parser: remove first_param from parse_param_general

[ImadRedwan]

Motivation

Currently, parse_param_general handles both the general function parameters and the optional parsing of the self receiver as the first parameter.

This mixes two distinct responsibilities:

• Parsing the self receiver (which has unique grammar and semantics),
• Parsing general parameters (typed, optionally named).

This makes the function harder to understand and harder to test.

---

What This PR Does

• Moves the parsing of the self parameter out of parse_param_general.
• Handles it explicitly in parse_fn_params before entering the general parameters loop.
• Removes the first_param boolean and its branching logic.
• Keeps req_name as-is (required name enforcement still makes sense).

---

Behavior After Refactor

Behavior is functionally identical. The only change is structural:

---

Tests

• Existing parser tests still pass.
• Closure syntax and self receiver handling remain correct.
• No behavioral changes were introduced.

---

Benefits

• Cleaner separation of concerns: self parsing is specialized and now handled separately.
• Improved readability: General parameters no longer have to reason about self.

Also parsing the self parameter can be extracted into a small inline function to isolate its logic from regular parameter parsing.

Reviewer Notes

• Feel free to suggest if self parsing should be refactored further.
• This cleanup was kept deliberately minimal to avoid breaking existing logic or tests.

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

             return Ok(ThinVec::new());
         }
 
-
         // Possibly parse `self`. Recover if we parsed it and it wasn't allowed here.
         let self_attrs = self.parse_outer_attributes()?;
         let self_param = if let Some(mut param) = this.parse_self_param()? {
Diff in /checkout/compiler/rustc_parse/src/parser/item.rs:2963:
             param.attrs = self_attrs;
             param
         };
-        
 
         let (mut params, _) = self.parse_paren_comma_seq(|p| {
             p.recover_vcs_conflict_marker();
Diff in /checkout/compiler/rustc_parse/src/parser/item.rs:2984:
                 // Create a placeholder argument for proper arg count (issue #34264).
                 Ok(dummy_arg(Ident::new(sym::dummy, lo.to(p.prev_token.span)), guar))
             });
-            
+
             param
         })?;