1 files changed, 80 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/neg_multiply.rs b/src/tools/clippy/clippy_lints/src/neg_multiply.rs
new file mode 100644
index 000000000..b087cfb36
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/neg_multiply.rs
@@ -0,0 +1,80 @@
+use clippy_utils::consts::{self, Constant};
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet_with_applicability;
+use clippy_utils::sugg::has_enclosing_paren;
+use if_chain::if_chain;
+use rustc_ast::util::parser::PREC_PREFIX;
+use rustc_errors::Applicability;
+use rustc_hir::{BinOpKind, Expr, ExprKind, UnOp};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::source_map::Span;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for multiplication by -1 as a form of negation.
+    ///
+    /// ### Why is this bad?
+    /// It's more readable to just negate.
+    ///
+    /// ### Known problems
+    /// This only catches integers (for now).
+    ///
+    /// ### Example
+    /// ```rust,ignore
+    /// let a = x * -1;
+    /// ```
+    ///
+    /// Use instead:
+    /// ```rust,ignore
+    /// let a = -x;
+    /// ```
+    #[clippy::version = "pre 1.29.0"]
+    pub NEG_MULTIPLY,
+    style,
+    "multiplying integers by `-1`"
+}
+
+declare_lint_pass!(NegMultiply => [NEG_MULTIPLY]);
+
+impl<'tcx> LateLintPass<'tcx> for NegMultiply {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
+        if let ExprKind::Binary(ref op, left, right) = e.kind {
+            if BinOpKind::Mul == op.node {
+                match (&left.kind, &right.kind) {
+                    (&ExprKind::Unary(..), &ExprKind::Unary(..)) => {},
+                    (&ExprKind::Unary(UnOp::Neg, lit), _) => check_mul(cx, e.span, lit, right),
+                    (_, &ExprKind::Unary(UnOp::Neg, lit)) => check_mul(cx, e.span, lit, left),
+                    _ => {},
+                }
+            }
+        }
+    }
+}
+
+fn check_mul(cx: &LateContext<'_>, span: Span, lit: &Expr<'_>, exp: &Expr<'_>) {
+    if_chain! {
+        if let ExprKind::Lit(ref l) = lit.kind;
+        if consts::lit_to_mir_constant(&l.node, cx.typeck_results().expr_ty_opt(lit)) == Constant::Int(1);
+        if cx.typeck_results().expr_ty(exp).is_integral();
+
+        then {
+            let mut applicability = Applicability::MachineApplicable;
+            let snip = snippet_with_applicability(cx, exp.span, "..", &mut applicability);
+            let suggestion = if exp.precedence().order() < PREC_PREFIX && !has_enclosing_paren(&snip) {
+                format!("-({})", snip)
+            } else {
+                format!("-{}", snip)
+            };
+            span_lint_and_sugg(
+                    cx,
+                    NEG_MULTIPLY,
+                    span,
+                    "this multiplication by -1 can be written more succinctly",
+                    "consider using",
+                    suggestion,
+                    applicability,
+                );
+        }
+    }
+}