From 4acf3baecda9c5080bcfdc1ebb4560fbb2a4567e Mon Sep 17 00:00:00 2001
From: Martin Pool <mbp@sourcefrog.net>
Date: Fri, 20 Jun 2025 13:18:05 -0700
Subject: [PATCH 1/2] libtest: expose --fail-fast

---
 library/test/src/cli.rs          |  4 +++-
 src/doc/rustc/src/tests/index.md | 10 ++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/library/test/src/cli.rs b/library/test/src/cli.rs
index 8840714a66238..d19141b8c2c73 100644
--- a/library/test/src/cli.rs
+++ b/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(
@@ -270,6 +271,7 @@ fn parse_opts_impl(matches: getopts::Matches) -> OptRes {
     let exact = matches.opt_present("exact");
     let list = matches.opt_present("list");
     let skip = matches.opt_strs("skip");
+    let fail_fast = matches.opt_present("fail-fast");
 
     let bench_benchmarks = matches.opt_present("bench");
     let run_tests = !bench_benchmarks || matches.opt_present("test");
@@ -307,7 +309,7 @@ fn parse_opts_impl(matches: getopts::Matches) -> OptRes {
         skip,
         time_options,
         options,
-        fail_fast: false,
+        fail_fast,
     };
 
     Ok(test_opts)
diff --git a/src/doc/rustc/src/tests/index.md b/src/doc/rustc/src/tests/index.md
index 12de69a4c9eec..7033184e52aa9 100644
--- a/src/doc/rustc/src/tests/index.md
+++ b/src/doc/rustc/src/tests/index.md
@@ -158,6 +158,16 @@ 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.
+
 #### `--test-threads` _NUM_THREADS_
 
 Sets the number of threads to use for running tests in parallel. By default,

From 547c72901ad145b40d7271e3ddef2b4ac0c53619 Mon Sep 17 00:00:00 2001
From: Martin Pool <mbp@sourcefrog.net>
Date: Fri, 20 Jun 2025 13:27:53 -0700
Subject: [PATCH 2/2] Make fail-fast unstable

---
 library/test/src/cli.rs          | 2 +-
 src/doc/rustc/src/tests/index.md | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/library/test/src/cli.rs b/library/test/src/cli.rs
index d19141b8c2c73..ea91ebc4e9875 100644
--- a/library/test/src/cli.rs
+++ b/library/test/src/cli.rs
@@ -262,6 +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)?;
@@ -271,7 +272,6 @@ fn parse_opts_impl(matches: getopts::Matches) -> OptRes {
     let exact = matches.opt_present("exact");
     let list = matches.opt_present("list");
     let skip = matches.opt_strs("skip");
-    let fail_fast = matches.opt_present("fail-fast");
 
     let bench_benchmarks = matches.opt_present("bench");
     let run_tests = !bench_benchmarks || matches.opt_present("test");
diff --git a/src/doc/rustc/src/tests/index.md b/src/doc/rustc/src/tests/index.md
index 7033184e52aa9..ab96fbcd3178e 100644
--- a/src/doc/rustc/src/tests/index.md
+++ b/src/doc/rustc/src/tests/index.md
@@ -168,6 +168,8 @@ 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,