Refactor: Split self-param logic from `parse_param_general`

[ImadRedwan]

Summary

The function parse_param_general currently handles two responsibilities:

1. Parsing the general function parameter syntax (like x: T, _x: T, etc.)
2. Optionally parsing the self receiver (e.g. self, &mut self, self: Box<Self>, etc.)

This makes the function harder to follow and less composable.

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Observation

This function is only used in one place: Parser::parse_fn_params.
Because of this, it is safe and straightforward to remove the first_param flag and instead handle the optional self parameter directly in parse_fn_params.

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Suggested Refactor

Move the self parsing logic out of parse_param_general and into parse_fn_params.

This means:

• Add a try_parse_self_param() function to explicitly attempt parsing self as the first parameter.
• If successful, push the resulting param into the ThinVec before looping over regular parameters.
• Remove the first_param boolean from parse_param_general.
• Keep req_name logic intact (used to require named params).

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Benefits

• Simpler and more maintainable code.
• Clearer separation of responsibilities.
• Easier to test general parameter logic.
• More idiomatic parser design (handle special cases separately).

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Challenge

A key complication is that self parsing must occur after the ( token has been consumed — it is not syntactically valid before that point.
As a result, while extracting self-param parsing into a separate function (like try_parse_self_param) seems clean in theory, it doesn't buy us much in practice, because:

• It cannot be called before parse_fn_params sees the opening (.
• parse_param_general is only called after the paren is parsed anyway.
• Therefore, the decision logic for "is this the first parameter?" and "can it be self?" may remain right there.

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Notes

This was previously attempted in PR #144198 by myself, but I closed due to mentioned challange.
This issue tracks reintroducing that refactor cleanly.

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Related Files

• Parser::parse_fn_params
• Parser::parse_param_general

[nnethercote]

parse_param_general has a call to self.collect_tokens() that is important. That call would need to be duplicated for the self param parsing if the self param parsing is moved. I don't think this would be an improvement over the current code structure. (See also #145088.) So I'm going to close this issue: on the surface it's a reasonable idea, but it doesn't work out in practice.