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

Author: thiagoarrais

clippy_lints/src/floating_point_arithmetic.rs

@@ -573,6 +573,50 @@ fn check_custom_abs(cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
     }
 }
 
+fn are_same_base_logs(cx: &LateContext<'_, '_>, expr_a: &Expr<'_>, expr_b: &Expr<'_>) -> bool {
+    if_chain! {
+        if let ExprKind::MethodCall(PathSegment { ident: method_name_a, .. }, _, ref args_a) = expr_a.kind;
+        if let ExprKind::MethodCall(PathSegment { ident: method_name_b, .. }, _, ref args_b) = expr_b.kind;
+        then {
+            return method_name_a.as_str() == method_name_b.as_str() &&
+                args_a.len() == args_b.len() &&
+                (
+                    ["ln", "log2", "log10"].contains(&&*method_name_a.as_str()) ||
+                    method_name_a.as_str() == "log" && args_a.len() == 2 && are_exprs_equal(cx, &args_a[1], &args_b[1])
+                );
+        }
+    }
+
+    false
+}
+
+fn check_log_division(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 are_same_base_logs(cx, lhs, rhs);
+        if let ExprKind::MethodCall(_, _, ref largs) = lhs.kind;
+        if let ExprKind::MethodCall(_, _, ref rargs) = rhs.kind;
+        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 +636,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for FloatingPointArithmetic {
             check_expm1(cx, expr);
             check_mul_add(cx, expr);
             check_custom_abs(cx, expr);
+            check_log_division(cx, expr);
         }
     }
 }

tests/ui/floating_point_logbase.fixed

@@ -0,0 +1,16 @@
+// run-rustfix
+#![warn(clippy::suboptimal_flops)]
+
+fn main() {
+    let x = 3f32;
+    let y = 5f32;
+    let _ = x.log(y);
+    let _ = x.log(y);
+    let _ = x.log(y);
+    let _ = x.log(y);
+    // Cases where the lint shouldn't be applied
+    let _ = x.ln() / y.powf(3.2);
+    let _ = x.powf(3.2) / y.powf(3.2);
+    let _ = x.powf(3.2) / y.ln();
+    let _ = x.log(5f32) / y.log(7f32);
+}

tests/ui/floating_point_logbase.rs

@@ -0,0 +1,16 @@
+// run-rustfix
+#![warn(clippy::suboptimal_flops)]
+
+fn main() {
+    let x = 3f32;
+    let y = 5f32;
+    let _ = x.ln() / y.ln();
+    let _ = x.log2() / y.log2();
+    let _ = x.log10() / y.log10();
+    let _ = x.log(5f32) / y.log(5f32);
+    // Cases where the lint shouldn't be applied
+    let _ = x.ln() / y.powf(3.2);
+    let _ = x.powf(3.2) / y.powf(3.2);
+    let _ = x.powf(3.2) / y.ln();
+    let _ = x.log(5f32) / y.log(7f32);
+}

tests/ui/floating_point_logbase.stderr

@@ -0,0 +1,28 @@
+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: division of logarithms can be calculated more efficiently and accurately
+  --> $DIR/floating_point_logbase.rs:8:13
+   |
+LL |     let _ = x.log2() / y.log2();
+   |             ^^^^^^^^^^^^^^^^^^^ help: consider using: `x.log(y)`
+
+error: division of logarithms can be calculated more efficiently and accurately
+  --> $DIR/floating_point_logbase.rs:9:13
+   |
+LL |     let _ = x.log10() / y.log10();
+   |             ^^^^^^^^^^^^^^^^^^^^^ help: consider using: `x.log(y)`
+
+error: division of logarithms can be calculated more efficiently and accurately
+  --> $DIR/floating_point_logbase.rs:10:13
+   |
+LL |     let _ = x.log(5f32) / y.log(5f32);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `x.log(y)`
+
+error: aborting due to 4 previous errors
+