Parser: recover from ::: to :: in delegations

compiler/rustc_parse/src/parser/item.rs

@@ -707,7 +707,7 @@ impl<'a> Parser<'a> {
             })
         };
 
-        let (ident, item_kind) = if self.eat(&token::PathSep) {
+        let (ident, item_kind) = if self.eat_path_sep() {
             let suffixes = if self.eat(&token::BinOp(token::Star)) {
                 None
             } else {

tests/ui/parser/triple-colon-delegation.fixed

@@ -0,0 +1,44 @@
+//@ run-rustfix
+
+#![feature(fn_delegation)]
+#![allow(incomplete_features, unused)]
+
+trait Trait {
+    fn foo(&self) {}
+}
+
+struct F;
+impl Trait for F {}
+
+pub mod to_reuse {
+    pub fn bar() {}
+}
+
+mod fn_to_other {
+    use super::*;
+
+    reuse Trait::foo; //~ ERROR path separator must be a double colon
+    reuse to_reuse::bar; //~ ERROR path separator must be a double colon
+}
+
+impl Trait for u8 {}
+
+struct S(u8);
+
+mod to_import {
+    pub fn check(arg: &u8) -> &u8 { arg }
+}
+
+impl Trait for S {
+    reuse Trait::* { //~ ERROR path separator must be a double colon
+        use to_import::check;
+
+        let _arr = Some(self.0).map(|x| [x * 2; 3]);
+        check(&self.0)
+    }
+}
+
+fn main() {
+    let s = S(0);
+    s.foo();
+}

tests/ui/parser/triple-colon-delegation.rs

@@ -0,0 +1,44 @@
+//@ run-rustfix
+
+#![feature(fn_delegation)]
+#![allow(incomplete_features, unused)]
+
+trait Trait {
+    fn foo(&self) {}
+}
+
+struct F;
+impl Trait for F {}
+
+pub mod to_reuse {
+    pub fn bar() {}
+}
+
+mod fn_to_other {
+    use super::*;
+
+    reuse Trait:::foo; //~ ERROR path separator must be a double colon
+    reuse to_reuse:::bar; //~ ERROR path separator must be a double colon
+}
+
+impl Trait for u8 {}
+
+struct S(u8);
+
+mod to_import {
+    pub fn check(arg: &u8) -> &u8 { arg }
+}
+
+impl Trait for S {
+    reuse Trait:::* { //~ ERROR path separator must be a double colon
+        use to_import::check;
+
+        let _arr = Some(self.0).map(|x| [x * 2; 3]);
+        check(&self.0)
+    }
+}
+
+fn main() {
+    let s = S(0);
+    s.foo();
+}

tests/ui/parser/triple-colon-delegation.stderr

@@ -0,0 +1,38 @@
+error: path separator must be a double colon
+  --> $DIR/triple-colon-delegation.rs:20:18
+   |
+LL |     reuse Trait:::foo;
+   |                  ^
+   |
+help: use a double colon instead
+   |
+LL -     reuse Trait:::foo;
+LL +     reuse Trait::foo;
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon-delegation.rs:21:21
+   |
+LL |     reuse to_reuse:::bar;
+   |                     ^
+   |
+help: use a double colon instead
+   |
+LL -     reuse to_reuse:::bar;
+LL +     reuse to_reuse::bar;
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon-delegation.rs:33:18
+   |
+LL |     reuse Trait:::* {
+   |                  ^
+   |
+help: use a double colon instead
+   |
+LL -     reuse Trait:::* {
+LL +     reuse Trait::* {
+   |
+
+error: aborting due to 3 previous errors
+