rust-lang-rust-tests

name: rust-lang-rust-tests description: Test writing conventions for rust-lang/rust contributions — naming, headers, directives, verification, PR descriptions keywords: rust-lang, rust compiler, regression test, ICE test, E-needs-test, compiletest, build-pass, check-pass, tests/ui, tests/crashes related: rust-lang-rust-pre-push, oss-repo-recon, pre-pr-review, pr-contribution-excellence, git-commit-discipline

Writing Tests for rust-lang/rust

Reference: https://rustc-dev-guide.rust-lang.org/tests/best-practices.html

Mandatory: Verify Before Submitting

ALWAYS compile the test reproducer locally with rustc +nightly before committing. The compiler evolves fast — reproducers from issues may no longer behave as described. A build-pass test that doesn't compile will fail CI and waste reviewer time.

# For build-pass / check-pass tests: must exit 0
rustc +nightly <test-file>.rs 2>&1; echo "Exit: $?"

# For error tests: verify the exact errors match what you expect
rustc +nightly <test-file>.rs 2>&1

If the reproducer no longer compiles as the issue describes, investigate what changed. The test may need to become an error test instead of build-pass, or the reproducer may need adapting.

Test Naming

Name tests after what they exercise, not just the issue number.

# BAD — issue number alone or leading
issue-149035.rs
ice-issue-149035.rs

# GOOD — descriptive, issue number at end
generic-const-exprs-link-dead-code-ice-149035.rs
transmute-from-min-generic-const-args-ice-150457.rs
recursive-lazy-type-alias-ice-152633.rs

# Also acceptable — descriptive without issue number
generic-const-exprs-link-dead-code-ice.rs

Exception: tests/crashes/ uses issue numbers only (e.g., 149035.rs). This is the canonical convention for that directory.

Test Header

Use //! inner doc comments. Follow this template from the rustc-dev-guide:

//! Brief summary of what the test exercises.
//!
//! Optional: remarks on related tests, fix mechanism, etc.
//!
//! Regression test for <https://github.com/rust-lang/rust/issues/NNNNN>.

Rules:

• First line: what the test exercises (not "Regression test for..." — that goes at the end)
• Issue links as full URLs in angle brackets: <https://github.com/rust-lang/rust/issues/NNNNN>
• Keep it concise. Don't over-explain.

Compiletest Directives

Add a brief remark explaining why each non-obvious directive is needed:

//@ build-pass (ICE was in codegen during mono item collection)
//@ compile-flags: -Clink-dead-code=true (required to trigger the codegen path)
//@ ignore-wasi (wasi codegens the main symbol differently)

Attributes

• Do NOT add #![allow(incomplete_features)] — compiletest auto-injects -A incomplete_features
• Do NOT add #![allow(dead_code)] — compiletest auto-injects -A unused for pass tests
• DO use #![feature(...)] as needed
• Attribute order: #![feature(...)] first, then any #![allow(...)] if actually needed

Choosing Test Type

For error tests, generate .stderr with --bless or verify against actual rustc +nightly output. Never hand-craft .stderr files without verification.

PR Descriptions

Write like a human. Short, direct, no filler.

# BAD — sounds like an LLM
Add a `build-pass` regression test for #149035. Using `-Clink-dead-code=true`
with `generic_const_exprs` and `min_generic_const_args` previously caused an
ICE (`erroneous constant missed by mono item collection`). This was fixed by
#152129, but no regression test was added for this specific scenario. The test
exercises the exact reproducer from the issue report.

# GOOD — sounds like a person
Regression test for #149035.

The ICE in mono item collection with `-Clink-dead-code=true` was fixed by
#152129 but had no test. Added one.

Closes #149035

Key tells that flag LLM-generated text:

• Backtick-heavy formatting in prose
• Parenthetical explanations after every term
• "This was fixed by X, but no Y was added for this specific scenario"
• "The test exercises the exact reproducer from the issue report"
• Perfect grammar with no contractions

Instead: use contractions, be terse, skip the obvious.

Checklist

Before submitting a test PR to rust-lang/rust:

1. Compiled reproducer locally with rustc +nightly — behavior matches expected
2. Test named after what it exercises, not just issue number
3. //! header with brief description and full issue URL
4. Correct test type (build-pass vs check-pass vs error test)
5. No unnecessary #![allow(incomplete_features)] or #![allow(dead_code)]
6. .stderr generated from actual compiler output (if applicable)
7. PR description is short, human-sounding, no LLM tells
8. No issue links (#NNN) in commit messages
9. No manual r? — let triagebot assign