Rollup merge of #135731 - frank-king:feature/pin-borrow, r=eholk,traviscross

Implement parsing of pinned borrows

This PR implements part of #130494.

EDIT: It introduces `&pin mut $place` and `&pin const $place` as sugars for `std::pin::pin!($place)` and its shared reference equivalent, except that `$place` will not be moved when borrowing. The borrow check will be in charge of enforcing places cannot be moved or mutably borrowed since being pinned till dropped.

### Implementation steps:
- [x] parse the `&pin mut $place` and `&pin const $place` syntaxes
- [ ] borrowck of `&pin mut|const`
- [ ] support autoref of `&pin mut|const` when needed

compiler/rustc_parse/src/parser/expr.rs

@@ -847,7 +847,7 @@ impl<'a> Parser<'a> {
         self.dcx().emit_err(errors::LifetimeInBorrowExpression { span, lifetime_span: lt_span });
     }
 
-    /// Parse `mut?` or `raw [ const | mut ]`.
+    /// Parse `mut?` or `[ raw | pin ] [ const | mut ]`.
     fn parse_borrow_modifiers(&mut self) -> (ast::BorrowKind, ast::Mutability) {
         if self.check_keyword(exp!(Raw)) && self.look_ahead(1, Token::is_mutability) {
             // `raw [ const | mut ]`.
@@ -855,6 +855,11 @@ impl<'a> Parser<'a> {
             assert!(found_raw);
             let mutability = self.parse_const_or_mut().unwrap();
             (ast::BorrowKind::Raw, mutability)
+        } else if let Some((ast::Pinnedness::Pinned, mutbl)) = self.parse_pin_and_mut() {
+            // `pin [ const | mut ]`.
+            // `pin` has been gated in `self.parse_pin_and_mut()` so we don't
+            // need to gate it here.
+            (ast::BorrowKind::Pin, mutbl)
         } else {
             // `mut?`
             (ast::BorrowKind::Ref, self.parse_mutability())