let unnecessary_cast work for trivial non_literal expressions

Signed-off-by: TennyZhuang <zty0826@gmail.com>

4 files changed, 37 insertions, 2 deletions

diff --git a/clippy_lints/src/casts/unnecessary_cast.rs b/clippy_lints/src/casts/unnecessary_cast.rs
index 031cedb9072..071fab1165d 100644
--- a/clippy_lints/src/casts/unnecessary_cast.rs
+++ b/clippy_lints/src/casts/unnecessary_cast.rs
@@ -31,8 +31,10 @@ pub(super) fn check<'tcx>(
         }
     }
 
+    let cast_str = snippet_opt(cx, cast_expr.span).unwrap_or_default();
+
     if let Some(lit) = get_numeric_literal(cast_expr) {
-        let literal_str = snippet_opt(cx, cast_expr.span).unwrap_or_default();
+        let literal_str = cast_str;
 
         if_chain! {
             if let LitKind::Int(n, _) = lit.node;
@@ -81,6 +83,17 @@ pub(super) fn check<'tcx>(
                 }
             },
         }
+    } else if cast_from.kind() == cast_to.kind() && !in_external_macro(cx.sess(), expr.span) {
+        span_lint_and_sugg(
+            cx,
+            UNNECESSARY_CAST,
+            expr.span,
+            &format!("casting to the same type is unnecessary (`{cast_from}` -> `{cast_to}`)"),
+            "try",
+            cast_str,
+            Applicability::MachineApplicable,
+        );
+        return true;
     }
 
     false
diff --git a/tests/ui/unnecessary_cast.fixed b/tests/ui/unnecessary_cast.fixed
index 70ec3e94380..94dc9642726 100644
--- a/tests/ui/unnecessary_cast.fixed
+++ b/tests/ui/unnecessary_cast.fixed
@@ -103,4 +103,12 @@ mod fixable {
         #[allow(clippy::precedence)]
         let _: f64 = -8.0_f64.exp(); // should suggest `-8.0_f64.exp()` here not to change code behavior
     }
+
+    fn issue_9562_non_literal() {
+        fn foo() -> f32 {
+            0.
+        }
+
+        let _num = foo();
+    }
 }
diff --git a/tests/ui/unnecessary_cast.rs b/tests/ui/unnecessary_cast.rs
index 36c1a87fab6..e5150256f69 100644
--- a/tests/ui/unnecessary_cast.rs
+++ b/tests/ui/unnecessary_cast.rs
@@ -103,4 +103,12 @@ mod fixable {
         #[allow(clippy::precedence)]
         let _: f64 = -(8.0 as f64).exp(); // should suggest `-8.0_f64.exp()` here not to change code behavior
     }
+
+    fn issue_9562_non_literal() {
+        fn foo() -> f32 {
+            0.
+        }
+
+        let _num = foo() as f32;
+    }
 }
diff --git a/tests/ui/unnecessary_cast.stderr b/tests/ui/unnecessary_cast.stderr
index a52b92c339c..e5c3dd5e53f 100644
--- a/tests/ui/unnecessary_cast.stderr
+++ b/tests/ui/unnecessary_cast.stderr
@@ -174,5 +174,11 @@ error: casting float literal to `f64` is unnecessary
 LL |         let _: f64 = -(8.0 as f64).exp(); // should suggest `-8.0_f64.exp()` here not to change code behavior
    |                       ^^^^^^^^^^^^ help: try: `8.0_f64`
 
-error: aborting due to 29 previous errors
+error: casting to the same type is unnecessary (`f32` -> `f32`)
+  --> $DIR/unnecessary_cast.rs:112:20
+   |
+LL |         let _num = foo() as f32;
+   |                    ^^^^^^^^^^^^ help: try: `foo()`
+
+error: aborting due to 30 previous errors