auto merge of #15336 : jakub-/rust/diagnostics, r=brson

This is a continuation of @brson's work from https://github.com/rust-lang/rust/pull/12144.

This implements the minimal scaffolding that allows mapping diagnostic messages to alpha-numeric codes, which could improve the searchability of errors. In addition, there's a new compiler option, `--explain {code}` which takes an error code and prints out a somewhat detailed explanation of the error. Example:

```rust
fn f(x: Option<bool>) {
	match x {
		Some(true) | Some(false) => (),
		None => (),
		Some(true) => ()
	}
}
```

```shell
[~/rust]$ ./build/x86_64-apple-darwin/stage2/bin/rustc ./diagnostics.rs --crate-type dylib
diagnostics.rs:5:3: 5:13 error: unreachable pattern [E0001] (pass `--explain E0001` to see a detailed explanation)
diagnostics.rs:5 		Some(true) => ()
                 		^~~~~~~~~~
error: aborting due to previous error
[~/rust]$ ./build/x86_64-apple-darwin/stage2/bin/rustc --explain E0001

    This error suggests that the expression arm corresponding to the noted pattern
    will never be reached as for all possible values of the expression being matched,
    one of the preceeding patterns will match.

    This means that perhaps some of the preceeding patterns are too general, this
    one is too specific or the ordering is incorrect.

```

I've refrained from migrating many errors to actually use the new macros as it can be done in an incremental fashion but if we're happy with the approach, it'd be good to do all of them sooner rather than later.

Originally, I was going to make libdiagnostics a separate crate but that's posing some interesting challenges with semi-circular dependencies. In particular, librustc would have a plugin-phase dependency on libdiagnostics, which itself depends on librustc. Per my conversation with @alexcrichton, it seems like the snapshotting process would also have to change. So for now the relevant modules from libdiagnostics are included using `#[path = ...] mod`.

src/test/compile-fail/issue-11692.rs

@@ -10,9 +10,9 @@
 
 fn main() {
     print!(test!());
-    //~^ ERROR: macro undefined: 'test'
+    //~^ ERROR: macro undefined: 'test!'
     //~^^ ERROR: format argument must be a string literal
 
     concat!(test!());
-    //~^ ERROR: macro undefined: 'test'
+    //~^ ERROR: macro undefined: 'test!'
 }

src/test/compile-fail/rustc-diagnostics-1.rs

@@ -0,0 +1,28 @@
+// Copyright 2014 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.
+
+#![feature(rustc_diagnostic_macros)]
+
+__register_diagnostic!(E0001)
+__register_diagnostic!(E0003)
+
+fn main() {
+    __diagnostic_used!(E0002);
+    //~^ ERROR unknown diagnostic code E0002
+
+    __diagnostic_used!(E0001);
+    //~^ NOTE previous invocation
+
+    __diagnostic_used!(E0001);
+    //~^ WARNING diagnostic code E0001 already used
+}
+
+__build_diagnostic_array!(DIAGNOSTICS)
+//~^ WARN diagnostic code E0003 never used

src/test/compile-fail/rustc-diagnostics-2.rs

@@ -0,0 +1,20 @@
+// Copyright 2014 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.
+
+#![feature(rustc_diagnostic_macros)]
+
+__register_diagnostic!(E0001)
+__register_diagnostic!(E0001)
+//~^ ERROR diagnostic code E0001 already registered
+
+fn main() {
+}
+
+__build_diagnostic_array!(DIAGNOSTICS)

src/test/compile-fail/rustc-diagnostics-3.rs

@@ -0,0 +1,20 @@
+// Copyright 2014 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.
+
+__register_diagnostic!(E0001)
+//~^ ERROR macro undefined: '__register_diagnostic!'
+
+fn main() {
+    __diagnostic_used!(E0001);
+    //~^ ERROR macro undefined: '__diagnostic_used!'
+}
+
+__build_diagnostic_array!(DIAGNOSTICS)
+//~^ ERROR macro undefined: '__build_diagnostic_array!'