls ? receiver_tyty : root;
     auto candidates
-      = PathProbeType::Probe (root, expr.get_method_name ().get_segment (),
+      = PathProbeType::Probe (probe_type,
+			      expr.get_method_name ().get_segment (),
 			      probe_impls, probe_bounds,
 			      ignore_mandatory_trait_items);
     if (candidates.empty ())

gcc/testsuite/rust/compile/torture/issue-808.rs

@@ -0,0 +1,23 @@
+pub trait Foo {
+    type Target;
+    // { dg-warning "unused name" "" { target *-*-* } .-1 }
+
+    fn bar(&self) -> &Self::Target;
+    // { dg-warning "unused name .self." "" { target *-*-* } .-1 }
+    // { dg-warning "unused name .Foo::bar." "" { target *-*-* } .-2 }
+}
+
+impl<T> Foo for &T {
+    type Target = T;
+
+    fn bar(&self) -> &T {
+        *self
+    }
+}
+
+pub fn main() {
+    let a: i32 = 123;
+    let b: &i32 = &a;
+
+    b.bar();
+}