feat: type alias

[sinato]

This PR adds the ability to use type alias in Mun. (maybe close #110 )

The following code should work properly.

type Foo = u32;
fn main() {
    let a: Foo = 1;
}

And the following code should raise a type mismatch.

type Foo = u32;
fn main() {
    let a: Foo = 1.0;
}

error: mismatched type
 --> main.mun:3:17
  |
3 |     let a: Foo = 1.0;
  |                  ^^^ expected `u32`, found `{float}`
  |

NOTE: The following code does not work properly yet.

fn main() {
   type Foo = u32;
   let a: Foo = 10;
}

• lexer: add the type keyword
• parsing: add parsing of type TypeName = OtherTypeName
• hir: add type aliases as type definition
• tests: add test for all of the above stages
  • lexer: crates/mun_syntax/src/tests/snapshots/mun_syntax__tests__lexer__keywords.snap
  • parser: crates/mun_syntax/src/tests/snapshots/mun_syntax__tests__parser__type_alias_def.snap
  • HIR: crates/mun_hir/src/ty/snapshots/mun_hir__ty__tests__infer_type_alias.snap

[codecov]

Codecov Report

Merging #251 into master will increase coverage by 0.24%.
The diff coverage is 87.80%.

@@            Coverage Diff             @@
##           master     #251      +/-   ##
==========================================
+ Coverage   78.50%   78.75%   +0.24%     
==========================================
  Files         215      215              
  Lines       12709    12857     +148     
==========================================
+ Hits         9977    10125     +148     
  Misses       2732     2732              

Impacted Files	Coverage Δ
crates/mun_codegen/src/ir/file_group.rs	100.00% <ø> (ø)
crates/mun_hir/src/ty.rs	83.33% <0.00%> (-0.67%)	⬇️
crates/mun_syntax/src/lib.rs	69.56% <ø> (ø)
crates/mun_syntax/src/tests/lexer.rs	100.00% <ø> (ø)
crates/mun_hir/src/ty/infer.rs	71.00% <57.14%> (+0.06%)	⬆️
crates/mun_syntax/src/syntax_kind/generated.rs	65.45% <75.00%> (+0.14%)	⬆️
crates/mun_syntax/src/ast/generated.rs	72.11% <76.47%> (+0.07%)	⬆️
crates/mun_hir/src/diagnostics.rs	61.00% <83.33%> (+1.30%)	⬆️
crates/mun_hir/src/code_model.rs	79.73% <83.87%> (+1.23%)	⬆️
crates/mun_hir/src/adt.rs	90.00% <89.47%> (-0.25%)	⬇️
... and 18 more

---

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[baszalmstra]

Thanks for the PR!

Im unable to test this right now, but does it give a proper error in this case?:

type Foo = Bar; // Unknown type `Bar`

[Wodann]

Overall very impressive that you were able to largely pulled off such a complex task as your first PR.

There are still some corner cases that are not being handled. Could you have a look at those?

[sinato]

Thank you for the review. I have applied the review suggestions.
I will clean up the history after the review.

[Wodann]

Nice. Thank you for the quick turn around! 🙂

To polish, there is just some missing documentation left, and we are going to locally do a stress test to see if we can discover syntax that breaks the type aliasing.

[sinato]

If there is anything I can (or should) do to help with the stress test, please let me know.

[Wodann]

You can try to use type in weird ways or unexpected places. E.g. fn type Foo = u32; or (type Foo =u32)

If you install the Mun language server for VSCode, you should get continuous feedback to pinpoint errors and crashes more easily.

[baszalmstra]

I did a bit of stress testing and found this issue:

#[test]
fn recursive_alias() {
    infer_snapshot(
        r#"
    struct Foo {}
    type Foo = Foo;

    type A = B
    type B = A;
    "#,
    )
}

Both cases seem to result in a cycle in the salsa database.

Also, would you be able create a PR on this repo: https://github.com/mun-lang/vscode-extension to support type aliases in the syntax highlighting?

[sinato]

Both cases seem to result in a cycle in the salsa database.

Thank you for testing! Fixed 00d23e8
I referred to the following commit. rust-lang/rust-analyzer@1c622e9

Also, would you be able create a PR on this repo: https://github.com/mun-lang/vscode-extension to support type aliases in the syntax highlighting?

Of course. I'll fix it.

[Wodann]

Very quick fix; nice! There seems to be a merge conflict, but other than that I am happy to merge this. As far as I am concerned, you can go ahead with the rebase 🙂

[sinato]

Rebase completed.

[baszalmstra]

Your solution for recursive types is still incomplete. There are no errors reported for the recursive aliases which will crash the codegen. The codegen assumes that if there are no errors the HIR is in a valid state. You can add this test to crates/mun_codegen/src/test.rs to witness the problem:

#[test]
fn recursive_alias() {
    test_snapshot(
        r#"
    type Alias = Alias;

    fn foo() -> Alias {
    }
        "#,
    )
}

I see there is actually a comment that states we need to implement cyclic types. 😅

This also doesn't throw an error but does break the compiler:

#[test]
fn recursive_struct() {
    test_snapshot(
        r#"
    struct(value) Struct {
        a: Struct
    }
        "#,
    )
}

@Wodann WDYT we should do? Merge this PR and file an issue? Or should at least the alias cycle thing be fixed?

[Wodann]

@Wodann WDYT we should do? Merge this PR and file an issue? Or should at least the alias cycle thing be fixed?
@sinato do you think you can come up with a fix for the alias cycle based on @baszalmstra's comments or would you like some help with it? As we are aware of the issue before merging, I'd say that we should ideally resolve it first.

[sinato]

@Wodann Maybe there are better implementations, but I think the fix would be as 44ae87d .
I'd be happy to get some advice on this fix.

[Wodann]

Thanks for solving the issue with cyclic type references. This seems like a good enough approach for now.

I found one typo, could you fix that? Then I am happy to merge it 🙂

[baszalmstra]

Yes! Fine for me too!

[sinato]

Thank you for your comment! Fix typo and squashed the commits.

[baszalmstra]

Thank you very much for your contribution and the quick turnaround regarding comments! Great job! 👍👍

[Wodann]

Yes, thank you so much, @sinato! I can't wait for your next contribution; it's been a pleasure collaborating. We could really use another regular contributor 🙂

[sinato]

It was a very nice experience. Thank you for your kind comments!