Suggest swapping the order of ref and box

compiler/rustc_parse/src/errors.rs

@@ -137,6 +137,14 @@ pub(crate) enum InvalidVariableDeclarationSub {
     UseLetNotVar(#[primary_span] Span),
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_switch_ref_box_order)]
+pub(crate) struct SwitchRefBoxOrder {
+    #[primary_span]
+    #[suggestion(applicability = "machine-applicable", code = "box ref")]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_invalid_comparison_operator)]
 pub(crate) struct InvalidComparisonOperator {

compiler/rustc_parse/src/parser/pat.rs

@@ -5,8 +5,8 @@ use crate::errors::{
     ExpectedCommaAfterPatternField, GenericArgsInPatRequireTurbofishSyntax,
     InclusiveRangeExtraEquals, InclusiveRangeMatchArrow, InclusiveRangeNoEnd, InvalidMutInPattern,
     PatternOnWrongSideOfAt, RefMutOrderIncorrect, RemoveLet, RepeatedMutInPattern,
-    TopLevelOrPatternNotAllowed, TopLevelOrPatternNotAllowedSugg, TrailingVertNotAllowed,
-    UnexpectedLifetimeInPattern, UnexpectedVertVertBeforeFunctionParam,
+    SwitchRefBoxOrder, TopLevelOrPatternNotAllowed, TopLevelOrPatternNotAllowedSugg,
+    TrailingVertNotAllowed, UnexpectedLifetimeInPattern, UnexpectedVertVertBeforeFunctionParam,
     UnexpectedVertVertInPattern,
 };
 use crate::{maybe_recover_from_interpolated_ty_qpath, maybe_whole};
@@ -374,6 +374,12 @@ impl<'a> Parser<'a> {
         } else if self.eat_keyword(kw::Mut) {
             self.parse_pat_ident_mut(syntax_loc)?
         } else if self.eat_keyword(kw::Ref) {
+            if self.check_keyword(kw::Box) {
+                // Suggest `box ref` and quit parsing pattern to prevent series of
+                // misguided diagnostics from later stages of the compiler.
+                let span = self.prev_token.span.to(self.token.span);
+                return Err(self.sess.create_err(SwitchRefBoxOrder { span }));
+            }
             // Parse ref ident @ pat / ref mut ident @ pat
             let mutbl = self.parse_mutability();
             self.parse_pat_ident(BindingAnnotation(ByRef::Yes, mutbl), syntax_loc)?