Handle incorrect and ambiguous use of pin sugar better

compiler/rustc_parse/src/parser/ty.rs

@@ -530,7 +530,10 @@ impl<'a> Parser<'a> {
     fn parse_borrowed_pointee(&mut self) -> PResult<'a, TyKind> {
         let and_span = self.prev_token.span;
         let mut opt_lifetime = self.check_lifetime().then(|| self.expect_lifetime());
-        let (pinned, mut mutbl) = self.parse_pin_and_mut();
+        let (pinned, mut mutbl) = match self.parse_pin_and_mut() {
+            Some(pin_mut) => pin_mut,
+            None => (Pinnedness::Not, self.parse_mutability()),
+        };
         if self.token.is_lifetime() && mutbl == Mutability::Mut && opt_lifetime.is_none() {
             // A lifetime is invalid here: it would be part of a bare trait bound, which requires
             // it to be followed by a plus, but we disallow plus in the pointee type.
@@ -572,31 +575,24 @@ impl<'a> Parser<'a> {
     /// Parses `pin` and `mut` annotations on references.
     ///
     /// It must be either `pin const` or `pin mut`.
-    pub(crate) fn parse_pin_and_mut(&mut self) -> (Pinnedness, Mutability) {
-        let pinned = if self.eat(&TokenKind::Ident(sym::pin, IdentIsRaw::No)) {
-            Pinnedness::Pinned
-        } else {
-            Pinnedness::Not
-        };
-
-        if pinned == Pinnedness::Pinned {
-            self.psess.gated_spans.gate(sym::pin_ergonomics, self.prev_token.span);
-        }
-
-        match pinned {
-            Pinnedness::Pinned => {
-                if self.eat_keyword(kw::Const) {
-                    (pinned, Mutability::Not)
-                } else if self.eat_keyword(kw::Mut) {
-                    (pinned, Mutability::Mut)
+    pub(crate) fn parse_pin_and_mut(&mut self) -> Option<(Pinnedness, Mutability)> {
+        if self.token.is_ident_named(sym::pin) {
+            let result = self.look_ahead(1, |token| {
+                if token.is_keyword(kw::Const) {
+                    Some((Pinnedness::Pinned, Mutability::Not))
+                } else if token.is_keyword(kw::Mut) {
+                    Some((Pinnedness::Pinned, Mutability::Mut))
                 } else {
-                    // FIXME: Emit an error here
-
-                    // self.dcx().emit_err();
-                    (pinned, Mutability::Not)
+                    None
                 }
+            });
+            if result.is_some() {
+                self.bump();
+                self.bump();
             }
-            Pinnedness::Not => (pinned, self.parse_mutability()),
+            result
+        } else {
+            None
         }
     }
 

tests/ui/async-await/pin-sugar-ambiguity.rs

@@ -0,0 +1,15 @@
+//@ check-pass
+#![feature(pin_ergonomics)]
+#![allow(dead_code, incomplete_features)]
+
+// Handle the case where there's ambiguity between pin as a contextual keyword and pin as a path.
+
+struct Foo;
+
+mod pin {
+    pub struct Foo;
+}
+
+fn main() {
+    let _x: &pin ::Foo = &pin::Foo;
+}

tests/ui/async-await/pin-sugar-no-const.rs

@@ -0,0 +1,8 @@
+#![feature(pin_ergonomics)]
+#![allow(incomplete_features)]
+
+// Makes sure we don't accidentally accept `&pin Foo` without the `const` keyword.
+
+fn main() {
+    let _x: &pin i32 = todo!(); //~ ERROR found `i32`
+}

tests/ui/async-await/pin-sugar-no-const.stderr

@@ -0,0 +1,15 @@
+error: expected one of `!`, `(`, `::`, `;`, `<`, or `=`, found `i32`
+  --> $DIR/pin-sugar-no-const.rs:7:18
+   |
+LL |     let _x: &pin i32 = todo!();
+   |           -      ^^^ expected one of `!`, `(`, `::`, `;`, `<`, or `=`
+   |           |
+   |           while parsing the type for `_x`
+   |
+help: there is a keyword `in` with a similar name
+   |
+LL |     let _x: &in i32 = todo!();
+   |              ~~
+
+error: aborting due to 1 previous error
+

tests/ui/feature-gates/feature-gate-pin_ergonomics.rs

@@ -4,12 +4,13 @@ use std::pin::Pin;
 
 struct Foo;
 
-fn foo(_: Pin<&mut Foo>) {
-}
+fn foo(_: &pin mut Foo) {}
 
 fn bar(mut x: Pin<&mut Foo>) {
     foo(x);
     foo(x); //~ ERROR use of moved value: `x`
 }
 
+fn baz(_: &pin Foo) {}
+
 fn main() {}