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,
 }
 
@@ -1356,7 +1354,30 @@ impl<'a> Parser<'a> {
         };
         self.recover_from_inc_dec(operand_expr, kind, op_span)
     }
+    pub(super) fn recover_from_prefix_decrement(
+        &mut self,
+        operand_expr: P<Expr>,
+        op_span: Span,
+        start_stmt: bool,
+    ) -> PResult<'a, P<Expr>> {
+        let standalone = if start_stmt { IsStandalone::Standalone } else { IsStandalone::Subexpr };
+        let kind = IncDecRecovery { standalone, op: IncOrDec::Dec, fixity: UnaryFixity::Pre };
+        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

@@ -281,6 +281,16 @@ impl<'a> Parser<'a> {
                 lhs = self.recover_from_postfix_increment(lhs, op_span, starts_stmt)?;
                 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()
+            {
+                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:
@@ -550,10 +560,7 @@ impl<'a> Parser<'a> {
             token::Not => make_it!(this, attrs, |this, _| this.parse_unary_expr(lo, UnOp::Not)),
             // `~expr`
             token::Tilde => make_it!(this, attrs, |this, _| this.recover_tilde_expr(lo)),
-            // `-expr`
-            token::BinOp(token::Minus) => {
-                make_it!(this, attrs, |this, _| this.parse_unary_expr(lo, UnOp::Neg))
-            }
+
             // `*expr`
             token::BinOp(token::Star) => {
                 make_it!(this, attrs, |this, _| this.parse_unary_expr(lo, UnOp::Deref))
@@ -595,6 +602,24 @@ impl<'a> Parser<'a> {
                 let operand_expr = this.parse_dot_or_call_expr(Default::default())?;
                 this.recover_from_prefix_increment(operand_expr, pre_span, starts_stmt)
             }
+            // Recover from `--x`:
+            token::BinOp(token::Minus)
+                if this.look_ahead(1, |t| *t == token::BinOp(token::Minus)) =>
+            {
+                let starts_stmt = this.prev_token == token::Semi
+                    || this.prev_token == token::CloseDelim(Delimiter::Brace);
+                let pre_span = this.token.span.to(this.look_ahead(1, |t| t.span));
+                // Eat both `-`s.
+                this.bump();
+                this.bump();
+
+                let operand_expr = this.parse_dot_or_call_expr(Default::default())?;
+                this.recover_from_prefix_decrement(operand_expr, pre_span, starts_stmt)
+            }
+            // `-expr`
+            token::BinOp(token::Minus) => {
+                make_it!(this, attrs, |this, _| this.parse_unary_expr(lo, UnOp::Neg))
+            }
             token::Ident(..) if this.token.is_keyword(kw::Box) => {
                 make_it!(this, attrs, |this, _| this.parse_box_expr(lo))
             }

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

@@ -0,0 +1,52 @@
+fn test1() {
+    let mut i = 0;
+    let _ = i + --i; //~ ERROR Rust has no prefix decrement operator
+}
+
+fn test2() {
+    let mut i = 0;
+    let _ = --i + i; //~ ERROR Rust has no prefix decrement operator
+}
+
+fn test3() {
+    let mut i = 0;
+    let _ = --i + --i; //~ ERROR Rust has no prefix decrement operator
+}
+
+fn test4() {
+    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 test5() {
+    let mut i = 0;
+    let _ = i-- + i; //~ ERROR Rust has no postfix decrement operator
+}
+
+fn test6() {
+    let mut i = 0;
+    let _ = i-- + i--; //~ ERROR Rust has no postfix decrement operator
+}
+
+fn test7() {
+    let mut i = 0;
+    let _ = --i + i--; //~ ERROR Rust has no prefix decrement operator
+}
+
+fn test8() {
+    let mut i = 0;
+    let _ = i-- + --i; //~ ERROR Rust has no postfix decrement operator
+}
+
+fn test9() {
+    let mut i = 0;
+    let _ = (1 + 2 + i)--; //~ ERROR Rust has no postfix decrement operator
+}
+
+fn test10() {
+    let mut i = 0;
+    let _ = (i-- + 1) + 2; //~ ERROR Rust has no postfix decrement operator
+}
+
+fn main() { }

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

@@ -0,0 +1,107 @@
+error: Rust has no prefix decrement operator
+  --> $DIR/issue-dec.rs:3:17
+   |
+LL |     let _ = i + --i;
+   |                 ^^ not a valid prefix operator
+   |
+help: use `-= 1` instead
+   |
+LL |     let _ = i + { i -= 1; i };
+   |                 ~   +++++++++
+
+error: Rust has no prefix decrement operator
+  --> $DIR/issue-dec.rs:8:13
+   |
+LL |     let _ = --i + i;
+   |             ^^ not a valid prefix operator
+   |
+help: use `-= 1` instead
+   |
+LL |     let _ = { i -= 1; i } + i;
+   |             ~   +++++++++
+
+error: Rust has no prefix decrement operator
+  --> $DIR/issue-dec.rs:13:13
+   |
+LL |     let _ = --i + --i;
+   |             ^^ not a valid prefix operator
+   |
+help: use `-= 1` instead
+   |
+LL |     let _ = { i -= 1; i } + --i;
+   |             ~   +++++++++
+
+error: Rust has no postfix decrement operator
+  --> $DIR/issue-dec.rs:18:18
+   |
+LL |     let _ = i + i--;
+   |                  ^^ not a valid postfix operator
+
+error: Rust has no postfix decrement operator
+  --> $DIR/issue-dec.rs:24:14
+   |
+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-dec.rs:29:14
+   |
+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 prefix decrement operator
+  --> $DIR/issue-dec.rs:34:13
+   |
+LL |     let _ = --i + i--;
+   |             ^^ not a valid prefix operator
+   |
+help: use `-= 1` instead
+   |
+LL |     let _ = { i -= 1; i } + i--;
+   |             ~   +++++++++
+
+error: Rust has no postfix decrement operator
+  --> $DIR/issue-dec.rs:39:14
+   |
+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-dec.rs:44:24
+   |
+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-dec.rs:49:15
+   |
+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: aborting due to 10 previous errors
+