Auto merge of #139114 - m-ou-se:super-let-pin, r=davidtwco

Implement `pin!()` using `super let`

Tracking issue for super let: https://github.com/rust-lang/rust/issues/139076

This uses `super let` to implement `pin!()`.

This means we can remove [the hack](https://github.com/rust-lang/rust/pull/138717) we had to put in to fix https://github.com/rust-lang/rust/issues/138596.

It also means we can remove the original hack to make `pin!()` work, which used a questionable public-but-unstable field rather than a proper private field.

While `super let` is still unstable and subject to change, it seems safe to assume that future Rust will always have a way to express `pin!()` in a compatible way, considering `pin!()` is already stable.

It'd help [the experiment](https://github.com/rust-lang/rust/issues/139076) to have `pin!()` use `super let`, so we can get some more experience with it.

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -2959,21 +2959,27 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             }
         }
 
-        let mut err = self.path_does_not_live_long_enough(borrow_span, &format!("`{name}`"));
+        let name = if borrow_span.in_external_macro(self.infcx.tcx.sess.source_map()) {
+            // Don't name local variables in external macros.
+            "value".to_string()
+        } else {
+            format!("`{name}`")
+        };
+
+        let mut err = self.path_does_not_live_long_enough(borrow_span, &name);
 
         if let Some(annotation) = self.annotate_argument_and_return_for_borrow(borrow) {
             let region_name = annotation.emit(self, &mut err);
 
             err.span_label(
                 borrow_span,
-                format!("`{name}` would have to be valid for `{region_name}`..."),
+                format!("{name} would have to be valid for `{region_name}`..."),
             );
 
             err.span_label(
                 drop_span,
                 format!(
-                    "...but `{}` will be dropped here, when the {} returns",
-                    name,
+                    "...but {name} will be dropped here, when the {} returns",
                     self.infcx
                         .tcx
                         .opt_item_name(self.mir_def_id().to_def_id())
@@ -3011,7 +3017,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             }
         } else {
             err.span_label(borrow_span, "borrowed value does not live long enough");
-            err.span_label(drop_span, format!("`{name}` dropped here while still borrowed"));
+            err.span_label(drop_span, format!("{name} dropped here while still borrowed"));
 
             borrow_spans.args_subdiag(&mut err, |args_span| {
                 crate::session_diagnostics::CaptureArgLabel::Capture {

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -95,7 +95,9 @@ impl<'tcx> BorrowExplanation<'tcx> {
                         && let hir::def::Res::Local(hir_id) = p.res
                         && let hir::Node::Pat(pat) = tcx.hir_node(hir_id)
                     {
-                        err.span_label(pat.span, format!("binding `{ident}` declared here"));
+                        if !ident.span.in_external_macro(tcx.sess.source_map()) {
+                            err.span_label(pat.span, format!("binding `{ident}` declared here"));
+                        }
                     }
                 }
             }

compiler/rustc_span/src/symbol.rs

@@ -2217,7 +2217,6 @@ symbols! {
         unsafe_extern_blocks,
         unsafe_fields,
         unsafe_no_drop_flag,
-        unsafe_pin_internals,
         unsafe_pinned,
         unsafe_unpin,
         unsize,