Fix span edition for 2024 RPIT coming from an external macro

This fixes a problem where code generated by an external macro with an
RPIT would end up using the call-site edition instead of the macro's
edition for the RPIT. When used from a 2024 crate, this caused the code
to change behavior to the 2024 capturing rules, which we don't want.

This was caused by the impl-trait lowering code would replace the span
with one marked with `DesugaringKind::OpaqueTy` desugaring. However, it
was also overriding the edition of the span with the edition of the
local crate. Instead it should be using the edition of the span itself.

Fixes #132917

Author: ehuss

compiler/rustc_ast_lowering/src/lib.rs

@@ -738,7 +738,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         allow_internal_unstable: Option<Lrc<[Symbol]>>,
     ) -> Span {
         self.tcx.with_stable_hashing_context(|hcx| {
-            span.mark_with_reason(allow_internal_unstable, reason, self.tcx.sess.edition(), hcx)
+            span.mark_with_reason(allow_internal_unstable, reason, span.edition(), hcx)
         })
     }
 

tests/ui/impl-trait/precise-capturing/external-macro.rs

@@ -6,24 +6,21 @@
 //@ aux-crate: no_use_macro=no-use-macro.rs
 //@ edition: 2024
 //@ compile-flags:-Z unstable-options
+//@ check-pass
 
 no_use_pm::pm_rpit!{}
-//~^ ERROR: cannot borrow `x` as mutable
 
 no_use_macro::macro_rpit!{}
-//~^ ERROR: cannot borrow `x` as mutable
 
 fn main() {
     let mut x = vec![];
     x.push(1);
 
     let element = test_pm(&x);
     x.push(2);
-    //~^ ERROR: cannot borrow `x` as mutable
     println!("{element}");
 
     let element = test_mbe(&x);
     x.push(2);
-    //~^ ERROR: cannot borrow `x` as mutable
     println!("{element}");
 }