Rollup merge of #104199 - SarthakSingh31:issue-97417-1, r=cjgillot

Keep track of the start of the argument block of a closure

This removes a call to `tcx.sess.source_map()` from [compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs](https://github.com/rust-lang/rust/compare/master...SarthakSingh31:issue-97417-1?expand=1#diff-8406bbc0d0b43d84c91b1933305df896ecdba0d1f9269e6744f13d87a2ab268a) as required by #97417.

VsCode automatically applied `rustfmt` to the files I edited under `src/tools`. I can undo that if its a problem.

r? `@cjgillot`

compiler/rustc_ast/src/ast.rs

@@ -1312,8 +1312,10 @@ pub struct Closure {
     pub movability: Movability,
     pub fn_decl: P<FnDecl>,
     pub body: P<Expr>,
-    /// The span of the argument block `|...|`.
+    /// The span of the declaration block: 'move |...| -> ...'
     pub fn_decl_span: Span,
+    /// The span of the argument block `|...|`
+    pub fn_arg_span: Span,
 }
 
 /// Limit types of a range (inclusive or exclusive)