Rollup merge of #125117 - dev-ardi:improve-parser, r=wesleywiser,fmease

Improve parser

Fixes #124935.

- Add a few more help diagnostics to incorrect semicolons
- Overall improved that function
- Addded a few comments
- Renamed diff_marker fns to git_diff_marker

compiler/rustc_ast/src/ast.rs

@@ -3262,6 +3262,7 @@ pub enum ItemKind {
 }
 
 impl ItemKind {
+    /// "a" or "an"
     pub fn article(&self) -> &'static str {
         use ItemKind::*;
         match self {