Add "build-finished" JSON message.

src/cargo/core/compiler/job_queue.rs

@@ -70,12 +70,11 @@ use super::job::{
 use super::timings::Timings;
 use super::{BuildContext, BuildPlan, CompileMode, Context, Unit};
 use crate::core::{PackageId, TargetKind};
-use crate::util;
 use crate::util::diagnostic_server::{self, DiagnosticPrinter};
-use crate::util::Queue;
-use crate::util::{internal, profile, CargoResult, CargoResultExt, ProcessBuilder};
-use crate::util::{Config, DependencyQueue};
-use crate::util::{Progress, ProgressStyle};
+use crate::util::machine_message::{self, Message as _};
+use crate::util::{self, internal, profile};
+use crate::util::{CargoResult, CargoResultExt, ProcessBuilder};
+use crate::util::{Config, DependencyQueue, Progress, ProgressStyle, Queue};
 
 /// This structure is backed by the `DependencyQueue` type and manages the
 /// queueing of compilation steps for each package. Packages enqueue units of
@@ -701,6 +700,13 @@ impl<'a, 'cfg> DrainState<'a, 'cfg> {
 
         let time_elapsed = util::elapsed(cx.bcx.config.creation_time().elapsed());
         self.timings.finished(cx.bcx, &error)?;
+        if cx.bcx.build_config.emit_json() {
+            let msg = machine_message::BuildFinished {
+                success: error.is_none(),
+            }
+            .to_json_string();
+            cx.bcx.config.shell().stdout_println(msg);
+        }
 
         if let Some(e) = error {
             Err(e)

src/cargo/util/machine_message.rs

@@ -90,3 +90,14 @@ impl<'a> Message for TimingInfo<'a> {
         "timing-info"
     }
 }
+
+#[derive(Serialize)]
+pub struct BuildFinished {
+    pub success: bool,
+}
+
+impl Message for BuildFinished {
+    fn reason(&self) -> &str {
+        "build-finished"
+    }
+}

src/doc/src/reference/external-tools.md

@@ -217,6 +217,30 @@ may be found in [the chapter on build scripts](build-scripts.md).
 }
 ```
 
+#### Build finished
+
+The "build-finished" message is emitted at the end of the build.
+
+```javascript
+{
+    /* The "reason" indicates the kind of message. */
+    "reason": "build-finished",
+    /* Whether or not the build finished successfully. */
+    "success": true,
+}
+````
+
+This message can be helpful for tools to know when to stop reading JSON
+messages. Commands such as `cargo test` or `cargo run` can produce additional
+output after the build has finished. This message lets a tool know that Cargo
+will not produce additional JSON messages, but there may be additional output
+that may be generated afterwards (such as the output generated by the program
+executed by `cargo run`).
+
+> Note: There is experimental nightly-only support for JSON output for tests,
+> so additional test-specific JSON messages may begin arriving after the
+> "build-finished" message if that is enabled.
+
 ### Custom subcommands
 
 Cargo is designed to be extensible with new subcommands without having to modify

tests/testsuite/bench.rs

@@ -1628,6 +1628,8 @@ fn json_artifact_includes_executable_for_benchmark() {
                     "src_path": "[..]/foo/benches/benchmark.rs"
                 }
             }
+
+            {"reason": "build-finished", "success": true}
         "#,
         )
         .run();

tests/testsuite/build.rs

@@ -3169,6 +3169,8 @@ fn compiler_json_error_format() {
         "filenames": "{...}",
         "fresh": $FRESH
     }
+
+    {"reason": "build-finished", "success": true}
 "#
         .replace("$FRESH", fresh)
     };
@@ -3249,6 +3251,8 @@ fn message_format_json_forward_stderr() {
         "filenames": "{...}",
         "fresh": false
     }
+
+    {"reason": "build-finished", "success": true}
 "#,
         )
         .run();

tests/testsuite/message_format.rs

@@ -64,7 +64,10 @@ fn cargo_renders() {
 
     p.cargo("build --message-format json-render-diagnostics")
         .with_status(101)
-        .with_stdout("{\"reason\":\"compiler-artifact\",[..]")
+        .with_stdout(
+            "{\"reason\":\"compiler-artifact\",[..]\n\
+             {\"reason\":\"build-finished\",\"success\":false}",
+        )
         .with_stderr_contains(
             "\
 [COMPILING] bar [..]

tests/testsuite/test.rs

@@ -3363,6 +3363,8 @@ fn json_artifact_includes_test_flag() {
         "filenames":["[..]/foo-[..]"],
         "fresh": false
     }
+
+    {"reason": "build-finished", "success": true}
 "#,
         )
         .run();
@@ -3395,6 +3397,8 @@ fn json_artifact_includes_executable_for_library_tests() {
                     "src_path": "[..]/foo/src/lib.rs"
                 }
             }
+
+            {"reason": "build-finished", "success": true}
         "#,
         )
         .run();
@@ -3429,6 +3433,8 @@ fn json_artifact_includes_executable_for_integration_tests() {
                     "src_path": "[..]/foo/tests/integration_test.rs"
                 }
             }
+
+            {"reason": "build-finished", "success": true}
         "#,
         )
         .run();