Lint logbase for log2 and log10

Author: thiagoarrais

clippy_lints/src/floating_point_arithmetic.rs

@@ -575,6 +575,18 @@ fn check_custom_abs(cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
     }
 }
 
+fn are_same_base_logs(expr_a: &Expr<'_>, expr_b: &Expr<'_>) -> bool {
+    if_chain! {
+        if let ExprKind::MethodCall(PathSegment { ident: method_name_a, .. }, _, _) = expr_a.kind;
+        if let ExprKind::MethodCall(PathSegment { ident: method_name_b, .. }, _, _) = expr_b.kind;
+        then {
+            return method_name_a.as_str() == method_name_b.as_str() && ["ln", "log2", "log10"].contains(&&*method_name_a.as_str());
+        }
+    }
+
+    false
+}
+
 fn check_logbase(cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
     // check if expression of the form x.logN() / y.logN()
     if_chain! {
@@ -585,9 +597,9 @@ fn check_logbase(cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
             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();
+        if are_same_base_logs(lhs, rhs);
+        if let ExprKind::MethodCall(_, _, ref largs) = lhs.kind;
+        if let ExprKind::MethodCall(_, _, ref rargs) = rhs.kind;
         then {
             span_lint_and_sugg(
                 cx,

tests/ui/floating_point_logbase.fixed

@@ -5,4 +5,10 @@ fn main() {
     let x = 3f32;
     let y = 5f32;
     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();
 }

tests/ui/floating_point_logbase.rs

@@ -5,4 +5,10 @@ fn main() {
     let x = 3f32;
     let y = 5f32;
     let _ = x.ln() / y.ln();
+    let _ = x.log2() / y.log2();
+    let _ = x.log10() / y.log10();
+    // 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();
 }

tests/ui/floating_point_logbase.stderr

@@ -6,5 +6,17 @@ LL |     let _ = x.ln() / y.ln();
    |
    = note: `-D clippy::suboptimal-flops` implied by `-D warnings`
 
-error: aborting due to previous error
+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: aborting due to 3 previous errors