Rollup merge of #140228 - fmease:revert-overzealous-colon-recovery, r=jieyouxu

Revert overzealous parse recovery for single colons in paths

Basically manually reverts #136808, cc ``@chenyukang`` ``@estebank.``

Reopens #129273.
Fixes [after beta backport] #140227.

2 files changed, 18 insertions, 13 deletions

diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 39251f1ce27..4be8a90368d 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -2058,6 +2058,17 @@ impl<'a> Parser<'a> {
         }
         self.expect_field_ty_separator()?;
         let ty = self.parse_ty()?;
+        if self.token == token::Colon && self.look_ahead(1, |&t| t != token::Colon) {
+            self.dcx()
+                .struct_span_err(self.token.span, "found single colon in a struct field type path")
+                .with_span_suggestion_verbose(
+                    self.token.span,
+                    "write a path separator here",
+                    "::",
+                    Applicability::MaybeIncorrect,
+                )
+                .emit();
+        }
         let default = if self.token == token::Eq {
             self.bump();
             let const_expr = self.parse_expr_anon_const()?;
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index 1a02d45f0e3..1093e4f4af0 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -248,19 +248,13 @@ impl<'a> Parser<'a> {
             segments.push(segment);
 
             if self.is_import_coupler() || !self.eat_path_sep() {
-                let ok_for_recovery = self.may_recover()
-                    && match style {
-                        PathStyle::Expr => true,
-                        PathStyle::Type if let Some((ident, _)) = self.prev_token.ident() => {
-                            self.token == token::Colon
-                                && ident.as_str().chars().all(|c| c.is_lowercase())
-                                && self.token.span.lo() == self.prev_token.span.hi()
-                                && self
-                                    .look_ahead(1, |token| self.token.span.hi() == token.span.lo())
-                        }
-                        _ => false,
-                    };
-                if ok_for_recovery
+                // IMPORTANT: We can *only ever* treat single colons as typo'ed double colons in
+                // expression contexts (!) since only there paths cannot possibly be followed by
+                // a colon and still form a syntactically valid construct. In pattern contexts,
+                // a path may be followed by a type annotation. E.g., `let pat:ty`. In type
+                // contexts, a path may be followed by a list of bounds. E.g., `where ty:bound`.
+                if self.may_recover()
+                    && style == PathStyle::Expr // (!)
                     && self.token == token::Colon
                     && self.look_ahead(1, |token| token.is_ident() && !token.is_reserved_ident())
                 {