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 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. These comments serve to direct compiletest on if or how to run the test, what behavior to expect, and more. See header commands and the test suite documentation below for more details on these annotations.

See the Adding new tests chapter for a tutorial on creating a new test, and the Running tests chapter on how to run the test suite.

Test suites

All of the tests are in the src/test directory. The tests are organized into "suites", with each suite in a separate subdirectory. Each test suite behaves a little differently, with different compiler behavior and different checks for correctness. For example, the src/test/incremental directory contains tests for incremental compilation. The various suites are defined in src/tools/compiletest/src/ in the pub enum Mode declaration.

The following test suites are available, with links for more information:

  • ui — tests that check the stdout/stderr from the compilation and/or running the resulting executable
  • ui-fulldepsui tests which require a linkable build of rustc (such as using extern crate rustc_span; or used as a plugin)
  • pretty — tests for pretty printing
  • incremental — tests incremental compilation behavior
  • debuginfo — tests for debuginfo generation running debuggers
  • codegen — tests for code generation
  • codegen-units — tests for codegen unit partitioning
  • assembly — verifies assembly output
  • mir-opt — tests for MIR generation
  • run-make — general purpose tests using a Makefile
  • run-make-fulldepsrun-make tests which require a linkable build of rustc, or the rust demangler
  • run-pass-valgrind — tests run with Valgrind
  • Rustdoc tests:
    • rustdoc — tests for rustdoc, making sure that the generated files contain the expected documentation.
    • rustdoc-gui — tests for rustdoc's GUI using a web browser.
    • rustdoc-js — tests to ensure the rustdoc search is working as expected.
    • rustdoc-js-std — tests to ensure the rustdoc search is working as expected (run specifically on the std docs).
    • rustdoc-json — tests on the JSON output of rustdoc.
    • rustdoc-ui — tests on the terminal output of rustdoc.

Pretty-printer tests

The tests in src/test/pretty exercise the "pretty-printing" functionality of rustc. The -Z unpretty CLI option for rustc causes it to translate the input source into various different formats, such as the Rust source after macro expansion.

The pretty-printer tests have several header commands described below. These commands can significantly change the behavior of the test, but the default behavior without any commands is to:

  1. Run rustc -Zunpretty=normal on the source file
  2. Run rustc -Zunpretty=normal on the output of the previous step
  3. The output of the previous two steps should be the same.
  4. Run rustc -Zno-codegen on the output to make sure that it can type check (this is similar to running cargo check)

If any of the commands above fail, then the test fails.

The header commands for pretty-printing tests are:

  • pretty-mode specifies the mode pretty-print tests should run in (that is, the argument to -Zunpretty). The default is normal if not specified.

  • pretty-compare-only causes a pretty test to only compare the pretty-printed output (stopping after step 3 from above). It will not try to compile the expanded output to type check it. This is needed for a pretty-mode that does not expand to valid Rust, or for other situations where the expanded output cannot be compiled.

  • pretty-expanded allows a pretty test to also check that the expanded output can be type checked. That is, after the steps above, it does two more steps:

    1. Run rustc -Zunpretty=expanded on the original source
    2. Run rustc -Zno-codegen on the expanded output to make sure that it can type check

    This is needed because not all code can be compiled after being expanded. Pretty tests should specify this if they can. An example where this cannot be used is if the test includes println!. That macro expands to reference private internal functions of the standard library that cannot be called directly without the fmt_internals feature gate.

    More history about this may be found in #23616.

  • pp-exact is used to ensure a pretty-print test results in specific output. If specified without a value, then it means the pretty-print output should match the original source. If specified with a value, as in // pp-exact:foo.pp, it will ensure that the pretty-printed output matches the contents of the given file. Otherwise, if pp-exact is not specified, then the pretty-printed output will be pretty-printed one more time, and the output of the two pretty-printing rounds will be compared to ensure that the pretty-printed output converges to a steady state.

Incremental tests

The tests in src/test/incremental exercise incremental compilation. They use revision headers to tell compiletest to run the compiler in a series of steps. Compiletest starts with an empty directory with the -C incremental flag, and then runs the compiler for each revision, reusing the incremental results from previous steps. The revisions should start with:

  • rpass — the test should compile and run successfully
  • rfail — the test should compile successfully, but the executable should fail to run
  • cfail — the test should fail to compile

To make the revisions unique, you should add a suffix like rpass1 and rpass2.

To simulate changing the source, compiletest also passes a --cfg flag with the current revision name. For example, this will run twice, simulating changing a function:

// revisions: rpass1 rpass2

fn foo() {

fn foo() {

fn main() { foo(); }

cfail tests support the forbid-output header to specify that a certain substring must not appear anywhere in the compiler output. This can be useful to ensure certain errors do not appear, but this can be fragile as error messages change over time, and a test may no longer be checking the right thing but will still pass.

cfail tests support the should-ice header to specify that a test should cause an Internal Compiler Error (ICE). This is a highly specialized header to check that the incremental cache continues to work after an ICE.

Debuginfo tests

The tests in src/test/debuginfo test debuginfo generation. They build a program, launch a debugger, and issue commands to the debugger. A single test can work with cdb, gdb, and lldb.

Most tests should have the // compile-flags: -g header or something similar to generate the appropriate debuginfo.

To set a breakpoint on a line, add a // #break comment on the line.

The debuginfo tests consist of a series of debugger commands along with "check" lines which specify output that is expected from the debugger.

The commands are comments of the form // $DEBUGGER-command:$COMMAND where $DEBUGGER is the debugger being used and $COMMAND is the debugger command to execute. The debugger values can be:

  • cdb
  • gdb
  • gdbg — GDB without Rust support (versions older than 7.11)
  • gdbr — GDB with Rust support
  • lldb
  • lldbg — LLDB without Rust support
  • lldbr — LLDB with Rust support (this no longer exists)

The command to check the output are of the form // $DEBUGGER-check:$OUTPUT where $OUTPUT is the output to expect.

For example, the following will build the test, start the debugger, set a breakpoint, launch the program, inspect a value, and check what the debugger prints:

// compile-flags: -g

// lldb-command: run
// lldb-command: print foo
// lldb-check: $0 = 123

fn main() {
    let foo = 123;
    b(); // #break

fn b() {}

The following header commands are available to disable a test based on the debugger currently being used:

  • min-cdb-version: 10.0.18317.1001 — ignores the test if the version of cdb is below the given version
  • min-gdb-version: 8.2 — ignores the test if the version of gdb is below the given version
  • ignore-gdb-version: 9.2 — ignores the test if the version of gdb is equal to the given version
  • ignore-gdb-version: 7.11.90 - 8.0.9 — ignores the test if the version of gdb is in a range (inclusive)
  • min-lldb-version: 310 — ignores the test if the version of lldb is below the given version
  • rust-lldb — ignores the test if lldb is not contain the Rust plugin. NOTE: The "Rust" version of LLDB doesn't exist anymore, so this will always be ignored. This should probably be removed.

Codegen tests

The tests in src/test/codegen test LLVM code generation. They compile the test with the --emit=llvm-ir flag to emit LLVM IR. They then run the LLVM FileCheck tool. The test is annotated with various // CHECK comments to check the generated code. See the FileCheck documentation for a tutorial and more information.

See also the assembly tests for a similar set of tests.

Assembly tests

The tests in src/test/assembly test LLVM assembly output. They compile the test with the --emit=asm flag to emit a .s file with the assembly output. They then run the LLVM FileCheck tool.

Each test should be annotated with the // assembly-output: header with a value of either emit-asm or ptx-linker to indicate the type of assembly output.

Then, they should be annotated with various // CHECK comments to check the assembly output. See the FileCheck documentation for a tutorial and more information.

See also the codegen tests for a similar set of tests.

Codegen-units tests

The tests in src/test/codegen-units test the monomorphization collector and CGU partitioning.

These tests work by running rustc with a flag to print the result of the monomorphization collection pass, and then special annotations in the file are used to compare against that.

Each test should be annotated with the // compile-flags:-Zprint-mono-items=VAL header with the appropriate VAL to instruct rustc to print the monomorphization information.

Then, the test should be annotated with comments of the form //~ MONO_ITEM name where name is the monomorphized string printed by rustc like fn <u32 as Trait>::foo.

To check for CGU partitioning, a comment of the form //~ MONO_ITEM name @@ cgu where cgu is a space separated list of the CGU names and the linkage information in brackets. For example: //~ MONO_ITEM static function::FOO @@ statics[Internal]

Mir-opt tests

The tests in src/test/mir-opt check parts of the generated MIR to make sure it is generated correctly and is doing the expected optimizations. Check out the MIR Optimizations chapter for more.

Compiletest will build the test with several flags to dump the MIR output and set a baseline for optimizations:

  • -Copt-level=1
  • -Zdump-mir=all
  • -Zmir-opt-level=4
  • -Zvalidate-mir
  • -Zdump-mir-exclude-pass-number

The test should be annotated with // EMIT_MIR comments that specify files that will contain the expected MIR output. You can use test --bless to create the initial expected files.

There are several forms the EMIT_MIR comment can take:

  • // EMIT_MIR $MIR_PATH.mir — This will check that the given filename matches the exact output from the MIR dump. For example, my_test.main.SimplifyCfg-elaborate-drops.after.mir will load that file from the test directory, and compare it against the dump from rustc.

    Checking the "after" file (which is after optimization) is useful if you are interested in the final state after an optimization. Some rare cases may want to use the "before" file for completeness.

  • // EMIT_MIR $MIR_PATH.diff — where $MIR_PATH is the filename of the MIR dump, such as my_test_name.my_function.EarlyOtherwiseBranch. Compiletest will diff the .before.mir and .after.mir files, and compare the diff output to the expected .diff file from the EMIT_MIR comment.

    This is useful if you want to see how an optimization changes the MIR.

  • // EMIT_MIR $ or $MIR_PATH.html — These are special cases for other MIR outputs (via -Z dump-mir-graphviz and -Z dump-mir-spanview) that will check that the output matches the given file.

By default 32 bit and 64 bit targets use the same dump files, which can be problematic in the presence of pointers in constants or other bit width dependent things. In that case you can add // EMIT_MIR_FOR_EACH_BIT_WIDTH to your test, causing separate files to be generated for 32bit and 64bit systems.

Run-make tests

The tests in src/test/run-make are general-purpose tests using Makefiles which provide the ultimate in flexibility. These should be used as a last resort. If possible, you should use one of the other test suites. If there is some minor feature missing which you need for your test, consider extending compiletest to add a header command for what you need. However, sometimes just running a bunch of commands is really what you need, run-make is here to the rescue!

Each test should be in a separate directory with a Makefile indicating the commands to run. There is a Makefile which you can include which provides a bunch of utilities to make it easier to run commands and compare outputs. Take a look at some of the other tests for some examples on how to get started.

Valgrind tests

The tests in src/test/run-pass-valgrind are for use with Valgrind. These are currently vestigial, as Valgrind is no longer used in CI. These may be removed in the future.

Building auxiliary crates

It is common that some tests require additional auxiliary crates to be compiled. There are two headers to assist with that:

  • aux-build
  • aux-crate

aux-build will build a separate crate from the named source file. The source file should be in a directory called auxiliary beside the test file.

// aux-build:

extern crate my_helper;
// ... You can use my_helper.

The aux crate will be built as a dylib if possible (unless on a platform that does not support them, or the no-prefer-dynamic header is specified in the aux file). The -L flag is used to find the extern crates.

aux-crate is very similar to aux-build; however, it uses the --extern flag to link to the extern crate. That allows you to specify the additional syntax of the --extern flag, such as renaming a dependency. For example, // will compile auxiliary/ and make it available under then name foo within the test. This is similar to how Cargo does dependency renaming.

Auxiliary proc-macro

If you want a proc-macro dependency, then there currently is some ceremony needed. Place the proc-macro itself in a file like auxiliary/ with the following structure:

// force-host
// no-prefer-dynamic

#![crate_type = "proc-macro"]

extern crate proc_macro;
use proc_macro::TokenStream;

pub fn foo(input: TokenStream) -> TokenStream {

The force-host is needed because proc-macros are loaded in the host compiler, and no-prefer-dynamic is needed to tell compiletest to not use prefer-dynamic which is not compatible with proc-macros. The #![crate_type] attribute is needed to specify the correct crate-type.

Then in your test, you can build with with aux-build:

// aux-build:

extern crate my_proc_macro;

fn main() {


Certain classes of tests support "revisions" (as of February 2022, this includes UI, assembly, codegen, incremental, and rustdoc UI tests, though incremental tests are somewhat different). Revisions allow a single test file to be used for multiple tests. This is done by adding a special header at the top of the file:

// revisions: foo bar baz

This will result in the test being compiled (and tested) three times, once with --cfg foo, once with --cfg bar, and once with --cfg baz. You can therefore use #[cfg(foo)] etc within the test to tweak each of these results.

You can also customize headers and expected error messages to a particular revision. To do this, add [foo] (or bar, baz, etc) after the // comment, like so:

// A flag to pass in only for cfg `foo`:
//[foo]compile-flags: -Z verbose

fn test_foo() {
    let x: usize = 32_u32; //[foo]~ ERROR mismatched types

Note that not all headers have meaning when customized to a revision. For example, the ignore-test header (and all "ignore" headers) currently only apply to the test as a whole, not to particular revisions. The only headers that are intended to really work when customized to a revision are error patterns and compiler flags.

Compare modes

Compiletest can be run in different modes, called compare modes, which can be used to compare the behavior of all tests with different compiler flags enabled. This can help highlight what differences might appear with certain flags, and check for any problems that might arise.

To run the tests in a different mode, you need to pass the --compare-mode CLI flag:

./ test src/test/ui --compare-mode=nll

The possible compare modes are:

  • nll — Runs in the "true" NLL mode with -Zborrowck=mir. See UI compare modes for more.
  • polonius — Runs with Polonius with -Zpolonius -Zborrowck=mir, and reuses the nll stderr files.
  • chalk — Runs with Chalk with -Zchalk.
  • split-dwarf — Runs with unpacked split-DWARF with -Csplit-debuginfo=unpacked.
  • split-dwarf-single — Runs with packed split-DWARF with -Csplit-debuginfo=packed.

In CI, compare modes are only used in one Linux builder, and only with the following settings:

  • src/test/ui: Uses nll mode.
  • src/test/debuginfo: Uses split-dwarf mode. This helps ensure that none of the debuginfo tests are affected when enabling split-DWARF.

Note that compare modes are separate to revisions. All revisions are tested when running ./ test src/test/ui, however compare-modes must be manually run individually via the --compare-mode flag.