Compiletest - Rust Compiler Development Guide (original) (raw)

Rust Compiler Development Guide

Compiletest

Introduction
Test suites
Building auxiliary crates
- Auxiliary proc-macro
Revisions
- Ignoring unused revision names
Compare modes

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.

Note for macOS users

For macOS users, SIP (System Integrity Protection) may consistently check the compiled binary by sending network requests to Apple, so you may get a huge performance degradation when running tests.

You can resolve it by tweaking the following settings: Privacy & Security -> Developer Tools -> Add Terminal (Or VsCode, etc.).

compiletest may check test code for compile-time or run-time success/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 compileteston if or how to run the test, what behavior to expect, and more. Seedirectives and the test suite documentation below for more details on these annotations.

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

Arguments can be passed to compiletest using --test-args or by placing them after --, e.g.

x test --test-args --force-rerun
x test -- --force-rerun

Additionally, bootstrap accepts several common arguments directly, e.g.

x test --no-capture --force-rerun --run --pass.

Compiletest itself tries to avoid running tests when the artifacts that are involved (mainly the compiler) haven't changed. You can use x test --test-args --force-rerun to rerun a test even when none of the inputs have changed.

Test suites

All of the tests are in the tests 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 tests/incremental directory contains tests for incremental compilation. The various suites are defined insrc/tools/compiletest/src/common.rs in the pub enum Mode declaration.

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

Compiler-specific test suites

Test suite	Purpose
ui	Check the stdout/stderr snapshots from the compilation and/or running the resulting executable
ui-fulldeps	ui tests which require a linkable build of rustc (such as using extern crate rustc_span; or used as a plugin)
pretty	Check pretty printing
incremental	Check incremental compilation behavior
debuginfo	Check debuginfo generation running debuggers
codegen	Check code generation
codegen-units	Check codegen unit partitioning
assembly	Check assembly output
mir-opt	Check MIR generation and optimizations
coverage	Check coverage instrumentation
coverage-run-rustdoc	coverage tests that also run instrumented doctests

General purpose test suite

run-make are general purpose tests using Rust programs.

Rustdoc test suites

See Rustdoc tests for more details.

Test suite	Purpose
rustdoc	Check rustdoc generated files contain the expected documentation
rustdoc-gui	Check rustdoc's GUI using a web browser
rustdoc-js	Check rustdoc search is working as expected
rustdoc-js-std	Check rustdoc search is working as expected specifically on the std docs
rustdoc-json	Check JSON output of rustdoc
rustdoc-ui	Check terminal output of rustdoc

Pretty-printer tests

The tests in tests/pretty exercise the "pretty-printing" functionality ofrustc. 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 directives described below. These commands can significantly change the behavior of the test, but the default behavior without any commands is to:

Run rustc -Zunpretty=normal on the source file.
Run rustc -Zunpretty=normal on the output of the previous step.
The output of the previous two steps should be the same.
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 directives 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.
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 tests/incremental exercise incremental compilation. They userevisions directive 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 andrpass2.

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

#[cfg(rpass1)]
fn foo() {
    println!("one");
}

#[cfg(rpass2)]
fn foo() {
    println!("two");
}

fn main() { foo(); }

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

Debuginfo tests

The tests in tests/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 directive 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 // <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>D</mi><mi>E</mi><mi>B</mi><mi>U</mi><mi>G</mi><mi>G</mi><mi>E</mi><mi>R</mi><mo>−</mo><mi>c</mi><mi>o</mi><mi>m</mi><mi>m</mi><mi>a</mi><mi>n</mi><mi>d</mi><mo>:</mo></mrow><annotation encoding="application/x-tex">DEBUGGER-command:</annotation></semantics></math>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 // <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>D</mi><mi>E</mi><mi>B</mi><mi>U</mi><mi>G</mi><mi>G</mi><mi>E</mi><mi>R</mi><mo>−</mo><mi>c</mi><mi>h</mi><mi>e</mi><mi>c</mi><mi>k</mi><mo>:</mo></mrow><annotation encoding="application/x-tex">DEBUGGER-check:</annotation></semantics></math>DEBUGGER−check:OUTPUTwhere $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 directives 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.

By passing the --debugger option to compiletest, you can specify a single debugger to run tests with. For example, ./x test tests/debuginfo -- --debugger gdb will only test GDB commands.

Note on running lldb debuginfo tests locally

If you want to run lldb debuginfo tests locally, then currently on Windows it is required that:

You have Python 3.10 installed.

You have python310.dll available in your PATH env var. This is not provided by the standard Python installer you obtain from python.org; you need to add this to PATH manually.

Otherwise the lldb debuginfo tests can produce crashes in mysterious ways.

Note on acquiring cdb.exe on Windows 11

cdb.exe is acquired alongside a suitable "Windows 11 SDK" which is part of the "Desktop Development with C++" workload profile in a Visual Studio installer (e.g. Visual Studio 2022 installer).

HOWEVER this is not sufficient by default alone. If you need cdb.exe, you must go to Installed Apps, find the newest "Windows Software Development Kit" (and yes, this can still say Windows 10.0.22161.3233 even though the OS is called Windows 11). You must then click "Modify" -> "Change" and then selected "Debugging Tools for Windows" in order to acquire cdb.exe.

Codegen tests

The tests in tests/codegen test LLVM code generation. They compile the test with the --emit=llvm-ir flag to emit LLVM IR. They then run the LLVMFileCheck 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.

If you need to work with #![no_std] cross-compiling tests, consult theminicore test auxiliary chapter.

Assembly tests

The tests in tests/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: directive 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.

If you need to work with #![no_std] cross-compiling tests, consult theminicore test auxiliary chapter.

Codegen-units tests

The tests in tests/codegen-units test themonomorphization 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 directive 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 @@ cguwhere 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 tests/mir-opt check parts of the generated MIR to make sure it is generated correctly and is doing the expected optimizations. Check out theMIR 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 x 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 <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>M</mi><mi>I</mi><msub><mi>R</mi><mi>P</mi></msub><mi>A</mi><mi>T</mi><mi>H</mi><mi mathvariant="normal">.</mi><mi>d</mi><mi>i</mi><mi>f</mi><mi>f</mi><mi mathvariant="normal">‘</mi><mtext>—</mtext><mi>w</mi><mi>h</mi><mi>e</mi><mi>r</mi><mi>e</mi><mi mathvariant="normal">‘</mi></mrow><annotation encoding="application/x-tex">MIR_PATH.diff — where </annotation></semantics></math>MIRPATH.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 $MIR_PATH.dot — When using specific flags that dump additional MIR data (e.g. -Z dump-mir-graphviz to produce .dot files), this 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 tests/run-make are general-purpose tests using Rust recipes, which are small programs (rmake.rs) allowing arbitrary Rust code such asrustc invocations, and is supported by a run_make_support library. Using Rust recipes provide the ultimate in flexibility.

run-make tests should be used if no other test suites better suit your needs.

Using Rust recipes

Each test should be in a separate directory with a rmake.rs Rust program, called the recipe. A recipe will be compiled and executed by compiletest with the run_make_support library linked in.

If you need new utilities or functionality, consider extending and improving therun_make_support library.

Compiletest directives like //@ only-<target> or //@ ignore-<target> are supported in rmake.rs, like in UI tests. However, revisions or building auxiliary via directives are not currently supported.

rmake.rs and run-make-support may not use any nightly/unstable features, as they must be compilable by a stage 0 rustc that may be a beta or even stable rustc.

Quickly check if rmake.rs tests can be compiled

You can quickly check if rmake.rs tests can be compiled without having to build stage1 rustc by forcing rmake.rs to be compiled with the stage0 compiler:

$ COMPILETEST_FORCE_STAGE0=1 x test --stage 0 tests/run-make/<test-name>

Of course, some tests will not successfully run in this way.

Using rust-analyzer with rmake.rs

Like other test programs, the rmake.rs scripts used by run-make tests do not have rust-analyzer integration by default.

To work around this when working on a particular test, temporarily create aCargo.toml file in the test's directory (e.g. tests/run-make/sysroot-crates-are-unstable/Cargo.toml) with these contents:

Be careful not to add this `Cargo.toml` or its `Cargo.lock` to your actual PR!

# Convince cargo that this isn't part of an enclosing workspace.
[workspace]

[package]
name = "rmake"
version = "0.1.0"
edition = "2021"

[dependencies]
run_make_support = { path = "../../../src/tools/run-make-support" }

[[bin]]
name = "rmake"
path = "rmake.rs"

Then add a corresponding entry to "rust-analyzer.linkedProjects"(e.g. in .vscode/settings.json):

"rust-analyzer.linkedProjects": [
  "tests/run-make/sysroot-crates-are-unstable/Cargo.toml"
],

Coverage tests

The tests in tests/coverage are shared by multiple test modes that test coverage instrumentation in different ways. Running the coverage test suite will automatically run each test in all of the different coverage modes.

Each mode also has an alias to run the coverage tests in just that mode:

./x test coverage # runs all of tests/coverage in all coverage modes
./x test tests/coverage # same as above

./x test tests/coverage/if.rs # runs the specified test in all coverage modes

./x test coverage-map # runs all of tests/coverage in "coverage-map" mode only
./x test coverage-run # runs all of tests/coverage in "coverage-run" mode only

./x test coverage-map -- tests/coverage/if.rs # runs the specified test in "coverage-map" mode only

If a particular test should not be run in one of the coverage test modes for some reason, use the //@ ignore-coverage-map or //@ ignore-coverage-rundirectives.

coverage-map suite

In coverage-map mode, these tests verify the mappings between source code regions and coverage counters that are emitted by LLVM. They compile the test with --emit=llvm-ir, then use a custom tool (src/tools/coverage-dump) to extract and pretty-print the coverage mappings embedded in the IR. These tests don't require the profiler runtime, so they run in PR CI jobs and are easy to run/bless locally.

These coverage map tests can be sensitive to changes in MIR lowering or MIR optimizations, producing mappings that are different but produce identical coverage reports.

As a rule of thumb, any PR that doesn't change coverage-specific code shouldfeel free to re-bless the coverage-map tests as necessary, without worrying about the actual changes, as long as the coverage-run tests still pass.

coverage-run suite

In coverage-run mode, these tests perform an end-to-end test of coverage reporting. They compile a test program with coverage instrumentation, run that program to produce raw coverage data, and then use LLVM tools to process that data into a human-readable code coverage report.

Instrumented binaries need to be linked against the LLVM profiler runtime, socoverage-run tests are automatically skipped unless the profiler runtime is enabled in bootstrap.toml:

# bootstrap.toml
[build]
profiler = true

This also means that they typically don't run in PR CI jobs, though they do run as part of the full set of CI jobs used for merging.

coverage-run-rustdoc suite

The tests in tests/coverage-run-rustdoc also run instrumented doctests and include them in the coverage report. This avoids having to build rustdoc when only running the main coverage suite.

Crashes tests

tests/crashes serve as a collection of tests that are expected to cause the compiler to ICE, panic or crash in some other way, so that accidental fixes are tracked. This was formally done at https://github.com/rust-lang/glacier but doing it inside the rust-lang/rust testsuite is more convenient.

It is imperative that a test in the suite causes rustc to ICE, panic or crash crash in some other way. A test will "pass" if rustc exits with an exit status other than 1 or 0.

If you want to see verbose stdout/stderr, you need to setCOMPILETEST_VERBOSE_CRASHES=1, e.g.

$ COMPILETEST_VERBOSE_CRASHES=1 ./x test tests/crashes/999999.rs --stage 1

When adding crashes from https://github.com/rust-lang/rust/issues, the issue number should be noted in the file name (12345.rs should suffice) and also inside the file include a //@ known-bug: #4321 directive.

If you happen to fix one of the crashes, please move it to a fitting subdirectory in tests/ui and give it a meaningful name. Please add a doc comment at the top of the file explaining why this test exists, even better if you can briefly explain how the example causes rustc to crash previously and what was done to prevent rustc to ICE/panic/crash.

Adding

Fixes #NNNNN
Fixes #MMMMM

to the description of your pull request will ensure the corresponding tickets be closed automatically upon merge.

Make sure that your fix actually fixes the root cause of the issue and not just a subset first. The issue numbers can be found in the file name or the //@ known-bug directive inside the test file.

Building auxiliary crates

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

aux-build
aux-crate
aux-bin
aux-codegen-backend
proc-macro

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: my-helper.rs

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 to make the crate be available as an extern prelude. That allows you to specify the additional syntax of the --extern flag, such as renaming a dependency. For example, // aux-crate:foo=bar.rs will compileauxiliary/bar.rs and make it available under then name foo within the test. This is similar to how Cargo does dependency renaming.

aux-bin is similar to aux-build but will build a binary instead of a library. The binary will be available in auxiliary/bin relative to the working directory of the test.

aux-codegen-backend is similar to aux-build, but will then pass the compiled dylib to -Zcodegen-backend when building the main file. This will only work for tests in tests/ui-fulldeps, since it requires the use of compiler crates.

Auxiliary proc-macro

If you want a proc-macro dependency, then you can use the proc-macrodirective. This directive behaves just like aux-build, i.e. that you should place the proc-macro test auxiliary file under a auxiliary folder under the same parent folder as the main test file. However, it also has four additional preset behavior compared to aux-build for the proc-macro test auxiliary:

The aux test file is built with --crate-type=proc-macro.
The aux test file is built without -C prefer-dynamic, i.e. it will not try to produce a dylib for the aux crate.
The aux crate is made available to the test file via extern prelude with--extern <aux_crate_name>. Note that since UI tests default to edition 2015, you still need to specify extern <aux_crate_name> unless the main test file is using an edition that is 2018 or newer if you want to use the aux crate name in a use import.
The proc_macro crate is made available as an extern prelude module. Same edition 2015 vs newer edition distinction for extern proc_macro; applies.

For example, you might have a test tests/ui/cat/meow.rs and proc-macro auxiliary tests/ui/cat/auxiliary/whiskers.rs:

tests/ui/cat/
    meow.rs                 # main test file
    auxiliary/whiskers.rs   # auxiliary

// tests/ui/cat/meow.rs

//@ proc-macro: whiskers.rs

extern crate whiskers; // needed as ui test defaults to edition 2015

fn main() {
  whiskers::identity!();
}

// tests/ui/cat/auxiliary/whiskers.rs

extern crate proc_macro;
use proc_macro::*;

#[proc_macro]
pub fn identity(ts: TokenStream) -> TokenStream {
    ts
}

Note: The proc-macro header currently does not work with thebuild-aux-doc header for rustdoc tests. In that case, you will need to use the aux-build header, and use #![crate_type="proc_macro"], and //@ force-host and //@ no-prefer-dynamic headers in the proc-macro.

Revisions

Revisions allow a single test file to be used for multiple tests. This is done by adding a special directive 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 directives and expected error messages to a particular revision. To do this, add [revision-name] after the //@ for directives, and after // for UI error annotations, like so:

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

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

Multiple revisions can be specified in a comma-separated list, such as//[foo,bar,baz]~^.

In test suites that use the LLVM FileCheck tool, the current revision name is also registered as an additional prefix for FileCheck directives:

//@ revisions: NORMAL COVERAGE
//@[COVERAGE] compile-flags: -Cinstrument-coverage
//@[COVERAGE] needs-profiler-runtime

// COVERAGE:   @__llvm_coverage_mapping
// NORMAL-NOT: @__llvm_coverage_mapping

// CHECK: main
fn main() {}

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

The following test suites support revisions:

ui
assembly
codegen
coverage
debuginfo
rustdoc UI tests
incremental (these are special in that they inherently cannot be run in parallel)

Ignoring unused revision names

Normally, revision names mentioned in other directives and error annotations must correspond to an actual revision declared in a revisions directive. This is enforced by an ./x test tidy check.

If a revision name needs to be temporarily removed from the revision list for some reason, the above check can be suppressed by adding the revision name to an//@ unused-revision-names: header instead.

Specifying an unused name of * (i.e. //@ unused-revision-names: *) will permit any unused revision name to be mentioned.

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:

./x test tests/ui --compare-mode=chalk

The possible compare modes are:

polonius — Runs with Polonius with -Zpolonius.
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.

See UI compare modes for more information about how UI tests support different output for different modes.

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

tests/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 ./x test tests/ui, however compare-modes must be manually run individually via the --compare-mode flag.