Writing noirc_frontend Unit Tests

Tests live under compiler/noirc_frontend/src/tests/. The root module tests.rs defines helpers and declares submodules organized by topic.

Helpers in tests.rs

Importable from submodules via use crate::tests::{...}.

Compilation

Helper	Use when
assert_no_errors(src)	Program should compile without errors. Panics with "Expected no errors" and prints diagnostics. Returns Context.
assert_no_errors_without_report(src)	Same but doesn't print diagnostics on failure.
get_program_errors(src) -> Vec<CompilationError>	Get raw error list for manual inspection.
get_program_using_features(src, &[UnstableFeature])	Compile with specific unstable features enabled.

Error Checking with Inline Annotations

Helper	Use when
check_errors(src)	Program has inline error annotations (see below).
check_errors_with_stdlib(src, stdlib_src)	Same but prefixes stdlib snippets.
check_errors_using_features(src, &[UnstableFeature])	Same with unstable features.
check_monomorphization_error(src)	Error occurs during monomorphization, not elaboration.

Inline Error Annotation Syntax

Used with check_errors(). Error markers go on the line below the code that produces the error:

let src = r#"
    impl Foo::Bar { }
         ^^^^^^^^ Cannot define a trait impl on associated types
         ~~~~~~~~ secondary message here

    fn main() { }
"#;
check_errors(src);

• ^^^ = primary error span + message
• ~~~ = secondary error span + message
• Markers align character-by-character with the code line above

Helpers in test_utils.rs

Importable via use crate::test_utils::{...}. Also usable by other crates via the test_utils feature.

Helper	Use when
get_program(src)	Raw (ParsedModule, Context, Vec<CompilationError>). No stdlib.
get_program_with_options(src, GetProgramOptions)	Full control over compilation.
get_monomorphized(src)	Compile + monomorphize. Returns Result<Program, MonomorphizationError>.
get_monomorphized_with_stdlib(user_src, stdlib_src)	Monomorphize with stdlib prefix.
stdlib_src::ZEROED, EQ, ORD	Pre-written stdlib snippets for tests needing stdlib traits.

GetProgramOptions

GetProgramOptions {
    allow_parser_errors: bool,       // default: false
    allow_elaborator_errors: bool,   // default: false — needed for check_errors
    root_and_stdlib: bool,           // default: false — treat as stdlib (enables builtins)
    frontend_options: FrontendOptions,
}

Patterns

Regression test for a bug that should compile (already fixed)

/// Regression test for https://github.com/noir-lang/noir/issues/XXXXX
#[test]
fn descriptive_name() {
    let src = r#"
    // ... Noir program ...
    fn main() {}
    "#;
    assert_no_errors(src);
}

Reproduction test for a bug that should compile (not yet fixed)

/// TODO(https://github.com/noir-lang/noir/issues/XXXXX): remove should_panic once fixed
#[test]
#[should_panic(expected = "Expected no errors")]
fn descriptive_name() {
    let src = r#"
    // ... Noir program ...
    fn main() {}
    "#;
    assert_no_errors(src);
}

Test for expected compile error

#[test]
fn descriptive_name() {
    let src = r#"
    fn main() {
        let x: bool = 42;
                       ^^ expected type annotation
    }
    "#;
    check_errors(src);
}

Gotchas

• Warnings count as errors: get_program returns warnings (dead code, unused functions) in the Vec<CompilationError>. Unused items cause assert_no_errors to fail.
  • Fix: mark items pub to suppress "unused" warnings.
  • pub struct, pub fn for items not called from main().
  • If a struct is pub but never constructed, that's still a warning — add pub to fields too, or construct it.
  • Use the _ placeholder to suppress unused variable warnings (e.g., an unused x: u32 param becomes _x: u32)
• Always include fn main() {}: Programs without main will fail differently.
• No stdlib by default: get_program doesn't include the stdlib. Use check_errors_with_stdlib or root_and_stdlib: true if you need stdlib types/traits.
• Run command: cargo nextest run -p noirc_frontend -E 'test(test_name)'