Expand output and suggestions, fix tests

src/test/ui/feature-gate/issue-43106-gating-of-builtin-attrs.rs

@@ -309,20 +309,25 @@ mod bench {
 
 #[repr = "3900"]
 //~^ WARN unused attribute
+//~| WARN `repr` attribute isn't configurable with a literal
 mod repr {
     mod inner { #![repr="3900"] }
     //~^ WARN unused attribute
+    //~| WARN `repr` attribute isn't configurable with a literal
 
     #[repr = "3900"] fn f() { }
     //~^ WARN unused attribute
+    //~| WARN `repr` attribute isn't configurable with a literal
 
     struct S;
 
     #[repr = "3900"] type T = S;
     //~^ WARN unused attribute
+    //~| WARN `repr` attribute isn't configurable with a literal
 
     #[repr = "3900"] impl S { }
     //~^ WARN unused attribute
+    //~| WARN `repr` attribute isn't configurable with a literal
 }
 
 #[path = "3800"]

src/test/ui/suggestions/repr.rs

@@ -0,0 +1,28 @@
+// 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.
+
+// compile-pass
+
+#[repr]
+//^ WARN `repr` attribute must have a hint
+struct _A {}
+
+#[repr = "B"]
+//^ WARN `repr` attribute isn't configurable with a literal
+struct _B {}
+
+#[repr = "C"]
+//^ WARN `repr` attribute isn't configurable with a literal
+struct _C {}
+
+#[repr(C)]
+struct _D {}
+
+fn main() {}

src/test/ui/suggestions/repr.stderr

@@ -0,0 +1,24 @@
+warning: `repr` attribute must have a hint
+  --> $DIR/repr.rs:13:1
+   |
+LL | #[repr]
+   | ^^^^^^^ needs a hint
+   |
+   = help: valid hints include `#[repr(C)]`, `#[repr(packed)]` and `#[repr(rust)]`
+   = note: for more information, visit <https://doc.rust-lang.org/nomicon/other-reprs.html>
+
+warning: `repr` attribute isn't configurable with a literal
+  --> $DIR/repr.rs:17:1
+   |
+LL | #[repr = "B"]
+   | ^^^^^^^^^^^^^ needs a hint
+   |
+   = help: valid hints include `#[repr(C)]`, `#[repr(packed)]` and `#[repr(rust)]`
+   = note: for more information, visit <https://doc.rust-lang.org/nomicon/other-reprs.html>
+
+warning: `repr` attribute isn't configurable with a literal
+  --> $DIR/repr.rs:21:3
+   |
+LL | #[repr = "C"]
+   |   ^^^^^^^^^^ help: give `repr` a hint: `repr(C)`
+