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

src/librustc/session/config.rs

@@ -1468,6 +1468,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
          the same values as the target option of the same name"),
     allow_features: Option<Vec<String>> = (None, parse_opt_comma_list, [TRACKED],
         "only allow the listed language features to be enabled in code (space separated)"),
+    emit_directives: bool = (false, parse_bool, [UNTRACKED],
+        "emit build directives if producing JSON output"),
 }
 
 pub fn default_lib_output() -> CrateType {

src/librustc/session/mod.rs

@@ -1078,8 +1078,7 @@ fn default_emitter(
 
 pub enum DiagnosticOutput {
     Default,
-    Raw(Box<dyn Write + Send>),
-    Emitter(Box<dyn Emitter + Send + sync::Send>)
+    Raw(Box<dyn Write + Send>)
 }
 
 pub fn build_session_with_source_map(
@@ -1115,7 +1114,6 @@ pub fn build_session_with_source_map(
         DiagnosticOutput::Raw(write) => {
             default_emitter(&sopts, registry, &source_map, Some(write))
         }
-        DiagnosticOutput::Emitter(emitter) => emitter,
     };
 
     let diagnostic_handler = errors::Handler::with_emitter_and_flags(

src/librustc_codegen_llvm/lib.rs

@@ -300,7 +300,7 @@ impl CodegenBackend for LlvmCodegenBackend {
         sess: &Session,
         dep_graph: &DepGraph,
         outputs: &OutputFilenames,
-    ) -> Result<(), ErrorReported>{
+    ) -> Result<(), ErrorReported> {
         use rustc::util::common::time;
         let (codegen_results, work_products) =
             ongoing_codegen.downcast::

src/librustc_codegen_ssa/back/link.rs

@@ -46,11 +46,10 @@ pub fn remove(sess: &Session, path: &Path) {
 /// Performs the linkage portion of the compilation phase. This will generate all
 /// of the requested outputs for this compilation session.
 pub fn link_binary<'a, B: ArchiveBuilder<'a>>(sess: &'a Session,
-                          codegen_results: &CodegenResults,
-                          outputs: &OutputFilenames,
-                          crate_name: &str,
-                          target_cpu: &str) -> Vec<PathBuf> {
-    let mut out_filenames = Vec::new();
+                                              codegen_results: &CodegenResults,
+                                              outputs: &OutputFilenames,
+                                              crate_name: &str,
+                                              target_cpu: &str) {
     for &crate_type in sess.crate_types.borrow().iter() {
         // Ignore executable crates if we have -Z no-codegen, as they will error.
         let output_metadata = sess.opts.output_types.contains_key(&OutputType::Metadata);
@@ -64,13 +63,12 @@ pub fn link_binary<'a, B: ArchiveBuilder<'a>>(sess: &'a Session,
            bug!("invalid output type `{:?}` for target os `{}`",
                 crate_type, sess.opts.target_triple);
         }
-        let out_files = link_binary_output::<B>(sess,
-                                           codegen_results,
-                                           crate_type,
-                                           outputs,
-                                           crate_name,
-                                           target_cpu);
-        out_filenames.extend(out_files);
+        link_binary_output::<B>(sess,
+                                codegen_results,
+                                crate_type,
+                                outputs,
+                                crate_name,
+                                target_cpu);
     }
 
     // Remove the temporary object file and metadata if we aren't saving temps
@@ -97,22 +95,18 @@ pub fn link_binary<'a, B: ArchiveBuilder<'a>>(sess: &'a Session,
             }
         }
     }
-
-    out_filenames
 }
 
 fn link_binary_output<'a, B: ArchiveBuilder<'a>>(sess: &'a Session,
-                      codegen_results: &CodegenResults,
-                      crate_type: config::CrateType,
-                      outputs: &OutputFilenames,
-                      crate_name: &str,
-                      target_cpu: &str) -> Vec<PathBuf> {
+                                                 codegen_results: &CodegenResults,
+                                                 crate_type: config::CrateType,
+                                                 outputs: &OutputFilenames,
+                                                 crate_name: &str,
+                                                 target_cpu: &str) {
     for obj in codegen_results.modules.iter().filter_map(|m| m.object.as_ref()) {
         check_file_is_writeable(obj, sess);
     }
 
-    let mut out_filenames = vec![];
-
     if outputs.outputs.contains_key(&OutputType::Metadata) {
         let out_filename = filename_for_metadata(sess, crate_name, outputs);
         // To avoid races with another rustc process scanning the output directory,
@@ -125,10 +119,15 @@ fn link_binary_output<'a, B: ArchiveBuilder<'a>>(sess: &'a Session,
             .tempdir_in(out_filename.parent().unwrap())
             .unwrap_or_else(|err| sess.fatal(&format!("couldn't create a temp dir: {}", err)));
         let metadata = emit_metadata(sess, codegen_results, &metadata_tmpdir);
-        if let Err(e) = fs::rename(metadata, &out_filename) {
-            sess.fatal(&format!("failed to write {}: {}", out_filename.display(), e));
+        match fs::rename(&metadata, &out_filename) {
+            Ok(_) => {
+                if sess.opts.debugging_opts.emit_directives {
+                    sess.parse_sess.span_diagnostic.maybe_emit_json_directive(
+                        format!("metadata file written: {}", out_filename.display()));
+                }
+            }
+            Err(e) => sess.fatal(&format!("failed to write {}: {}", out_filename.display(), e)),
         }
-        out_filenames.push(out_filename);
     }
 
     let tmpdir = TempFileBuilder::new().prefix("rustc").tempdir().unwrap_or_else(|err|
@@ -158,14 +157,11 @@ fn link_binary_output<'a, B: ArchiveBuilder<'a>>(sess: &'a Session,
                 );
             }
         }
-        out_filenames.push(out_filename);
     }
 
     if sess.opts.cg.save_temps {
         let _ = tmpdir.into_path();
     }
-
-    out_filenames
 }
 
 // The third parameter is for env vars, used on windows to set up the

src/librustc_codegen_ssa/back/write.rs

@@ -1726,7 +1726,7 @@ impl SharedEmitter {
 }
 
 impl Emitter for SharedEmitter {
-    fn emit(&mut self, db: &DiagnosticBuilder<'_>) {
+    fn emit_diagnostic(&mut self, db: &DiagnosticBuilder<'_>) {
         drop(self.sender.send(SharedEmitterMessage::Diagnostic(Diagnostic {
             msg: db.message(),
             code: db.code.clone(),
@@ -1865,7 +1865,7 @@ impl<B: ExtraBackendMethods> OngoingCodegen<B> {
         self.wait_for_signal_to_codegen_item();
         self.check_for_errors(tcx.sess);
 
-        // These are generally cheap and won't through off scheduling.
+        // These are generally cheap and won't throw off scheduling.
         let cost = 0;
         submit_codegened_module_to_llvm(&self.backend, tcx, module, cost);
     }

src/librustc_errors/emitter.rs

@@ -50,7 +50,11 @@ const ANONYMIZED_LINE_NUM: &str = "LL";
 /// Emitter trait for emitting errors.
 pub trait Emitter {
     /// Emit a structured diagnostic.
-    fn emit(&mut self, db: &DiagnosticBuilder<'_>);
+    fn emit_diagnostic(&mut self, db: &DiagnosticBuilder<'_>);
+
+    /// Emit a JSON directive. The default is to do nothing; this should only
+    /// be emitted with --error-format=json.
+    fn maybe_emit_json_directive(&mut self, _directive: String) {}
 
     /// Checks if should show explanations about "rustc --explain"
     fn should_show_explain(&self) -> bool {
@@ -59,7 +63,7 @@ pub trait Emitter {
 }
 
 impl Emitter for EmitterWriter {
-    fn emit(&mut self, db: &DiagnosticBuilder<'_>) {
+    fn emit_diagnostic(&mut self, db: &DiagnosticBuilder<'_>) {
         let mut primary_span = db.span.clone();
         let mut children = db.children.clone();
         let mut suggestions: &[_] = &[];

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 {

src/libsyntax/json.rs

@@ -79,7 +79,7 @@ impl JsonEmitter {
 }
 
 impl Emitter for JsonEmitter {
-    fn emit(&mut self, db: &DiagnosticBuilder<'_>) {
+    fn emit_diagnostic(&mut self, db: &DiagnosticBuilder<'_>) {
         let data = Diagnostic::from_diagnostic_builder(db, self);
         let result = if self.pretty {
             writeln!(&mut self.dst, "{}", as_pretty_json(&data))
@@ -90,6 +90,18 @@ impl Emitter for JsonEmitter {
             panic!("failed to print diagnostics: {:?}", e);
         }
     }
+
+    fn maybe_emit_json_directive(&mut self, directive: String) {
+        let data = Directive { directive };
+        let result = if self.pretty {
+            writeln!(&mut self.dst, "{}", as_pretty_json(&data))
+        } else {
+            writeln!(&mut self.dst, "{}", as_json(&data))
+        };
+        if let Err(e) = result {
+            panic!("failed to print message: {:?}", e);
+        }
+    }
 }
 
 // The following data types are provided just for serialisation.
@@ -168,6 +180,12 @@ struct DiagnosticCode {
     explanation: Option<&'static str>,
 }
 
+#[derive(RustcEncodable)]
+struct Directive {
+    /// The directive itself.
+    directive: String,
+}
+
 impl Diagnostic {
     fn from_diagnostic_builder(db: &DiagnosticBuilder<'_>,
                                je: &JsonEmitter)
@@ -200,7 +218,7 @@ impl Diagnostic {
         let buf = BufWriter::default();
         let output = buf.clone();
         je.json_rendered.new_emitter(Box::new(buf), Some(je.sm.clone()), false)
-            .ui_testing(je.ui_testing).emit(db);
+            .ui_testing(je.ui_testing).emit_diagnostic(db);
         let output = Arc::try_unwrap(output.0).unwrap().into_inner().unwrap();
         let output = String::from_utf8(output).unwrap();
 

src/test/ui/emit-directives.rs

@@ -0,0 +1,12 @@
+// ignore-tidy-linelength
+// compile-flags:--emit=metadata --error-format=json -Z emit-directives
+// compile-pass
+//
+// Normalization is required to eliminated minor path and filename differences
+// across platforms.
+// normalize-stderr-test: "metadata file written: .*/emit-directives" -> "metadata file written: .../emit-directives"
+// normalize-stderr-test: "emit-directives(\.\w*)?/a(\.\w*)?" -> "emit-directives/a"
+
+// A very basic test for the emission of build directives in JSON output.
+
+fn main() {}

src/test/ui/emit-directives.stderr

@@ -0,0 +1 @@
+{"directive":"metadata file written: .../emit-directives/a"}

src/tools/compiletest/src/json.rs

@@ -17,6 +17,12 @@ struct Diagnostic {
     rendered: Option<String>,
 }
 
+#[derive(Deserialize)]
+struct Directive {
+    #[allow(dead_code)]
+    directive: String,
+}
+
 #[derive(Deserialize, Clone)]
 struct DiagnosticSpan {
     file_name: String,
@@ -67,16 +73,17 @@ pub fn extract_rendered(output: &str) -> String {
         .lines()
         .filter_map(|line| {
             if line.starts_with('{') {
-                match serde_json::from_str::<Diagnostic>(line) {
-                    Ok(diagnostic) => diagnostic.rendered,
-                    Err(error) => {
-                        print!(
-                            "failed to decode compiler output as json: \
-                             `{}`\nline: {}\noutput: {}",
-                            error, line, output
-                        );
-                        panic!()
-                    }
+                if let Ok(diagnostic) = serde_json::from_str::<Diagnostic>(line) {
+                    diagnostic.rendered
+                } else if let Ok(_directive) = serde_json::from_str::<Directive>(line) {
+                    // Swallow the directive.
+                    None
+                } else {
+                    print!(
+                        "failed to decode compiler output as json: line: {}\noutput: {}",
+                        line, output
+                    );
+                    panic!()
                 }
             } else {
                 // preserve non-JSON lines, such as ICEs