Test headers

Header commands are special comments that tell compiletest how to build and interpret a test. They must appear before the Rust source in the test. They may also appear in Makefiles for run-make tests.

They are normally put after the short comment that explains the point of this test. For example, this test uses the // compile-flags command to specify a custom flag to give to rustc when the test is compiled:

// Test the behavior of `0 - 1` when overflow checks are disabled.

// compile-flags: -C overflow-checks=off

fn main() {
    let x = 0 - 1;

Header commands can be standalone (like // run-pass) or take a value (like // compile-flags: -C overflow-checks=off).

Header commands

The following is a list of header commands. Commands are linked to sections the describe the command in more detail if available. This list may not be exhaustive. Header commands can generally be found by browsing the TestProps structure found in header.rs from the compiletest source.

  • Controlling pass/fail expectations
    • check-pass — building (no codegen) should pass
    • build-pass — building should pass
    • run-pass — running the test should pass
    • check-fail — building (no codegen) should fail (the default if no header)
    • build-fail — building should fail
    • run-fail — running should fail
    • ignore-pass — ignores the --pass flag
    • check-run-results — checks run-pass/fail-pass output
  • UI headers
  • Building auxiliary crates
    • aux-build
    • aux-crate
  • Pretty-printer headers
    • pretty-compare-only
    • pretty-expanded
    • pretty-mode
    • pp-exact
  • Ignoring tests
    • ignore-X
    • only-X
    • needs-X
    • no-system-llvm
    • min-llvm-versionX
    • min-system-llvm-version
    • ignore-llvm-version
    • ignore-llvm-version
  • Environment variable headers
    • rustc-env
    • exec-env
    • unset-rustc-env
  • Miscellaneous headers
    • compile-flags — adds compiler flags
    • run-flags — adds flags to executable tests
    • edition — sets the edition
    • failure-status — expected exit code
    • should-fail — testing compiletest itself
    • gate-test-X — feature gate testing
    • error-pattern — errors not on a line
    • incremental — incremental tests not in the incremental test-suite
    • no-prefer-dynamic — don't use -C prefer-dynamic, don't build as a dylib
    • force-host — build only for the host target
    • revisions — compile multiple times
    • forbid-output — incremental cfail rejects output pattern
    • should-ice — incremental cfail should ICE
    • known-bug — indicates that the test is for a known bug that has not yet been fixed
  • Assembly headers
    • assembly-output — the type of assembly output to check

Ignoring tests

These header commands are used to ignore the test in some situations, which means the test won't be compiled or run.

  • ignore-X where X is a target detail or stage will ignore the test accordingly (see below)
  • only-X is like ignore-X, but will only run the test on that target or stage
  • ignore-test always ignores the test. This can be used to temporarily disable a test if it is currently not working, but you want to keep it in tree to re-enable it later.

Some examples of X in ignore-X or only-X:

  • A full target triple: aarch64-apple-ios
  • Architecture: aarch64, arm, asmjs, mips, wasm32, x86_64, x86, ...
  • OS: android, emscripten, freebsd, ios, linux, macos, windows, ...
  • Environment (fourth word of the target triple): gnu, msvc, musl
  • WASM: wasm32-bare matches wasm32-unknown-unknown. emscripten also matches that target as well as the emscripten targets.
  • Pointer width: 32bit, 64bit
  • Endianness: endian-big
  • Stage: stage0, stage1, stage2
  • Channel: stable, beta
  • When cross compiling: cross-compile
  • When remote testing is used: remote
  • When debug-assertions are enabled: debug
  • When particular debuggers are being tested: cdb, gdb, lldb
  • Specific compare modes: compare-mode-nll, compare-mode-polonius, compare-mode-chalk, compare-mode-split-dwarf, compare-mode-split-dwarf-single

The following header commands will check rustc build settings and target settings:

  • needs-asm-support — ignores if it is running on a target that doesn't have stable support for asm!
  • needs-profiler-support — ignores if profiler support was not enabled for the target (profiler = true in rustc's config.toml)
  • needs-sanitizer-support — ignores if the sanitizer support was not enabled for the target (sanitizers = true in rustc's config.toml)
  • needs-sanitizer-{address,hwaddress,leak,memory,thread} — ignores if the corresponding sanitizer is not enabled for the target (AddressSanitizer, hardware-assisted AddressSanitizer, LeakSanitizer, MemorySanitizer or ThreadSanitizer respectively)
  • needs-run-enabled — ignores if it is a test that gets executed, and running has been disabled. Running tests can be disabled with the x.py test --run=never flag, or running on fuchsia.
  • needs-unwind — ignores if the target does not support unwinding
  • needs-rust-lld — ignores if the rust lld support is not enabled (rust.lld = true in config.toml)

The following header commands will check LLVM support:

  • no-system-llvm — ignores if the system llvm is used
  • min-llvm-version: 13.0 — ignored if the LLVM version is less than the given value
  • min-system-llvm-version: 12.0 — ignored if using a system LLVM and its version is less than the given value
  • ignore-llvm-version: 9.0 — ignores a specific LLVM version
  • ignore-llvm-version: 7.0 - 9.9.9 — ignores LLVM versions in a range (inclusive)
  • needs-llvm-components: powerpc — ignores if the specific LLVM component was not built. Note: The test will fail on CI if the component does not exist.
  • needs-matching-clang — ignores if the version of clang does not match the LLVM version of rustc. These tests are always ignored unless a special environment variable is set (which is only done in one CI job).

See also Debuginfo tests for headers for ignoring debuggers.

Environment variable headers

The following headers affect environment variables.

  • rustc-env is an environment variable to set when running rustc of the form KEY=VALUE.
  • exec-env is an environment variable to set when executing a test of the form KEY=VALUE.
  • unset-rustc-env specifies an environment variable to unset when running rustc.

Miscellaneous headers

The following headers are generally available, and not specific to particular test suites.

  • compile-flags passes extra command-line args to the compiler, e.g. compile-flags -g which forces debuginfo to be enabled.
  • run-flags passes extra args to the test if the test is to be executed.
  • edition controls the edition the test should be compiled with (defaults to 2015). Example usage: // edition:2018.
  • failure-status specifies the numeric exit code that should be expected for tests that expect an error. If this is not set, the default is 1.
  • should-fail indicates that the test should fail; used for "meta testing", where we test the compiletest program itself to check that it will generate errors in appropriate scenarios. This header is ignored for pretty-printer tests.
  • gate-test-X where X is a feature marks the test as "gate test" for feature X. Such tests are supposed to ensure that the compiler errors when usage of a gated feature is attempted without the proper #![feature(X)] tag. Each unstable lang feature is required to have a gate test. This header is actually checked by tidy, it is not checked by compiletest.
  • error-pattern checks the diagnostics just like the ERROR annotation without specifying error line. This is useful when the error doesn't give any span. See error-pattern.
  • incremental runs the test with the -C incremental flag and an empty incremental directory. This should be avoided when possible; you should use an incremental mode test instead. Incremental mode tests support running the compiler multiple times and verifying that it can load the generated incremental cache. This flag is for specialized circumstances, like checking the interaction of codegen unit partitioning with generating an incremental cache.
  • no-prefer-dynamic will force an auxiliary crate to be built as an rlib instead of a dylib. When specified in a test, it will remove the use of -C prefer-dynamic. This can be useful in a variety of circumstances. For example, it can prevent a proc-macro from being built with the wrong crate type. Or if your test is specifically targeting behavior of other crate types, it can be used to prevent building with the wrong crate type.
  • force-host will force the test to build for the host platform instead of the target. This is useful primarily for auxiliary proc-macros, which need to be loaded by the host compiler.


Headers values support substituting a few variables which will be replaced with their corresponding value. For example, if you need to pass a compiler flag with a path to a specific file, something like the following could work:

// compile-flags: --remap-path-prefix={{src-base}}=/the/src

Where the sentinel {{src-base}} will be replaced with the appropriate path described below:

  • {{cwd}}: The directory where compiletest is run from. This may not be the root of the checkout, so you should avoid using it where possible.
    • Examples: /path/to/rust, /path/to/build/root
  • {{src-base}}: The directory where the test is defined. This is equivalent to $DIR for output normalization.
    • Example: /path/to/rust/src/test/ui/error-codes
  • {{build-base}}: The base directory where the test's output goes. This is equivalent to $TEST_BUILD_DIR for output normalization.
    • Example: /path/to/rust/build/x86_64-unknown-linux-gnu/test/ui

See src/test/ui/commandline-argfile.rs for an example of a test that uses this substitution.

Adding a new header command

One would add a new header command if there is a need to define some test property or behavior on an individual, test-by-test basis. A header command property serves as the header command's backing store (holds the command's current value) at runtime.

To add a new header command property:

  1. Look for the pub struct TestProps declaration in src/tools/compiletest/src/header.rs and add the new public property to the end of the declaration.
  2. Look for the impl TestProps implementation block immediately following the struct declaration and initialize the new property to its default value.

Adding a new header command parser

When compiletest encounters a test file, it parses the file a line at a time by calling every parser defined in the Config struct's implementation block, also in src/tools/compiletest/src/header.rs (note that the Config struct's declaration block is found in src/tools/compiletest/src/common.rs). TestProps's load_from() method will try passing the current line of text to each parser, which, in turn typically checks to see if the line begins with a particular commented (//) header command such as // must-compile-successfully or // failure-status. Whitespace after the comment marker is optional.

Parsers will override a given header command property's default value merely by being specified in the test file as a header command or by having a parameter value specified in the test file, depending on the header command.

Parsers defined in impl Config are typically named parse_<header_command> (note kebab-case <header-command> transformed to snake-case <header_command>). impl Config also defines several 'low-level' parsers which make it simple to parse common patterns like simple presence or not (parse_name_directive()), header-command:parameter(s) (parse_name_value_directive()), optional parsing only if a particular cfg attribute is defined (has_cfg_prefix()) and many more. The low-level parsers are found near the end of the impl Config block; be sure to look through them and their associated parsers immediately above to see how they are used to avoid writing additional parsing code unnecessarily.

As a concrete example, here is the implementation for the parse_failure_status() parser, in src/tools/compiletest/src/header.rs:

@@ -232,6 +232,7 @@ pub struct TestProps {
     // customized normalization rules
     pub normalize_stdout: Vec<(String, String)>,
     pub normalize_stderr: Vec<(String, String)>,
+    pub failure_status: i32,

 impl TestProps {
@@ -260,6 +261,7 @@ impl TestProps {
             run_pass: false,
             normalize_stdout: vec![],
             normalize_stderr: vec![],
+            failure_status: 101,

@@ -383,6 +385,10 @@ impl TestProps {
             if let Some(rule) = config.parse_custom_normalization(ln, "normalize-stderr") {
+            if let Some(code) = config.parse_failure_status(ln) {
+                self.failure_status = code;
+            }

         for key in &["RUST_TEST_NOCAPTURE", "RUST_TEST_THREADS"] {
@@ -488,6 +494,13 @@ impl Config {
         self.parse_name_directive(line, "pretty-compare-only")

+    fn parse_failure_status(&self, line: &str) -> Option<i32> {
+        match self.parse_name_value_directive(line, "failure-status") {
+            Some(code) => code.trim().parse::<i32>().ok(),
+            _ => None,
+        }
+    }

Implementing the behavior change

When a test invokes a particular header command, it is expected that some behavior will change as a result. What behavior, obviously, will depend on the purpose of the header command. In the case of failure-status, the behavior that changes is that compiletest expects the failure code defined by the header command invoked in the test, rather than the default value.

Although specific to failure-status (as every header command will have a different implementation in order to invoke behavior change) perhaps it is helpful to see the behavior change implementation of one case, simply as an example. To implement failure-status, the check_correct_failure_status() function found in the TestCx implementation block, located in src/tools/compiletest/src/runtest.rs, was modified as per below:

@@ -295,11 +295,14 @@ impl<'test> TestCx<'test> {

     fn check_correct_failure_status(&self, proc_res: &ProcRes) {
-        // The value the Rust runtime returns on failure
-        const RUST_ERR: i32 = 101;
-        if proc_res.status.code() != Some(RUST_ERR) {
+        let expected_status = Some(self.props.failure_status);
+        let received_status = proc_res.status.code();
+        if expected_status != received_status {
-                &format!("failure produced the wrong error: {}", proc_res.status),
+                &format!("Error: expected failure status ({:?}) but received status {:?}.",
+                         expected_status,
+                         received_status),
@@ -320,7 +323,6 @@ impl<'test> TestCx<'test> {

         let proc_res = self.exec_compiled_test();
         if !proc_res.status.success() {
             self.fatal_proc_rec("test run failed!", &proc_res);
@@ -499,7 +501,6 @@ impl<'test> TestCx<'test> {
-            panic!();

Note the use of self.props.failure_status to access the header command property. In tests which do not specify the failure status header command, self.props.failure_status will evaluate to the default value of 101 at the time of this writing. But for a test which specifies a header command of, for example, // failure-status: 1, self.props.failure_status will evaluate to 1, as parse_failure_status() will have overridden the TestProps default value, for that test specifically.