Parser: recover from ::: to ::

compiler/rustc_parse/messages.ftl

@@ -670,7 +670,7 @@ parse_parentheses_with_struct_fields = invalid `struct` delimiters or `fn` call
 parse_parenthesized_lifetime = parenthesized lifetime bounds are not supported
 parse_parenthesized_lifetime_suggestion = remove the parentheses
 
-parse_path_single_colon = path separator must be a double colon
+parse_path_double_colon = path separator must be a double colon
     .suggestion = use a double colon instead
 
 parse_pattern_method_param_without_body = patterns aren't allowed in methods without bodies

compiler/rustc_parse/src/errors.rs

@@ -1571,7 +1571,7 @@ pub(crate) struct ExpectedFnPathFoundFnKeyword {
 }
 
 #[derive(Diagnostic)]
-#[diag(parse_path_single_colon)]
+#[diag(parse_path_double_colon)]
 pub(crate) struct PathSingleColon {
     #[primary_span]
     pub span: Span,
@@ -1583,6 +1583,14 @@ pub(crate) struct PathSingleColon {
     pub type_ascription: bool,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_path_double_colon)]
+pub(crate) struct PathTripleColon {
+    #[primary_span]
+    #[suggestion(applicability = "maybe-incorrect", code = "", style = "verbose")]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_colon_as_semi)]
 pub(crate) struct ColonAsSemi {

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 {
@@ -1054,7 +1054,7 @@ impl<'a> Parser<'a> {
         {
             // `use *;` or `use ::*;` or `use {...};` or `use ::{...};`
             let mod_sep_ctxt = self.token.span.ctxt();
-            if self.eat(&token::PathSep) {
+            if self.eat_path_sep() {
                 prefix
                     .segments
                     .push(PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
@@ -1065,7 +1065,7 @@ impl<'a> Parser<'a> {
             // `use path::*;` or `use path::{...};` or `use path;` or `use path as bar;`
             prefix = self.parse_path(PathStyle::Mod)?;
 
-            if self.eat(&token::PathSep) {
+            if self.eat_path_sep() {
                 self.parse_use_tree_glob_or_nested()?
             } else {
                 // Recover from using a colon as path separator.

compiler/rustc_parse/src/parser/mod.rs

@@ -1562,12 +1562,25 @@ impl<'a> Parser<'a> {
         })
     }
 
+    /// Checks for `::` or, potentially, `:::` and then look ahead after it.
+    fn check_path_sep_and_look_ahead(&mut self, looker: impl Fn(&Token) -> bool) -> bool {
+        if self.check(&token::PathSep) {
+            if self.may_recover() && self.look_ahead(1, |t| t.kind == token::Colon) {
+                debug_assert!(!self.look_ahead(1, &looker), "Looker must not match on colon");
+                self.look_ahead(2, looker)
+            } else {
+                self.look_ahead(1, looker)
+            }
+        } else {
+            false
+        }
+    }
+
     /// `::{` or `::*`
     fn is_import_coupler(&mut self) -> bool {
-        self.check(&token::PathSep)
-            && self.look_ahead(1, |t| {
-                *t == token::OpenDelim(Delimiter::Brace) || *t == token::BinOp(token::Star)
-            })
+        self.check_path_sep_and_look_ahead(|t| {
+            matches!(t.kind, token::OpenDelim(Delimiter::Brace) | token::BinOp(token::Star))
+        })
     }
 
     // Debug view of the parser's token stream, up to `{lookahead}` tokens.

compiler/rustc_parse/src/parser/path.rs

@@ -16,7 +16,7 @@ use tracing::debug;
 
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{Parser, Restrictions, TokenType};
-use crate::errors::PathSingleColon;
+use crate::errors::{PathSingleColon, PathTripleColon};
 use crate::parser::{CommaRecoveryMode, RecoverColon, RecoverComma};
 use crate::{errors, maybe_whole};
 
@@ -210,7 +210,7 @@ impl<'a> Parser<'a> {
         let lo = self.token.span;
         let mut segments = ThinVec::new();
         let mod_sep_ctxt = self.token.span.ctxt();
-        if self.eat(&token::PathSep) {
+        if self.eat_path_sep() {
             segments.push(PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
         }
         self.parse_path_segments(&mut segments, style, ty_generics)?;
@@ -246,7 +246,7 @@ impl<'a> Parser<'a> {
             }
             segments.push(segment);
 
-            if self.is_import_coupler() || !self.eat(&token::PathSep) {
+            if self.is_import_coupler() || !self.eat_path_sep() {
                 if style == PathStyle::Expr
                     && self.may_recover()
                     && self.token == token::Colon
@@ -272,6 +272,18 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// Eat `::` or, potentially, `:::`.
+    #[must_use]
+    pub(super) fn eat_path_sep(&mut self) -> bool {
+        let result = self.eat(&token::PathSep);
+        if result && self.may_recover() {
+            if self.eat_noexpect(&token::Colon) {
+                self.dcx().emit_err(PathTripleColon { span: self.prev_token.span });
+            }
+        }
+        result
+    }
+
     pub(super) fn parse_path_segment(
         &mut self,
         style: PathStyle,
@@ -297,9 +309,7 @@ impl<'a> Parser<'a> {
 
         Ok(
             if style == PathStyle::Type && check_args_start(self)
-                || style != PathStyle::Mod
-                    && self.check(&token::PathSep)
-                    && self.look_ahead(1, |t| is_args_start(t))
+                || style != PathStyle::Mod && self.check_path_sep_and_look_ahead(is_args_start)
             {
                 // We use `style == PathStyle::Expr` to check if this is in a recursion or not. If
                 // it isn't, then we reset the unmatched angle bracket count as we're about to start
@@ -310,7 +320,8 @@ impl<'a> Parser<'a> {
 
                 // Generic arguments are found - `<`, `(`, `::<` or `::(`.
                 // First, eat `::` if it exists.
-                let _ = self.eat(&token::PathSep);
+                let _ = self.eat_path_sep();
+
                 let lo = self.token.span;
                 let args = if self.eat_lt() {
                     // `<'a, T, A = U>`

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
+

tests/ui/parser/triple-colon.fixed

@@ -0,0 +1,23 @@
+//@ run-rustfix
+
+#![allow(unused)]
+
+use ::std::{cell as _}; //~ ERROR path separator must be a double colon
+use std::cell::*; //~ ERROR path separator must be a double colon
+use std::cell::Cell; //~ ERROR path separator must be a double colon
+use std::{cell as _}; //~ ERROR path separator must be a double colon
+
+mod foo{
+    use ::{}; //~ ERROR path separator must be a double colon
+    use ::*; //~ ERROR path separator must be a double colon
+}
+
+fn main() {
+    let c: ::std::cell::Cell::<u8> = Cell::<u8>::new(0);
+    //~^ ERROR path separator must be a double colon
+    //~| ERROR path separator must be a double colon
+    //~| ERROR path separator must be a double colon
+    //~| ERROR path separator must be a double colon
+    //~| ERROR path separator must be a double colon
+    //~| ERROR path separator must be a double colon
+}

tests/ui/parser/triple-colon.rs

@@ -0,0 +1,23 @@
+//@ run-rustfix
+
+#![allow(unused)]
+
+use :::std::{cell as _}; //~ ERROR path separator must be a double colon
+use std::cell:::*; //~ ERROR path separator must be a double colon
+use std::cell:::Cell; //~ ERROR path separator must be a double colon
+use std:::{cell as _}; //~ ERROR path separator must be a double colon
+
+mod foo{
+    use :::{}; //~ ERROR path separator must be a double colon
+    use :::*; //~ ERROR path separator must be a double colon
+}
+
+fn main() {
+    let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+    //~^ ERROR path separator must be a double colon
+    //~| ERROR path separator must be a double colon
+    //~| ERROR path separator must be a double colon
+    //~| ERROR path separator must be a double colon
+    //~| ERROR path separator must be a double colon
+    //~| ERROR path separator must be a double colon
+}