Rollup merge of #108496 - nx2k3:issue-108495-dec, r=WaffleLapkin

fix #108495, postfix decrement and prefix decrement has no warning

Fixes #108495

2 files changed, 26 insertions, 2 deletions

diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index b4948dddcc9..33218d3397a 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -165,8 +165,6 @@ enum IsStandalone {
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 enum IncOrDec {
     Inc,
-    // FIXME: `i--` recovery isn't implemented yet
-    #[allow(dead_code)]
     Dec,
 }
 
@@ -1357,6 +1355,20 @@ impl<'a> Parser<'a> {
         self.recover_from_inc_dec(operand_expr, kind, op_span)
     }
 
+    pub(super) fn recover_from_postfix_decrement(
+        &mut self,
+        operand_expr: P<Expr>,
+        op_span: Span,
+        start_stmt: bool,
+    ) -> PResult<'a, P<Expr>> {
+        let kind = IncDecRecovery {
+            standalone: if start_stmt { IsStandalone::Standalone } else { IsStandalone::Subexpr },
+            op: IncOrDec::Dec,
+            fixity: UnaryFixity::Post,
+        };
+        self.recover_from_inc_dec(operand_expr, kind, op_span)
+    }
+
     fn recover_from_inc_dec(
         &mut self,
         base: P<Expr>,
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 33254d034c9..24d4c17f5d8 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -282,6 +282,18 @@ impl<'a> Parser<'a> {
                 continue;
             }
 
+            if self.prev_token == token::BinOp(token::Minus)
+                && self.token == token::BinOp(token::Minus)
+                && self.prev_token.span.between(self.token.span).is_empty()
+                && !self.look_ahead(1, |tok| tok.can_begin_expr())
+            {
+                let op_span = self.prev_token.span.to(self.token.span);
+                // Eat the second `-`
+                self.bump();
+                lhs = self.recover_from_postfix_decrement(lhs, op_span, starts_stmt)?;
+                continue;
+            }
+
             let op = op.node;
             // Special cases:
             if op == AssocOp::As {