Produce nice array lengths on a best effort basis

src/test/ui/did_you_mean/bad-assoc-ty.stderr

@@ -46,7 +46,7 @@ error[E0223]: ambiguous associated type
 LL | type A = [u8; 4]::AssocTy;
    |          ^^^^^^^^^^^^^^^^ ambiguous associated type
    |
-   = note: specify the type using the syntax `<[u8; <unevaluated[]>] as Trait>::AssocTy`
+   = note: specify the type using the syntax `<[u8; _] as Trait>::AssocTy`
 
 error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:15:10

src/test/ui/unevaluated_fixed_size_array_len.rs

@@ -0,0 +1,23 @@
+// Copyright 2018 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.
+
+// https://github.com/rust-lang/rust/issues/49208
+
+trait Foo {
+    fn foo();
+}
+
+impl Foo for [(); 1] {
+    fn foo() {}
+}
+
+fn main() {
+    <[(); 0] as Foo>::foo() //~ ERROR E0277
+}

src/test/ui/unevaluated_fixed_size_array_len.stderr

@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `[(); 0]: Foo` is not satisfied
+  --> $DIR/unevaluated_fixed_size_array_len.rs:22:5
+   |
+LL |     <[(); 0] as Foo>::foo() //~ ERROR E0277
+   |     ^^^^^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `[(); 0]`
+   |
+   = help: the following implementations were found:
+             <[(); 1] as Foo>
+note: required by `Foo::foo`
+  --> $DIR/unevaluated_fixed_size_array_len.rs:14:5
+   |
+LL |     fn foo();
+   |     ^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.