implement lint double_negations

compiler/rustc_lint/messages.ftl

@@ -78,6 +78,11 @@ lint_builtin_deprecated_attr_used = use of deprecated attribute `{$name}`: no lo
 lint_builtin_deref_nullptr = dereferencing a null pointer
     .label = this code causes undefined behavior when executed
 
+lint_builtin_double_negations = use of a double negation
+    .note = the prefix `--` could be misinterpreted as a decrement operator which exists in other languages
+    .note_decrement = use `-= 1` if you meant to decrement the value
+    .add_parens_suggestion = add parentheses for clarity
+
 lint_builtin_ellipsis_inclusive_range_patterns = `...` range patterns are deprecated
     .suggestion = use `..=` for an inclusive range
 

compiler/rustc_lint/src/builtin.rs

@@ -51,6 +51,7 @@ use crate::errors::BuiltinEllipsisInclusiveRangePatterns;
 use crate::lints::{
     BuiltinAnonymousParams, BuiltinConstNoMangle, BuiltinDeprecatedAttrLink,
     BuiltinDeprecatedAttrLinkSuggestion, BuiltinDeprecatedAttrUsed, BuiltinDerefNullptr,
+    BuiltinDoubleNegations, BuiltinDoubleNegationsAddParens,
     BuiltinEllipsisInclusiveRangePatternsLint, BuiltinExplicitOutlives,
     BuiltinExplicitOutlivesSuggestion, BuiltinFeatureIssueNote, BuiltinIncompleteFeatures,
     BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures, BuiltinKeywordIdents,
@@ -1557,6 +1558,58 @@ impl<'tcx> LateLintPass<'tcx> for TrivialConstraints {
     }
 }
 
+declare_lint! {
+    /// The `double_negations` lint detects expressions of the form `--x`.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// fn main() {
+    ///     let x = 1;
+    ///     let _b = --x;
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Negating something twice is usually the same as not negating it at all.
+    /// However, a double negation in Rust can easily be confused with the
+    /// prefix decrement operator that exists in many languages derived from C.
+    /// Use `-(-x)` if you really wanted to negate the value twice.
+    ///
+    /// To decrement a value, use `x -= 1` instead.
+    pub DOUBLE_NEGATIONS,
+    Warn,
+    "detects expressions of the form `--x`"
+}
+
+declare_lint_pass!(
+    /// Lint for expressions of the form `--x` that can be confused with C's
+    /// prefix decrement operator.
+    DoubleNegations => [DOUBLE_NEGATIONS]
+);
+
+impl EarlyLintPass for DoubleNegations {
+    #[inline]
+    fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &ast::Expr) {
+        // only lint on the innermost `--` in a chain of `-` operators,
+        // even if there are 3 or more negations
+        if let ExprKind::Unary(UnOp::Neg, ref inner) = expr.kind
+            && let ExprKind::Unary(UnOp::Neg, ref inner2) = inner.kind
+            && !matches!(inner2.kind, ExprKind::Unary(UnOp::Neg, _))
+        {
+            cx.emit_span_lint(DOUBLE_NEGATIONS, expr.span, BuiltinDoubleNegations {
+                add_parens: BuiltinDoubleNegationsAddParens {
+                    start_span: inner.span.shrink_to_lo(),
+                    end_span: inner.span.shrink_to_hi(),
+                },
+            });
+        }
+    }
+}
+
 declare_lint_pass!(
     /// Does nothing as a lint pass, but registers some `Lint`s
     /// which are used by other parts of the compiler.
@@ -1575,7 +1628,8 @@ declare_lint_pass!(
         UNSTABLE_FEATURES,
         UNREACHABLE_PUB,
         TYPE_ALIAS_BOUNDS,
-        TRIVIAL_BOUNDS
+        TRIVIAL_BOUNDS,
+        DOUBLE_NEGATIONS
     ]
 );
 

compiler/rustc_lint/src/lib.rs

@@ -181,6 +181,7 @@ early_lint_methods!(
             UnusedDocComment: UnusedDocComment,
             Expr2024: Expr2024,
             Precedence: Precedence,
+            DoubleNegations: DoubleNegations,
         ]
     ]
 );

compiler/rustc_lint/src/lints.rs

@@ -343,6 +343,24 @@ pub(crate) struct BuiltinTrivialBounds<'a> {
     pub predicate: Clause<'a>,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_builtin_double_negations)]
+#[note(lint_note)]
+#[note(lint_note_decrement)]
+pub(crate) struct BuiltinDoubleNegations {
+    #[subdiagnostic]
+    pub add_parens: BuiltinDoubleNegationsAddParens,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(lint_add_parens_suggestion, applicability = "maybe-incorrect")]
+pub(crate) struct BuiltinDoubleNegationsAddParens {
+    #[suggestion_part(code = "(")]
+    pub start_span: Span,
+    #[suggestion_part(code = ")")]
+    pub end_span: Span,
+}
+
 #[derive(LintDiagnostic)]
 pub(crate) enum BuiltinEllipsisInclusiveRangePatternsLint {
     #[diag(lint_builtin_ellipsis_inclusive_range_patterns)]

tests/ui/lint/lint-double-negations.rs

@@ -0,0 +1,9 @@
+//@ check-pass
+fn main() {
+    let x = 1;
+    -x;
+    -(-x);
+    --x; //~ WARN use of a double negation
+    ---x; //~ WARN use of a double negation
+    let _y = --(-x); //~ WARN use of a double negation
+}

tests/ui/lint/lint-double-negations.stderr

@@ -0,0 +1,42 @@
+warning: use of a double negation
+  --> $DIR/lint-double-negations.rs:6:5
+   |
+LL |     --x;
+   |     ^^^
+   |
+   = note: the prefix `--` could be misinterpreted as a decrement operator which exists in other languages
+   = note: use `-= 1` if you meant to decrement the value
+   = note: `#[warn(double_negations)]` on by default
+help: add parentheses for clarity
+   |
+LL |     -(-x);
+   |      +  +
+
+warning: use of a double negation
+  --> $DIR/lint-double-negations.rs:7:6
+   |
+LL |     ---x;
+   |      ^^^
+   |
+   = note: the prefix `--` could be misinterpreted as a decrement operator which exists in other languages
+   = note: use `-= 1` if you meant to decrement the value
+help: add parentheses for clarity
+   |
+LL |     --(-x);
+   |       +  +
+
+warning: use of a double negation
+  --> $DIR/lint-double-negations.rs:8:14
+   |
+LL |     let _y = --(-x);
+   |              ^^^^^^
+   |
+   = note: the prefix `--` could be misinterpreted as a decrement operator which exists in other languages
+   = note: use `-= 1` if you meant to decrement the value
+help: add parentheses for clarity
+   |
+LL |     let _y = -(-(-x));
+   |               +     +
+
+warning: 3 warnings emitted
+

tests/ui/lint/lint-type-overflow2.rs

@@ -4,6 +4,7 @@
 
 fn main() {
     let x2: i8 = --128; //~ ERROR literal out of range for `i8`
+    //~| WARN use of a double negation
 
     let x = -3.40282357e+38_f32; //~ ERROR literal out of range for `f32`
     let x =  3.40282357e+38_f32; //~ ERROR literal out of range for `f32`

tests/ui/lint/lint-type-overflow2.stderr

@@ -1,3 +1,17 @@
+warning: use of a double negation
+  --> $DIR/lint-type-overflow2.rs:6:18
+   |
+LL |     let x2: i8 = --128;
+   |                  ^^^^^
+   |
+   = note: the prefix `--` could be misinterpreted as a decrement operator which exists in other languages
+   = note: use `-= 1` if you meant to decrement the value
+   = note: `#[warn(double_negations)]` on by default
+help: add parentheses for clarity
+   |
+LL |     let x2: i8 = -(-128);
+   |                   +    +
+
 error: literal out of range for `i8`
   --> $DIR/lint-type-overflow2.rs:6:20
    |
@@ -13,36 +27,36 @@ LL | #![deny(overflowing_literals)]
    |         ^^^^^^^^^^^^^^^^^^^^
 
 error: literal out of range for `f32`
-  --> $DIR/lint-type-overflow2.rs:8:14
+  --> $DIR/lint-type-overflow2.rs:9:14
    |
 LL |     let x = -3.40282357e+38_f32;
    |              ^^^^^^^^^^^^^^^^^^
    |
    = note: the literal `3.40282357e+38_f32` does not fit into the type `f32` and will be converted to `f32::INFINITY`
 
 error: literal out of range for `f32`
-  --> $DIR/lint-type-overflow2.rs:9:14
+  --> $DIR/lint-type-overflow2.rs:10:14
    |
 LL |     let x =  3.40282357e+38_f32;
    |              ^^^^^^^^^^^^^^^^^^
    |
    = note: the literal `3.40282357e+38_f32` does not fit into the type `f32` and will be converted to `f32::INFINITY`
 
 error: literal out of range for `f64`
-  --> $DIR/lint-type-overflow2.rs:10:14
+  --> $DIR/lint-type-overflow2.rs:11:14
    |
 LL |     let x = -1.7976931348623159e+308_f64;
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: the literal `1.7976931348623159e+308_f64` does not fit into the type `f64` and will be converted to `f64::INFINITY`
 
 error: literal out of range for `f64`
-  --> $DIR/lint-type-overflow2.rs:11:14
+  --> $DIR/lint-type-overflow2.rs:12:14
    |
 LL |     let x =  1.7976931348623159e+308_f64;
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: the literal `1.7976931348623159e+308_f64` does not fit into the type `f64` and will be converted to `f64::INFINITY`
 
-error: aborting due to 5 previous errors
+error: aborting due to 5 previous errors; 1 warning emitted