5 files changed, 34 insertions, 18 deletions
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 35cf4c1b00d..6a6fb0eb9b5 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -478,6 +478,13 @@ pub(crate) struct ExpectedExpressionFoundLet {
     pub comparison: Option<MaybeComparison>,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_or_in_let_chain)]
+pub(crate) struct OrInLetChain {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Subdiagnostic, Clone, Copy)]
 #[multipart_suggestion(
     parse_maybe_missing_let,
diff --git a/compiler/rustc_parse/src/lib.rs b/compiler/rustc_parse/src/lib.rs
index 896e348a12d..e73d68e2037 100644
--- a/compiler/rustc_parse/src/lib.rs
+++ b/compiler/rustc_parse/src/lib.rs
@@ -4,13 +4,13 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
+#![cfg_attr(bootstrap, feature(let_chains))]
 #![feature(array_windows)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
 #![feature(debug_closure_helpers)]
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
-#![feature(let_chains)]
 #![feature(string_from_utf8_lossy_owned)]
 // tidy-alphabetical-end
 
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 370eb3f402d..f3b53971b29 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -4073,14 +4073,18 @@ impl MutVisitor for CondChecker<'_> {
         match e.kind {
             ExprKind::Let(_, _, _, ref mut recovered @ Recovered::No) => {
                 if let Some(reason) = self.forbid_let_reason {
-                    *recovered = Recovered::Yes(self.parser.dcx().emit_err(
-                        errors::ExpectedExpressionFoundLet {
+                    let error = match reason {
+                        NotSupportedOr(or_span) => {
+                            self.parser.dcx().emit_err(errors::OrInLetChain { span: or_span })
+                        }
+                        _ => self.parser.dcx().emit_err(errors::ExpectedExpressionFoundLet {
                             span,
                             reason,
                             missing_let: self.missing_let,
                             comparison: self.comparison,
-                        },
-                    ));
+                        }),
+                    };
+                    *recovered = Recovered::Yes(error);
                 } else if self.depth > 1 {
                     // Top level `let` is always allowed; only gate chains
                     match self.let_chains_policy {
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())
                 {