How to use test fixtures

Test fixtures in Rust are string literals that define a mini-project environment for IDE or compiler tests, using a specific syntax to declare files, crates, and cursor positions. You write the fixture inside a raw string literal, often annotated with #[rust_fixture], where lines starting with //- define file paths and dependencies, and $0 marks the cursor location for analysis. The test framework parses this string to build a temporary database and runs your test logic against the generated code structure.

#[test]
fn test_fixture_example() {
    let fixture = r#"
//- /main.rs crate:main deps:dep1
use dep1::foo;
fn main() { foo(); }

//- /lib.rs crate:dep1
pub fn foo() {}
"#;
    // Parse fixture and run test logic
    let (db, files) = RootDatabase::from_ra_fixture(fixture, MiniCore::default());
    assert!(db.is_valid());
}

Test fixtures are like mini-sandboxes written as text strings that let you simulate a whole project with multiple files and dependencies for a single test. Instead of creating real folders and files on your disk, you describe the project structure inside your test code, and the testing tool builds it instantly in memory. This is like setting up a temporary LEGO city just to test one specific rule, then tearing it down immediately after.