In JSON output, emit a directive after metadata is generated.

To implement pipelining, Cargo needs to know when metadata generation is
finished. This commit adds code to do that. Unfortunately, metadata file
writing currently occurs very late during compilation, so pipelining
won't produce a speed-up. Moving metadata file writing earlier will be a
follow-up.

The change involves splitting the existing `Emitter::emit` method in
two: `Emitter::emit_diagnostic` and `Emitter::emit_directive`.

The JSON directives look like this:
```
{"directive":"metadata file written: liba.rmeta"}
```
The functionality is behind the `-Z emit-directives` option, and also
requires `--error-format=json`.

1 files changed, 16 insertions, 6 deletions

diff --git a/src/librustc_errors/lib.rs b/src/librustc_errors/lib.rs
index 5d3861d9572..e173e1060cc 100644
--- a/src/librustc_errors/lib.rs
+++ b/src/librustc_errors/lib.rs
@@ -294,9 +294,16 @@ impl error::Error for ExplicitBug {
 pub use diagnostic::{Diagnostic, SubDiagnostic, DiagnosticStyledString, DiagnosticId};
 pub use diagnostic_builder::DiagnosticBuilder;
 
-/// A handler deals with errors; certain errors
-/// (fatal, bug, unimpl) may cause immediate exit,
-/// others log errors for later reporting.
+/// A handler deals with two kinds of compiler output.
+/// - Errors: certain errors (fatal, bug, unimpl) may cause immediate exit,
+///   others log errors for later reporting.
+/// - Directives: with --error-format=json, the compiler produces directives
+///   that indicate when certain actions have completed, which are useful for
+///   Cargo. They may change at any time and should not be considered a public
+///   API.
+///
+/// This crate's name (rustc_errors) doesn't encompass the directives, because
+/// directives were added much later.
 pub struct Handler {
     pub flags: HandlerFlags,
 
@@ -736,7 +743,7 @@ impl Handler {
     }
 
     pub fn force_print_db(&self, mut db: DiagnosticBuilder<'_>) {
-        self.emitter.borrow_mut().emit(&db);
+        self.emitter.borrow_mut().emit_diagnostic(&db);
         db.cancel();
     }
 
@@ -761,14 +768,17 @@ impl Handler {
         // Only emit the diagnostic if we haven't already emitted an equivalent
         // one:
         if self.emitted_diagnostics.borrow_mut().insert(diagnostic_hash) {
-            self.emitter.borrow_mut().emit(db);
+            self.emitter.borrow_mut().emit_diagnostic(db);
             if db.is_error() {
                 self.bump_err_count();
             }
         }
     }
-}
 
+    pub fn maybe_emit_json_directive(&self, directive: String) {
+        self.emitter.borrow_mut().maybe_emit_json_directive(directive);
+    }
+}
 
 #[derive(Copy, PartialEq, Clone, Hash, Debug, RustcEncodable, RustcDecodable)]
 pub enum Level {