libtest: expose --fail-fast as an unstable command-line option

library/test/src/cli.rs

@@ -57,6 +57,7 @@ fn optgroups() -> getopts::Options {
         .optflag("", "test", "Run tests and not benchmarks")
         .optflag("", "bench", "Run benchmarks instead of tests")
         .optflag("", "list", "List all tests and benchmarks")
+        .optflag("", "fail-fast", "Don't start new tests after the first failure")
         .optflag("h", "help", "Display this message")
         .optopt("", "logfile", "Write logs to the specified file (deprecated)", "PATH")
         .optflag(
@@ -261,6 +262,7 @@ fn parse_opts_impl(matches: getopts::Matches) -> OptRes {
     // Unstable flags
     let force_run_in_process = unstable_optflag!(matches, allow_unstable, "force-run-in-process");
     let exclude_should_panic = unstable_optflag!(matches, allow_unstable, "exclude-should-panic");
+    let fail_fast = unstable_optflag!(matches, allow_unstable, "fail-fast");
     let time_options = get_time_options(&matches, allow_unstable)?;
     let shuffle = get_shuffle(&matches, allow_unstable)?;
     let shuffle_seed = get_shuffle_seed(&matches, allow_unstable)?;
@@ -307,7 +309,7 @@ fn parse_opts_impl(matches: getopts::Matches) -> OptRes {
         skip,
         time_options,
         options,
-        fail_fast: false,
+        fail_fast,
     };
 
     Ok(test_opts)

src/doc/rustc/src/tests/index.md

@@ -158,6 +158,18 @@ unstable-options` flag. See [tracking issue
 
 The following options affect how tests are executed.
 
+#### `--fail-fast`
+
+Stops tests after the first failure.
+
+If running tests in parallel (which is the default), then tests that have already been started on
+other threads will be allowed to run to completion before the process exits.
+
+Note that when running tests in parallel, the test execution order is non-deterministic:
+if multiple tests would fail, the first failure encountered will be reported.
+
+⚠️ 🚧 This requires the `-Z unstable-options` flag.
+
 #### `--test-threads` _NUM_THREADS_
 
 Sets the number of threads to use for running tests in parallel. By default,