Lint x.ln() / y.ln() => x.log(y)

Author: thiagoarrais

clippy_lints/src/floating_point_arithmetic.rs

@@ -573,6 +573,33 @@ fn check_custom_abs(cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
     }
 }
 
+fn check_logbase(cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
+    // check if expression of the form x.logN() / y.logN()
+    if_chain! {
+        if let ExprKind::Binary(
+            Spanned {
+                node: BinOpKind::Div, ..
+            },
+            lhs,
+            rhs,
+        ) = &expr.kind;
+        if let ExprKind::MethodCall(PathSegment { ident: lmethod_name, .. }, _, ref largs) = lhs.kind;
+        if let ExprKind::MethodCall(PathSegment { ident: rmethod_name, .. }, _, ref rargs) = rhs.kind;
+        if rmethod_name.as_str()  == lmethod_name.as_str();
+        then {
+            span_lint_and_sugg(
+                cx,
+                SUBOPTIMAL_FLOPS,
+                expr.span,
+                "division of logarithms can be calculated more efficiently and accurately",
+                "consider using",
+                format!("{}.log({})", Sugg::hir(cx, &largs[0], ".."), Sugg::hir(cx, &rargs[0], ".."),),
+                Applicability::MachineApplicable,
+            );
+        }
+    }
+}
+
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for FloatingPointArithmetic {
     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'_>) {
         if let ExprKind::MethodCall(ref path, _, args) = &expr.kind {
@@ -592,6 +619,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for FloatingPointArithmetic {
             check_expm1(cx, expr);
             check_mul_add(cx, expr);
             check_custom_abs(cx, expr);
+            check_logbase(cx, expr);
         }
     }
 }

tests/ui/floating_point_logbase.fixed

@@ -0,0 +1,8 @@
+// run-rustfix
+#![warn(clippy::suboptimal_flops)]
+
+fn main() {
+    let x = 3f32;
+    let y = 5f32;
+    let _ = x.log(y);
+}

tests/ui/floating_point_logbase.rs

@@ -0,0 +1,8 @@
+// run-rustfix
+#![warn(clippy::suboptimal_flops)]
+
+fn main() {
+    let x = 3f32;
+    let y = 5f32;
+    let _ = x.ln() / y.ln();
+}

tests/ui/floating_point_logbase.stderr

@@ -0,0 +1,10 @@
+error: division of logarithms can be calculated more efficiently and accurately
+  --> $DIR/floating_point_logbase.rs:7:13
+   |
+LL |     let _ = x.ln() / y.ln();
+   |             ^^^^^^^^^^^^^^^ help: consider using: `x.log(y)`
+   |
+   = note: `-D clippy::suboptimal-flops` implied by `-D warnings`
+
+error: aborting due to previous error
+