Emit correctly-scoped JavaScript code for directly-matching case subjects

[justinlubin]

This PR fixes #5265. I added the code snippet from that issue as a test to language_test.gleam (with the dependence on the standard library removed for convenience). Now the JavaScript and Erlang compilation targets have the same behavior and pass this test. (Before this PR, the new test would fail under JavaScript only.)

It appears as if the root issue was that the body of the directly-matching branch was inserted directly into the scope of the case expression. I observed that commenting out lines 346–354 of decision.rs almost fixed the issue, but the resulting branch was emitted as part of a dangling else. (The generated code looked like <correct code> else { <correct code> } <correct code> with no if.) The main difference I saw between how the else branch is handled and how the lines above are handled is that the latter does not contain a call to self.inside_new_scope(). I figured that might be the missing piece, so I gave it a shot and it seems to work!

This is my first time looking at the internals of the Gleam compiler. Someone more experienced may want to check to make sure that no other code from the other paths in the switch function need to be ported over to the choices.is_empty() path.

Technically, this change affects the behavior of Gleam programs when compiled to JavaScript, and so it might therefore be considered backwards-incompatible.

• The changes in this PR have been discussed beforehand in an issue
• The issue for this PR has been linked
• Tests have been added for new behaviour
• The changelog has been updated for any user-facing changes

[lpil]

Lovely, thank you! Would you mind also adding a JavaScript codegen test please 🙏

[justinlubin]

Okay, I think I did it! I used the same test I added to language_test.gleam as a snapshot test in compiler-core/src/javascript/tests/case.rs.

[lpil]

Hello! I tried to rebase this on main but couldn't work out how to do it as your branch is also called main 😅 Could you rebase on main please

[justinlubin]

Hi! I rebased onto gleam-lang:main. I see that the language tests are now split from one file into many (nice!), so I created a new file for the language test I introduce. I also had to update gleam.toml for the language tests. It had a section called dev_dependencies (with underscore) but I think it needs to be dev-dependencies (with dash); at least, that's what I had to do on my machine to get Gleam to install gleeunit.

[giacomocavalieri]

Could you add the newline back?

[giacomocavalieri]

Looks good to me just some minor nitpick!

It had a section called dev_dependencies (with underscore) but I think it needs to be dev-dependencies (with dash); at least, that's what I had to do on my machine to get Gleam to install gleeunit.

If I recall correctly recently we switched to using dev_dependencies and dev-dependencies is considered deprecated, so it would be best to use that. I think the problem might be because the test is being run with the locally installed version?