Do not lint when cast is coming from signum method call

clippy_lints/src/casts/cast_possible_truncation.rs

@@ -1,12 +1,20 @@
 use clippy_utils::diagnostics::span_lint;
+use clippy_utils::expr_or_init;
 use clippy_utils::ty::is_isize_or_usize;
-use rustc_hir::Expr;
+use rustc_hir::{Expr, ExprKind};
 use rustc_lint::LateContext;
 use rustc_middle::ty::{self, FloatTy, Ty};
 
 use super::{utils, CAST_POSSIBLE_TRUNCATION};
 
-pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    // do not lint if cast comes from a `signum` function
+    if let ExprKind::MethodCall(path, ..) = expr_or_init(cx, cast_expr).kind {
+        if path.ident.name.as_str() == "signum" {
+            return;
+        }
+    }
+
     let msg = match (cast_from.is_integral(), cast_to.is_integral()) {
         (true, true) => {
             let from_nbits = utils::int_ty_to_nbits(cast_from, cx.tcx);

clippy_lints/src/casts/mod.rs

@@ -427,7 +427,7 @@ fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
             fn_to_numeric_cast::check(cx, expr, cast_expr, cast_from, cast_to);
             fn_to_numeric_cast_with_truncation::check(cx, expr, cast_expr, cast_from, cast_to);
             if cast_from.is_numeric() && cast_to.is_numeric() && !in_external_macro(cx.sess(), expr.span) {
-                cast_possible_truncation::check(cx, expr, cast_from, cast_to);
+                cast_possible_truncation::check(cx, expr, cast_expr, cast_from, cast_to);
                 cast_possible_wrap::check(cx, expr, cast_from, cast_to);
                 cast_precision_loss::check(cx, expr, cast_from, cast_to);
                 cast_lossless::check(cx, expr, cast_expr, cast_from, cast_to);

tests/ui/cast.rs

@@ -92,4 +92,12 @@ fn main() {
     (1i64).checked_rem_euclid(-1i64).unwrap() as u64;
     (1i64).checked_rem_euclid(-1i64).unwrap() as u128;
     (1isize).checked_rem_euclid(-1isize).unwrap() as usize;
+
+    // no lint for `cast_possible_truncation`
+    // with `signum` method call (see issue #5395)
+    let x: i64 = 5;
+    let _ = x.signum() as i32;
+
+    let s = x.signum();
+    let _ = s as i32;
 }