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

@@ -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.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
+}

tests/ui/parser/triple-colon.stderr

@@ -0,0 +1,146 @@
+error: path separator must be a double colon
+  --> $DIR/triple-colon.rs:5:7
+   |
+LL | use :::std::{cell as _};
+   |       ^
+   |
+help: use a double colon instead
+   |
+LL - use :::std::{cell as _};
+LL + use ::std::{cell as _};
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon.rs:6:16
+   |
+LL | use std::cell:::*;
+   |                ^
+   |
+help: use a double colon instead
+   |
+LL - use std::cell:::*;
+LL + use std::cell::*;
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon.rs:7:16
+   |
+LL | use std::cell:::Cell;
+   |                ^
+   |
+help: use a double colon instead
+   |
+LL - use std::cell:::Cell;
+LL + use std::cell::Cell;
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon.rs:8:10
+   |
+LL | use std:::{cell as _};
+   |          ^
+   |
+help: use a double colon instead
+   |
+LL - use std:::{cell as _};
+LL + use std::{cell as _};
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon.rs:11:11
+   |
+LL |     use :::{};
+   |           ^
+   |
+help: use a double colon instead
+   |
+LL -     use :::{};
+LL +     use ::{};
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon.rs:12:11
+   |
+LL |     use :::*;
+   |           ^
+   |
+help: use a double colon instead
+   |
+LL -     use :::*;
+LL +     use ::*;
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon.rs:16:14
+   |
+LL |     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+   |              ^
+   |
+help: use a double colon instead
+   |
+LL -     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+LL +     let c: ::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon.rs:16:20
+   |
+LL |     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+   |                    ^
+   |
+help: use a double colon instead
+   |
+LL -     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+LL +     let c: :::std::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon.rs:16:27
+   |
+LL |     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+   |                           ^
+   |
+help: use a double colon instead
+   |
+LL -     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+LL +     let c: :::std:::cell::Cell:::<u8> = Cell:::<u8>:::new(0);
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon.rs:16:34
+   |
+LL |     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+   |                                  ^
+   |
+help: use a double colon instead
+   |
+LL -     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+LL +     let c: :::std:::cell:::Cell::<u8> = Cell:::<u8>:::new(0);
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon.rs:16:48
+   |
+LL |     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+   |                                                ^
+   |
+help: use a double colon instead
+   |
+LL -     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+LL +     let c: :::std:::cell:::Cell:::<u8> = Cell::<u8>:::new(0);
+   |
+
+error: path separator must be a double colon
+  --> $DIR/triple-colon.rs:16:55
+   |
+LL |     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+   |                                                       ^
+   |
+help: use a double colon instead
+   |
+LL -     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>:::new(0);
+LL +     let c: :::std:::cell:::Cell:::<u8> = Cell:::<u8>::new(0);
+   |
+
+error: aborting due to 12 previous errors
+