Add --test-threads option to test binaries

This change allows parallelism of test runs to be specified by a
command line flag names --test-threads in addition to the existing
environment variable RUST_TEST_THREADS. Fixes #25636.

3 files changed, 26 insertions, 5 deletions

diff --git a/man/rustc.1 b/man/rustc.1
index fa61afd3be5..95650255c5d 100644
--- a/man/rustc.1
+++ b/man/rustc.1
@@ -264,7 +264,8 @@ which link to the standard library.
 .TP
 \fBRUST_TEST_THREADS\fR
 The test framework Rust provides executes tests in parallel. This variable sets
-the maximum number of threads used for this purpose.
+the maximum number of threads used for this purpose. This setting is overridden
+by the --test-threads option.
 
 .TP
 \fBRUST_TEST_NOCAPTURE\fR
diff --git a/src/libtest/lib.rs b/src/libtest/lib.rs
index 248f6f98650..710df24525e 100644
--- a/src/libtest/lib.rs
+++ b/src/libtest/lib.rs
@@ -300,6 +300,7 @@ pub struct TestOpts {
     pub nocapture: bool,
     pub color: ColorConfig,
     pub quiet: bool,
+    pub test_threads: Option<usize>,
 }
 
 impl TestOpts {
@@ -314,6 +315,7 @@ impl TestOpts {
             nocapture: false,
             color: AutoColor,
             quiet: false,
+            test_threads: None,
         }
     }
 }
@@ -331,6 +333,8 @@ fn optgroups() -> Vec<getopts::OptGroup> {
                           of stdout", "PATH"),
       getopts::optflag("", "nocapture", "don't capture stdout/stderr of each \
                                          task, allow printing directly"),
+      getopts::optopt("", "test-threads", "Number of threads used for running tests \
+                                           in parallel", "n_threads"),
       getopts::optflag("q", "quiet", "Display one character per test instead of one line"),
       getopts::optopt("", "color", "Configure coloring of output:
             auto   = colorize if stdout is a tty and tests are run on serially (default);
@@ -346,7 +350,8 @@ The FILTER string is tested against the name of all tests, and only those
 tests whose names contain the filter are run.
 
 By default, all tests are run in parallel. This can be altered with the
-RUST_TEST_THREADS environment variable when running tests (set it to 1).
+--test-threads flag or the RUST_TEST_THREADS environment variable when running
+tests (set it to 1).
 
 All tests have their standard output and standard error captured by default.
 This can be overridden with the --nocapture flag or setting RUST_TEST_NOCAPTURE
@@ -405,6 +410,18 @@ pub fn parse_opts(args: &[String]) -> Option<OptRes> {
         };
     }
 
+    let test_threads = match matches.opt_str("test-threads") {
+        Some(n_str) =>
+            match n_str.parse::<usize>() {
+                Ok(n) => Some(n),
+                Err(e) =>
+                    return Some(Err(format!("argument for --test-threads must be a number > 0 \
+                                             (error: {})", e)))
+            },
+        None =>
+            None,
+    };
+
     let color = match matches.opt_str("color").as_ref().map(|s| &**s) {
         Some("auto") | None => AutoColor,
         Some("always") => AlwaysColor,
@@ -426,6 +443,7 @@ pub fn parse_opts(args: &[String]) -> Option<OptRes> {
         nocapture: nocapture,
         color: color,
         quiet: quiet,
+        test_threads: test_threads,
     };
 
     Some(Ok(test_opts))
@@ -857,9 +875,10 @@ fn run_tests<F>(opts: &TestOpts, tests: Vec<TestDescAndFn>, mut callback: F) ->
             }
         });
 
-    // It's tempting to just spawn all the tests at once, but since we have
-    // many tests that run in other processes we would be making a big mess.
-    let concurrency = get_concurrency();
+    let concurrency = match opts.test_threads {
+        Some(n) => n,
+        None => get_concurrency(),
+    };
 
     let mut remaining = filtered_tests;
     remaining.reverse();
diff --git a/src/tools/compiletest/src/main.rs b/src/tools/compiletest/src/main.rs
index cefcc11486f..90641b5c476 100644
--- a/src/tools/compiletest/src/main.rs
+++ b/src/tools/compiletest/src/main.rs
@@ -310,6 +310,7 @@ pub fn test_opts(config: &Config) -> test::TestOpts {
             Err(_) => false
         },
         color: test::AutoColor,
+        test_threads: None,
     }
 }