Write JET tests

Use this skill when adding new JET tests or modifying existing test code.

Choose the test location

Place tests beside the component they cover and include new files from test/runtests.jl. Existing component directories include:

• test/abstractinterpret/
• test/analyzers/
• test/toplevel/
• test/ui/

Use a root-level file under test/ for cross-component integration or miscellaneous behavior when that matches the neighboring tests.

Test file structure

New test files should normally define an independent module with a test_ prefix matching the file name. For example, test/toplevel/test_feature.jl should follow this structure:

module test_feature

include("../setup.jl")

@testset "feature behavior" begin
    let source = "x = 1"
        result = report_text(source)
        @test isempty(result.res.toplevel_error_reports)
    end
    let source = "x ="
        result = report_text(source)
        @test !isempty(result.res.toplevel_error_reports)
    end
end

end # module test_feature

Include it from test/runtests.jl under the relevant component:

@testset "toplevel" begin
    @testset "feature.jl" include("toplevel/test_feature.jl")
end

Most existing tests follow this convention. Do not copy the known exception in test/abstractinterpret/test_inferenceerrorreport.jl, which currently depends on running in Main.

Shared test utilities

Subdirectory tests that need JET's report helpers generally include test/setup.jl with include("../setup.jl"). It loads Test, JET, and the helpers from test/interactive_utils.jl.

Do not include the shared setup automatically when direct using JET, Test or narrow imports are sufficient.

Organize test code

Use concise behavior-oriented @testset names. Group related cases into coherent @testset blocks so testrunner can select them by name. Nest @testset blocks when a larger behavior area has useful subgroups.

Keep using, import, helper functions, macros, and type definitions at module scope unless local placement is specifically required.

When an @testset contains multiple test units, use a separate let block for each unit to prevent variable collisions. A let block is unnecessary when the @testset contains only one unit because there is no later unit with which its variables can collide. A helper function or do block also provides local scope.

Prefer assertions on report types, fields, counts, and semantic properties. Reserve output-string assertions for formatting and UI behavior, such as tests under test/ui/, unless the exact rendered message is the behavior under test.

After writing tests

Use the run-test workflow and run the most specific relevant test before broader validation.