Description

Fixes #6391.

This PR fixes an issue with variable declarations whose initializers diverge because they contain return, continue or break expressions. In this case the initializer typechecks to Never, and the variable is inferred to have type Never in the subsequent (unreachable) code:

let mut x = return;
x = 42; // Illegal, since x has type Never

The problem is that this is the case even when the variable declaration has a type ascription:

let mut x : u32 = return;
x = 42; // Should be legal, since x has the type ascription u32

As part of the fix there is another slight change of behavior for code blocks that diverge. Until now the type of a code block has been determined based on whether it contains a diverging expression:

match { return; true } // Considered to have type Never because of `return;`
{ 
   // Type Never does not have a constructor
}

We now determine the type of a code block based on its implicit return:

match { return; true }
{
    // Type Boolean has two constructors
    true => ...,
    false => ...,
}

In principle this is a breaking change, but the only change in behavior is in code blocks that contain a return, break or continue, and then contains dead code after that expression, so I can't imagine anyone will be affected by it.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.