Implement name resolution for the IfLet expression.

[antego]

Addresses #1177.

Guidance from the ticket #1177:

You should be able to copy how we do name resolution for the if blocks from that file. Though if let statements add a new binding and scope level so you need to declare the pattern so you should be able to see how that is done from rust-ast-resolve-stmt.

I don't understand how to modify the block expression resolution so that it can handle the IfLet expression. For now, I copied the code from the MatchExpr resolution. Not sure how to test it either and what is the desired expected result of the name resolution so I just hope that reviewers will spot the errors.

I created this PR in order to get some guidance about how to proceed with it. Thanks!

[CohenArthur]

That looks good to me, but waiting for @philberty's expert opinion. One way to test this would be to add a test case using an if let construct with a pattern which depends on name resolution, something like:

mod a {
     enum Foo {
         Bar,
         Baz(i32),
      }
}

fn main() -> i32 {
    let value = a::Foo::Baz(15);
    if let a::Foo::Baz(value) = a {
        return 0;
    }
    
    1
}

[philberty]

I think this looks good. What sort of test-case are you using at the moment?

[antego]

What sort of test-case are you using at the moment?

@philberty still trying to compile the code from #1177

fn main() -> i32 {
    let mut res = 0;

    enum E {
        X(u8),
    }
    let v = E::X(4);
    if let E::X(n) = v {
        res = n;
    }

    0
}

It fails with

rust1: internal compiler error: in append_reference_for_def, at rust/resolve/rust-name-resolver.cc:200

I don't know if this error should be fixed by this PR or not.

[CohenArthur]

@antego I think this might be because you're not inserting a new definition? I'm not really sure as I've never worked on pattern resolution before. But the assertion is being triggered because the defId was not declared in this scope or an outer one. If we look at the ResolvePattern or PatternDeclaration class, they often include calls to resolver->insert_new_definition which might be what you need?

Sorry if this doesn't help :DD Trying to figure out something so you can progress!

[antego]

@CohenArthur appreciate the attempt but I still can't understand why MatchExpr works without calling the insert_new_definition.

While trying out different things I noticed that the part of the test code which causes the error is actually the line

    let v = E::X(4);

The minimal test case that fails with the error above is:

fn main() -> i32 {
    enum E {
        X(u8),
    }
    let v = E::X(4);

    0
}

looking into this

[antego]

This test case

enum E {
    X(u8),
}

fn main() -> i32 {
    let mut res = 0;
    let v = E::X(4);
    if let E::X(n) = v {
        res = n;
    }

    0
}

fails with the error

error: failed to type resolve expression
    8 |     if let E::X(n) = v {
      |     ^

Looks like the name resolution part is figured out and the type resolution is next?

[antego]

I guess the failing name resolution for enums should be addressed in another issue?

[antego]

@CohenArthur @philberty are you happy to merge this PR since the name resolution doesn't throw the error anymore?

[philberty]

bors try

[philberty]

@antego sorry this is my fault let me double-check the merge is ok first :)

[bors]

try

Build succeeded:

• build-and-check

[philberty]

bors r+

[bors]

Build succeeded:

• build-and-check