Auto merge of #5602 - ebroto:issue_3430, r=phansch

identity_op: allow `1 << 0`

I went for accepting `1 << 0` verbatim instead of something more general as it seems to be what everyone in the issue thread needed.

changelog: identity_op: allow `1 << 0` as it's a common pattern in bit manipulation code.

Fixes #3430

clippy_lints/src/identity_op.rs

@@ -1,4 +1,5 @@
-use rustc_hir::{BinOpKind, Expr, ExprKind};
+use if_chain::if_chain;
+use rustc_hir::{BinOp, BinOpKind, Expr, ExprKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
@@ -32,7 +33,10 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for IdentityOp {
         if e.span.from_expansion() {
             return;
         }
-        if let ExprKind::Binary(ref cmp, ref left, ref right) = e.kind {
+        if let ExprKind::Binary(cmp, ref left, ref right) = e.kind {
+            if is_allowed(cx, cmp, left, right) {
+                return;
+            }
             match cmp.node {
                 BinOpKind::Add | BinOpKind::BitOr | BinOpKind::BitXor => {
                     check(cx, left, 0, e.span, right.span);
@@ -54,6 +58,20 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for IdentityOp {
     }
 }
 
+fn is_allowed(cx: &LateContext<'_, '_>, cmp: BinOp, left: &Expr<'_>, right: &Expr<'_>) -> bool {
+    // `1 << 0` is a common pattern in bit manipulation code
+    if_chain! {
+        if let BinOpKind::Shl = cmp.node;
+        if let Some(Constant::Int(0)) = constant_simple(cx, cx.tables, right);
+        if let Some(Constant::Int(1)) = constant_simple(cx, cx.tables, left);
+        then {
+            return true;
+        }
+    }
+
+    false
+}
+
 #[allow(clippy::cast_possible_wrap)]
 fn check(cx: &LateContext<'_, '_>, e: &Expr<'_>, m: i8, span: Span, arg: Span) {
     if let Some(Constant::Int(v)) = constant_simple(cx, cx.tables, e) {

tests/ui/identity_op.rs

@@ -33,4 +33,9 @@ fn main() {
 
     let u: u8 = 0;
     u & 255;
+
+    1 << 0; // no error, this case is allowed, see issue 3430
+    42 << 0;
+    1 >> 0;
+    42 >> 0;
 }

tests/ui/identity_op.stderr

@@ -48,5 +48,23 @@ error: the operation is ineffective. Consider reducing it to `u`
 LL |     u & 255;
    |     ^^^^^^^
 
-error: aborting due to 8 previous errors
+error: the operation is ineffective. Consider reducing it to `42`
+  --> $DIR/identity_op.rs:38:5
+   |
+LL |     42 << 0;
+   |     ^^^^^^^
+
+error: the operation is ineffective. Consider reducing it to `1`
+  --> $DIR/identity_op.rs:39:5
+   |
+LL |     1 >> 0;
+   |     ^^^^^^
+
+error: the operation is ineffective. Consider reducing it to `42`
+  --> $DIR/identity_op.rs:40:5
+   |
+LL |     42 >> 0;
+   |     ^^^^^^^
+
+error: aborting due to 11 previous errors