Using compiletest commands to control test execution


compiletest is the main test harness of the Rust test suite. It allows test authors to organize large numbers of tests (the Rust compiler has many thousands), efficient test execution (parallel execution is supported), and allows the test author to configure behavior and expected results of both individual and groups of tests.

compiletest tests may check test code for success, for runtime failure, or for compile-time failure. Tests are typically organized as a Rust source file with annotations in comments before and/or within the test code, which serve to direct compiletest on if or how to run the test, what behavior to expect, and more. If you are unfamiliar with the compiler testing framework, see this chapter for additional background.

The tests themselves are typically (but not always) organized into "suites" – for example, run-fail, a folder holding tests that should compile successfully, but return a failure (non-zero status) at runtime, compile-fail, a folder holding tests that should fail to compile, and many more. The various suites are defined in src/tools/compiletest/src/ in the pub enum Mode declaration. And a good introduction to the different suites of compiler tests along with details about them can be found in Adding new tests.

Adding a new test file

Briefly, simply create your new test in the appropriate location under src/test. No registration of test files is necessary as compiletest will scan the src/test subfolder recursively, and will execute any Rust source files it finds as tests. See Adding new tests for a complete guide on how to add new tests.

Header Commands

Source file annotations which appear in comments near the top of the source file before any test code are known as header commands. These commands can instruct compiletest to ignore this test, set expectations on whether it is expected to succeed at compiling, or what the test's return code is expected to be. Header commands and inline //~ ERROR commands are described more fully here.

Adding a new header command

Header commands are defined in the TestProps struct in src/tools/compiletest/src/ At a high level, there are dozens of test properties defined here, all set to default values in the TestProp struct's impl block. Any test can override this default value by specifying the property in question as header command as a comment (//) in the test source file, before any source code.

Using a header command

Here is an example, specifying the must-compile-successfully header command, which takes no arguments, followed by the failure-status header command, which takes a single argument (which, in this case is a value of 1). failure-status is instructing compiletest to expect a failure status of 1 (rather than the current Rust default of 101). The header command and the argument list (if present) are typically separated by a colon:

// must-compile-successfully
// failure-status: 1


use std::io::{Error, ErrorKind};

fn main() -> Result<(), Box<Error>> {
    Err(Box::new(Error::new(ErrorKind::Other, "returned Box<Error> from main()")))

Adding a new header command property

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/ 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/ (note that the Config struct's declaration block is found in src/tools/compiletest/src/ 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/

@@ -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/, 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.