implement lint `double_negations`

1 files changed, 64 insertions, 11 deletions

diff --git a/compiler/rustc_lint/src/builtin.rs b/compiler/rustc_lint/src/builtin.rs
index 3da9ef3ff7c..c03de687a33 100644
--- a/compiler/rustc_lint/src/builtin.rs
+++ b/compiler/rustc_lint/src/builtin.rs
@@ -49,16 +49,16 @@ use rustc_trait_selection::traits::{self};
 use crate::errors::BuiltinEllipsisInclusiveRangePatterns;
 use crate::lints::{
     BuiltinAnonymousParams, BuiltinConstNoMangle, BuiltinDeprecatedAttrLink,
-    BuiltinDeprecatedAttrLinkSuggestion, BuiltinDerefNullptr,
-    BuiltinEllipsisInclusiveRangePatternsLint, BuiltinExplicitOutlives,
-    BuiltinExplicitOutlivesSuggestion, BuiltinFeatureIssueNote, BuiltinIncompleteFeatures,
-    BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures, BuiltinKeywordIdents,
-    BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc, BuiltinMutablesTransmutes,
-    BuiltinNoMangleGeneric, BuiltinNonShorthandFieldPatterns, BuiltinSpecialModuleNameUsed,
-    BuiltinTrivialBounds, BuiltinTypeAliasBounds, BuiltinUngatedAsyncFnTrackCaller,
-    BuiltinUnpermittedTypeInit, BuiltinUnpermittedTypeInitSub, BuiltinUnreachablePub,
-    BuiltinUnsafe, BuiltinUnstableFeatures, BuiltinUnusedDocComment, BuiltinUnusedDocCommentSub,
-    BuiltinWhileTrue, InvalidAsmLabel,
+    BuiltinDeprecatedAttrLinkSuggestion, BuiltinDerefNullptr, BuiltinDoubleNegations,
+    BuiltinDoubleNegationsAddParens, BuiltinEllipsisInclusiveRangePatternsLint,
+    BuiltinExplicitOutlives, BuiltinExplicitOutlivesSuggestion, BuiltinFeatureIssueNote,
+    BuiltinIncompleteFeatures, BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures,
+    BuiltinKeywordIdents, BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc,
+    BuiltinMutablesTransmutes, BuiltinNoMangleGeneric, BuiltinNonShorthandFieldPatterns,
+    BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds, BuiltinTypeAliasBounds,
+    BuiltinUngatedAsyncFnTrackCaller, BuiltinUnpermittedTypeInit, BuiltinUnpermittedTypeInitSub,
+    BuiltinUnreachablePub, BuiltinUnsafe, BuiltinUnstableFeatures, BuiltinUnusedDocComment,
+    BuiltinUnusedDocCommentSub, BuiltinWhileTrue, InvalidAsmLabel,
 };
 use crate::nonstandard_style::{MethodLateContext, method_context};
 use crate::{
@@ -1568,6 +1568,58 @@ 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.
@@ -1586,7 +1638,8 @@ declare_lint_pass!(
         UNSTABLE_FEATURES,
         UNREACHABLE_PUB,
         TYPE_ALIAS_BOUNDS,
-        TRIVIAL_BOUNDS
+        TRIVIAL_BOUNDS,
+        DOUBLE_NEGATIONS
     ]
 );