implement lint double_negation

Author: kadiwa4

compiler/rustc_lint/messages.ftl

@@ -79,6 +79,10 @@ 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_negation = use of a double negation
+lint_builtin_double_negation_help = consider adding parentheses between the negation operators or using the `-=` operator if you meant to decrement the value
+lint_builtin_double_negation_note = the prefix `--` could be misinterpreted as a decrement operator which exists in other languages
+
 lint_builtin_ellipsis_inclusive_range_patterns = `...` range patterns are deprecated
     .suggestion = use `..=` for an inclusive range
 

compiler/rustc_lint/src/builtin.rs

@@ -26,10 +26,10 @@ use crate::{
     lints::{
         BuiltinAnonymousParams, BuiltinBoxPointers, BuiltinConstNoMangle,
         BuiltinDeprecatedAttrLink, BuiltinDeprecatedAttrLinkSuggestion, BuiltinDeprecatedAttrUsed,
-        BuiltinDerefNullptr, BuiltinEllipsisInclusiveRangePatternsLint, BuiltinExplicitOutlives,
-        BuiltinExplicitOutlivesSuggestion, BuiltinFeatureIssueNote, BuiltinIncompleteFeatures,
-        BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures, BuiltinKeywordIdents,
-        BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc,
+        BuiltinDerefNullptr, BuiltinDoubleNegation, BuiltinEllipsisInclusiveRangePatternsLint,
+        BuiltinExplicitOutlives, BuiltinExplicitOutlivesSuggestion, BuiltinFeatureIssueNote,
+        BuiltinIncompleteFeatures, BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures,
+        BuiltinKeywordIdents, BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc,
         BuiltinMutablesTransmutes, BuiltinNamedAsmLabel, BuiltinNoMangleGeneric,
         BuiltinNonShorthandFieldPatterns, BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds,
         BuiltinTypeAliasGenericBounds, BuiltinTypeAliasGenericBoundsSuggestion,
@@ -1627,6 +1627,51 @@ impl<'tcx> LateLintPass<'tcx> for TrivialConstraints {
     }
 }
 
+declare_lint! {
+    /// The `double_negation` 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_NEGATION,
+    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.
+    DoubleNegation => [DOUBLE_NEGATION]
+);
+
+impl EarlyLintPass for DoubleNegation {
+    #[inline]
+    fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &ast::Expr) {
+        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_NEGATION, expr.span, BuiltinDoubleNegation);
+        }
+    }
+}
+
 declare_lint_pass!(
     /// Does nothing as a lint pass, but registers some `Lint`s
     /// which are used by other parts of the compiler.
@@ -1646,7 +1691,8 @@ declare_lint_pass!(
         UNSTABLE_FEATURES,
         UNREACHABLE_PUB,
         TYPE_ALIAS_BOUNDS,
-        TRIVIAL_BOUNDS
+        TRIVIAL_BOUNDS,
+        DOUBLE_NEGATION
     ]
 );
 

compiler/rustc_lint/src/lib.rs

@@ -170,6 +170,7 @@ early_lint_methods!(
             IncompleteInternalFeatures: IncompleteInternalFeatures,
             RedundantSemicolons: RedundantSemicolons,
             UnusedDocComment: UnusedDocComment,
+            DoubleNegation: DoubleNegation,
         ]
     ]
 );

compiler/rustc_lint/src/lints.rs

@@ -348,6 +348,12 @@ pub struct BuiltinTrivialBounds<'a> {
     pub predicate: Clause<'a>,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_builtin_double_negation)]
+#[note(lint_builtin_double_negation_note)]
+#[help(lint_builtin_double_negation_help)]
+pub struct BuiltinDoubleNegation;
+
 #[derive(LintDiagnostic)]
 pub enum BuiltinEllipsisInclusiveRangePatternsLint {
     #[diag(lint_builtin_ellipsis_inclusive_range_patterns)]

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

@@ -0,0 +1,30 @@
+warning: use of a double negation
+  --> $DIR/lint-double-negation.rs:6:5
+   |
+LL |     --x;
+   |     ^^^
+   |
+   = note: the prefix `--` could be misinterpreted as a decrement operator which exists in other languages
+   = help: consider adding parentheses between the negation operators or using the `-=` operator if you meant to decrement the value
+   = note: `#[warn(double_negation)]` on by default
+
+warning: use of a double negation
+  --> $DIR/lint-double-negation.rs:7:6
+   |
+LL |     ---x;
+   |      ^^^
+   |
+   = note: the prefix `--` could be misinterpreted as a decrement operator which exists in other languages
+   = help: consider adding parentheses between the negation operators or using the `-=` operator if you meant to decrement the value
+
+warning: use of a double negation
+  --> $DIR/lint-double-negation.rs:8:14
+   |
+LL |     let _y = --(-x);
+   |              ^^^^^^
+   |
+   = note: the prefix `--` could be misinterpreted as a decrement operator which exists in other languages
+   = help: consider adding parentheses between the negation operators or using the `-=` operator if you meant to decrement the value
+
+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 +1,13 @@
+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
+   = help: consider adding parentheses between the negation operators or using the `-=` operator if you meant to decrement the value
+   = note: `#[warn(double_negation)]` on by default
+
 error: literal out of range for `i8`
   --> $DIR/lint-type-overflow2.rs:6:20
    |
@@ -13,36 +23,36 @@ 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