Fix float_cmp false positive when comparing signum

f1.signum() == f2.signum()
f1.signum() != f2.signum()
should not trigger a warning.

clippy_lints/src/misc.rs

@@ -364,6 +364,12 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MiscLints {
                     if is_allowed(cx, left) || is_allowed(cx, right) {
                         return;
                     }
+
+                    // Allow comparing the results of signum()
+                    if is_signum(cx, left) && is_signum(cx, right) {
+                        return;
+                    }
+
                     if let Some(name) = get_item_name(cx, expr) {
                         let name = name.as_str();
                         if name == "eq"
@@ -493,6 +499,27 @@ fn is_allowed<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) -> bool {
     }
 }
 
+// Return true if `expr` is the result of `signum()` invoked on a float value.
+fn is_signum(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
+    // The negation of a signum is still a signum
+    if let ExprKind::Unary(UnNeg, ref child_expr) = expr.node {
+        return is_signum(cx, &child_expr);
+    }
+
+    if_chain! {
+        if let ExprKind::MethodCall(ref method_name, _, ref expressions) = expr.node;
+        if sym!(signum) == method_name.ident.name;
+        // Check that the receiver of the signum() is a float (expressions[0] is the receiver of
+        // the method call)
+        if is_float(cx, &expressions[0]);
+        then {
+            true
+        } else {
+            false
+        }
+    }
+}
+
 fn is_float(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
     matches!(walk_ptrs_ty(cx.tables.expr_ty(expr)).sty, ty::Float(_))
 }

tests/ui/float_cmp.rs

@@ -77,4 +77,23 @@ fn main() {
     let b: *const f32 = xs.as_ptr();
 
     assert_eq!(a, b); // no errors
+
+    // no errors - comparing signums is ok
+    let x32 = 3.21f32;
+    1.23f32.signum() == x32.signum();
+    1.23f32.signum() == -(x32.signum());
+    1.23f32.signum() == 3.21f32.signum();
+
+    1.23f32.signum() != x32.signum();
+    1.23f32.signum() != -(x32.signum());
+    1.23f32.signum() != 3.21f32.signum();
+
+    let x64 = 3.21f64;
+    1.23f64.signum() == x64.signum();
+    1.23f64.signum() == -(x64.signum());
+    1.23f64.signum() == 3.21f64.signum();
+
+    1.23f64.signum() != x64.signum();
+    1.23f64.signum() != -(x64.signum());
+    1.23f64.signum() != 3.21f64.signum();
 }