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

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>,

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 {

tests/ui/parser/issue-108495-dec.rs

@@ -0,0 +1,39 @@
+fn test0() {
+    let mut i = 0;
+    let _ = i + i--; //~ ERROR Rust has no postfix decrement operator
+    // won't suggest since we can not handle the precedences
+}
+
+fn test1() {
+    let mut i = 0;
+    let _ = i-- + i--; //~ ERROR Rust has no postfix decrement operator
+}
+
+fn test2() {
+    let mut i = 0;
+    let _ = --i + i--; //~ ERROR Rust has no postfix decrement operator
+}
+
+fn test3() {
+    let mut i = 0;
+    let _ = i-- + --i; //~ ERROR Rust has no postfix decrement operator
+}
+
+fn test4() {
+    let mut i = 0;
+    let _ = (1 + 2 + i)--; //~ ERROR Rust has no postfix decrement operator
+}
+
+fn test5() {
+    let mut i = 0;
+    let _ = (i-- + 1) + 2; //~ ERROR Rust has no postfix decrement operator
+}
+
+fn test6(){
+    let i=10;
+    while i != 0 {
+        i--; //~ ERROR Rust has no postfix decrement operator
+    }
+}
+
+fn main() {}

tests/ui/parser/issue-108495-dec.stderr

@@ -0,0 +1,69 @@
+error: Rust has no postfix decrement operator
+  --> $DIR/issue-108495-dec.rs:3:18
+   |
+LL |     let _ = i + i--;
+   |                  ^^ not a valid postfix operator
+
+error: Rust has no postfix decrement operator
+  --> $DIR/issue-108495-dec.rs:9:14
+   |
+LL |     let _ = i-- + i--;
+   |              ^^ not a valid postfix operator
+   |
+help: use `-= 1` instead
+   |
+LL |     let _ = { let tmp = i; i -= 1; tmp } + i--;
+   |             +++++++++++  ~~~~~~~~~~~~~~~
+
+error: Rust has no postfix decrement operator
+  --> $DIR/issue-108495-dec.rs:14:20
+   |
+LL |     let _ = --i + i--;
+   |                    ^^ not a valid postfix operator
+
+error: Rust has no postfix decrement operator
+  --> $DIR/issue-108495-dec.rs:19:14
+   |
+LL |     let _ = i-- + --i;
+   |              ^^ not a valid postfix operator
+   |
+help: use `-= 1` instead
+   |
+LL |     let _ = { let tmp = i; i -= 1; tmp } + --i;
+   |             +++++++++++  ~~~~~~~~~~~~~~~
+
+error: Rust has no postfix decrement operator
+  --> $DIR/issue-108495-dec.rs:24:24
+   |
+LL |     let _ = (1 + 2 + i)--;
+   |                        ^^ not a valid postfix operator
+   |
+help: use `-= 1` instead
+   |
+LL |     let _ = { let tmp = (1 + 2 + i); (1 + 2 + i) -= 1; tmp };
+   |             +++++++++++            ~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: Rust has no postfix decrement operator
+  --> $DIR/issue-108495-dec.rs:29:15
+   |
+LL |     let _ = (i-- + 1) + 2;
+   |               ^^ not a valid postfix operator
+   |
+help: use `-= 1` instead
+   |
+LL |     let _ = ({ let tmp = i; i -= 1; tmp } + 1) + 2;
+   |              +++++++++++  ~~~~~~~~~~~~~~~
+
+error: Rust has no postfix decrement operator
+  --> $DIR/issue-108495-dec.rs:35:10
+   |
+LL |         i--;
+   |          ^^ not a valid postfix operator
+   |
+help: use `-= 1` instead
+   |
+LL |         i -= 1;
+   |           ~~~~
+
+error: aborting due to 7 previous errors
+