refactor(parse): separate self parameter parsing from general parameter parsing

[xonx4l]

Fixes #144206

The changes-:
Moves self parameter parsing out of parse_param_general and into parse_fn_params
for better separation of concerns. This removes the first_param boolean and its
branching logic while maintaining identical behavior.

Benefits:

• Cleaner separation of concerns
• Improved readability and testability
• More maintainable code structure

No behavioral changes introduced.

[rustbot]

r? @nnethercote

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[nnethercote]

s/reamining_params/remaining_params/

[nnethercote]

Please fix the errors, thanks!

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[fmease]

(please also squash into one commit afterwards, thanks)

[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)

##[endgroup]
[TIMING] core::build_steps::tool::ToolBuild { build_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, tool: "tidy", path: "src/tools/tidy", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "", cargo_args: [], artifact_kind: Binary } -- 11.561
[TIMING] core::build_steps::tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_parse/src/parser/item.rs:2958:
                 .emit_err(errors::MissingFnParams { span: self.prev_token.span.shrink_to_hi() });
             return Ok(ThinVec::new());
         }
-        
+
         let mut params = ThinVec::new();
 
         //Parse the self parameter as first parameter
Diff in /checkout/compiler/rustc_parse/src/parser/item.rs:2965:
-        if let Some(mut self_param) = self.parse_self_param()?{
+        if let Some(mut self_param) = self.parse_self_param()? {
             let self_attrs = self.parse_outer_attributes()?;
             self_param.attrs = self.attrs;
             params.push(self_param);
Diff in /checkout/compiler/rustc_parse/src/parser/item.rs:2969:
-        }    
+        }
 
         let (mut remaining_params, _) = self.parse_paren_comma_seq(|p| {
             p.recover_vcs_conflict_marker();
fmt: checked 6299 files
Build completed unsuccessfully in 0:00:49
  local time: Mon Aug 11 10:47:35 UTC 2025
  network time: Mon, 11 Aug 2025 10:47:35 GMT
##[error]Process completed with exit code 1.

[xonx4l]

@rustbot ready

[nnethercote]

Suggested change

	//Parse the self parameter as first parameter
	// Parse the self parameter as first parameter

[nnethercote]

Suggested change

	if let Some(mut self_param) = self.parse_self_param()?{
	if let Some(mut self_param) = self.parse_self_param()? {

[nnethercote]

Having the parse_outer_attributes after the parse_self_param is suspicious.

[nnethercote]

Still more CI failures to be fixed.

@rustbot author

[bors]

☔ The latest upstream changes (presumably #145423) made this pull request unmergeable. Please resolve the merge conflicts.