feat: implement destructuring assignment by lowr · Pull Request #12428 · rust-la...

This is an attempt to implement destructuring assignments, or more specifically, type inference for assignee expressions.

I'm not sure if this is the right approach, so I don't even expect this to be merged (hence the branch name ) but rather want to propose one direction we could choose. I don't mind getting merged if this is good enough though!

Some notes on the implementation choices:

• Assignee expressions are not desugared on HIR level unlike rustc, but are inferred directly along with other expressions. This matches the processing of other syntaxes that are desugared in rustc but not in r-a. I find this reasonable because r-a only needs to infer types and it's easier to relate AST nodes and HIR nodes, so I followed it.
• Assignee expressions obviously resemble patterns, so type inference for each kind of pattern and its corresponding assignee expressions share a significant amount of logic. I tried to reuse the type inference functions for patterns by introducing PatLike trait which generalizes assignee expressions and patterns.
  • This is not the most elegant solution I suspect (and I really don't like the name of the trait!), but it's cleaner and the change is smaller than other ways I experimented, like making the functions generic without such trait, or making them take Either<ExprId, PatId> in place of PatId.

in case this is merged:
Closes #11532
Closes #11839
Closes #12322

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