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

Author: workingjubilee

src/expr.rs

@@ -2289,8 +2289,10 @@ fn rewrite_expr_addrof(
 ) -> RewriteResult {
     let operator_str = match (mutability, borrow_kind) {
         (ast::Mutability::Not, ast::BorrowKind::Ref) => "&",
+        (ast::Mutability::Not, ast::BorrowKind::Pin) => "&pin const ",
         (ast::Mutability::Not, ast::BorrowKind::Raw) => "&raw const ",
         (ast::Mutability::Mut, ast::BorrowKind::Ref) => "&mut ",
+        (ast::Mutability::Mut, ast::BorrowKind::Pin) => "&pin mut ",
         (ast::Mutability::Mut, ast::BorrowKind::Raw) => "&raw mut ",
     };
     rewrite_unary_prefix(context, operator_str, expr, shape)

tests/source/pin_sugar.rs

@@ -18,3 +18,13 @@ impl Foo {
 mut self) {}
     fn i(&pin      mut   self) {}
 }
+
+fn borrows() {
+    let mut foo = 0_i32;
+    let x: Pin<&mut _> = & pin 
+    mut    foo;
+
+    let x: Pin<&_> = &
+    pin                const 
+    foo;
+}

tests/target/pin_sugar.rs

@@ -16,3 +16,10 @@ impl Foo {
     fn h<'a>(&'a pin mut self) {}
     fn i(&pin mut self) {}
 }
+
+fn borrows() {
+    let mut foo = 0_i32;
+    let x: Pin<&mut _> = &pin mut foo;
+
+    let x: Pin<&_> = &pin const foo;
+}