Auto merge of #8144 - Gh0stm4chine:master, r=xFrednet

Add suggestion for neg_multiply lint

This fixes #8115 by adding a suggestion for [neg_multiply].

My first issue on Github, any feedback or input is welcome 😃

changelog: create a suggestion for `neg_multiply`

clippy_lints/src/neg_multiply.rs

@@ -1,6 +1,8 @@
 use clippy_utils::consts::{self, Constant};
-use clippy_utils::diagnostics::span_lint;
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet_with_applicability;
 use if_chain::if_chain;
+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};
@@ -18,12 +20,16 @@ declare_clippy_lint! {
     ///
     /// ### Example
     /// ```ignore
-    /// x * -1
+    /// // Bad
+    /// let a = x * -1;
+    ///
+    /// // Good
+    /// let b = -x;
     /// ```
     #[clippy::version = "pre 1.29.0"]
     pub NEG_MULTIPLY,
     style,
-    "multiplying integers with `-1`"
+    "multiplying integers by `-1`"
 }
 
 declare_lint_pass!(NegMultiply => [NEG_MULTIPLY]);
@@ -49,8 +55,19 @@ fn check_mul(cx: &LateContext<'_>, span: Span, lit: &Expr<'_>, exp: &Expr<'_>) {
         if let ExprKind::Lit(ref l) = lit.kind;
         if consts::lit_to_constant(&l.node, cx.typeck_results().expr_ty_opt(lit)) == Constant::Int(1);
         if cx.typeck_results().expr_ty(exp).is_integral();
+
         then {
-            span_lint(cx, NEG_MULTIPLY, span, "negation by multiplying with `-1`");
+            let mut applicability = Applicability::MachineApplicable;
+            let suggestion = format!("-{}", snippet_with_applicability(cx, exp.span, "..", &mut applicability));
+            span_lint_and_sugg(
+                    cx,
+                    NEG_MULTIPLY,
+                    span,
+                    "this multiplication by -1 can be written more succinctly",
+                    "consider using",
+                    suggestion,
+                    applicability,
+                );
         }
     }
 }

tests/ui/neg_multiply.fixed

@@ -0,0 +1,45 @@
+// run-rustfix
+#![warn(clippy::neg_multiply)]
+#![allow(clippy::no_effect, clippy::unnecessary_operation, clippy::precedence)]
+#![allow(unused)]
+
+use std::ops::Mul;
+
+struct X;
+
+impl Mul<isize> for X {
+    type Output = X;
+
+    fn mul(self, _r: isize) -> Self {
+        self
+    }
+}
+
+impl Mul<X> for isize {
+    type Output = X;
+
+    fn mul(self, _r: X) -> X {
+        X
+    }
+}
+
+fn main() {
+    let x = 0;
+
+    -x;
+
+    -x;
+
+    100 + -x;
+
+    -(100 + x);
+
+    -17;
+
+    0xcafe | -0xff00;
+
+    -1 * -1; // should be ok
+
+    X * -1; // should be ok
+    -1 * X; // should also be ok
+}

tests/ui/neg_multiply.rs

@@ -1,5 +1,7 @@
+// run-rustfix
 #![warn(clippy::neg_multiply)]
-#![allow(clippy::no_effect, clippy::unnecessary_operation)]
+#![allow(clippy::no_effect, clippy::unnecessary_operation, clippy::precedence)]
+#![allow(unused)]
 
 use std::ops::Mul;
 
@@ -28,6 +30,14 @@ fn main() {
 
     -1 * x;
 
+    100 + x * -1;
+
+    (100 + x) * -1;
+
+    -1 * 17;
+
+    0xcafe | 0xff00 * -1;
+
     -1 * -1; // should be ok
 
     X * -1; // should be ok

tests/ui/neg_multiply.stderr

@@ -1,16 +1,40 @@
-error: negation by multiplying with `-1`
+error: this multiplication by -1 can be written more succinctly
-  --> $DIR/neg_multiply.rs:27:5
+  --> $DIR/neg_multiply.rs:29:5
    |
 LL |     x * -1;
-   |     ^^^^^^
+   |     ^^^^^^ help: consider using: `-x`
    |
    = note: `-D clippy::neg-multiply` implied by `-D warnings`
 
-error: negation by multiplying with `-1`
+error: this multiplication by -1 can be written more succinctly
-  --> $DIR/neg_multiply.rs:29:5
+  --> $DIR/neg_multiply.rs:31:5
    |
 LL |     -1 * x;
-   |     ^^^^^^
+   |     ^^^^^^ help: consider using: `-x`
 
-error: aborting due to 2 previous errors
+error: this multiplication by -1 can be written more succinctly
+  --> $DIR/neg_multiply.rs:33:11
+   |
+LL |     100 + x * -1;
+   |           ^^^^^^ help: consider using: `-x`
+
+error: this multiplication by -1 can be written more succinctly
+  --> $DIR/neg_multiply.rs:35:5
+   |
+LL |     (100 + x) * -1;
+   |     ^^^^^^^^^^^^^^ help: consider using: `-(100 + x)`
+
+error: this multiplication by -1 can be written more succinctly
+  --> $DIR/neg_multiply.rs:37:5
+   |
+LL |     -1 * 17;
+   |     ^^^^^^^ help: consider using: `-17`
+
+error: this multiplication by -1 can be written more succinctly
+  --> $DIR/neg_multiply.rs:39:14
+   |
+LL |     0xcafe | 0xff00 * -1;
+   |              ^^^^^^^^^^^ help: consider using: `-0xff00`
+
+error: aborting due to 6 previous errors