Detect :: -> : typo when involving turbofish

src/libsyntax/parse/parser/diagnostics.rs

@@ -354,11 +354,11 @@ impl<'a> Parser<'a> {
     }
 
     pub fn maybe_annotate_with_ascription(
-        &self,
+        &mut self,
         err: &mut DiagnosticBuilder<'_>,
         maybe_expected_semicolon: bool,
     ) {
-        if let Some((sp, likely_path)) = self.last_type_ascription {
+        if let Some((sp, likely_path)) = self.last_type_ascription.take() {
             let sm = self.sess.source_map();
             let next_pos = sm.lookup_char_pos(self.token.span.lo());
             let op_pos = sm.lookup_char_pos(sp.hi());
@@ -1082,8 +1082,15 @@ impl<'a> Parser<'a> {
     }
 
     pub(super) fn could_ascription_be_path(&self, node: &ast::ExprKind) -> bool {
-        self.token.is_ident() &&
-            if let ast::ExprKind::Path(..) = node { true } else { false } &&
+        (self.token == token::Lt && // `foo:<bar`, likely a typoed turbofish.
+            self.look_ahead(1, |t| t.is_ident() && !t.is_reserved_ident())
+        ) ||
+            self.token.is_ident() &&
+            match node {
+                // `foo::` → `foo:` or `foo.bar::` → `foo.bar:`
+                ast::ExprKind::Path(..) | ast::ExprKind::Field(..) => true,
+                _ => false,
+            } &&
             !self.token.is_reserved_ident() &&           // v `foo:bar(baz)`
             self.look_ahead(1, |t| t == &token::OpenDelim(token::Paren)) ||
             self.look_ahead(1, |t| t == &token::Lt) &&     // `foo:bar<baz`

src/libsyntax/parse/parser/stmt.rs

@@ -397,6 +397,7 @@ impl<'a> Parser<'a> {
             }
             let stmt = match self.parse_full_stmt(false) {
                 Err(mut err) => {
+                    self.maybe_annotate_with_ascription(&mut err, false);
                     err.emit();
                     self.recover_stmt_(SemiColonMode::Ignore, BlockMode::Ignore);
                     Some(Stmt {

src/test/ui/suggestions/type-ascription-instead-of-path-2.rs

@@ -0,0 +1,5 @@
+fn main() -> Result<(), ()> {
+    vec![Ok(2)].into_iter().collect:<Result<Vec<_>,_>>()?;
+    //~^ ERROR expected `::`, found `(`
+    Ok(())
+}

src/test/ui/suggestions/type-ascription-instead-of-path-2.stderr

@@ -0,0 +1,13 @@
+error: expected `::`, found `(`
+  --> $DIR/type-ascription-instead-of-path-2.rs:2:55
+   |
+LL |     vec![Ok(2)].into_iter().collect:<Result<Vec<_>,_>>()?;
+   |                                    -                  ^ expected `::`
+   |                                    |
+   |                                    help: maybe write a path separator here: `::`
+   |
+   = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
+   = note: for more information, see https://github.com/rust-lang/rust/issues/23416
+
+error: aborting due to previous error
+