Auto merge of #4026 - cemiloten:working-on-#3981, r=flip1995

Attempt to fix #3981

Fixes #3981

Hi, hopefully this is correct, happy to have feedback.

changelog: Don't trigger `unnecessary_cast` inside a macro

3 files changed, 18 insertions, 3 deletions

diff --git a/clippy_lints/src/types.rs b/clippy_lints/src/types.rs
index 21b6fc2fee6..d69e1353418 100644
--- a/clippy_lints/src/types.rs
+++ b/clippy_lints/src/types.rs
@@ -1110,6 +1110,9 @@ fn fp_ty_mantissa_nbits(typ: Ty<'_>) -> u32 {
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Casts {
     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
+        if in_macro(expr.span) {
+            return;
+        }
         if let ExprKind::Cast(ref ex, _) = expr.node {
             let (cast_from, cast_to) = (cx.tables.expr_ty(ex), cx.tables.expr_ty(expr));
             lint_fn_to_numeric_cast(cx, expr, ex, cast_from, cast_to);
diff --git a/tests/ui/cast.rs b/tests/ui/cast.rs
index 914296644e6..16da099bd21 100644
--- a/tests/ui/cast.rs
+++ b/tests/ui/cast.rs
@@ -48,6 +48,18 @@ fn main() {
     1f32 as f32;
     false as bool;
     &1i32 as &i32;
+    // macro version
+    macro_rules! foo {
+        ($a:ident, $b:ident) => {
+            pub fn $a() -> $b {
+                1 as $b
+            }
+        };
+    }
+    foo!(a, i32);
+    foo!(b, f32);
+    foo!(c, f64);
+
     // casting integer literal to float is unnecessary
     100 as f32;
     100 as f64;
diff --git a/tests/ui/cast.stderr b/tests/ui/cast.stderr
index 28bcfacd858..42232808c9b 100644
--- a/tests/ui/cast.stderr
+++ b/tests/ui/cast.stderr
@@ -159,19 +159,19 @@ LL |     false as bool;
    |     ^^^^^^^^^^^^^
 
 error: casting integer literal to f32 is unnecessary
-  --> $DIR/cast.rs:52:5
+  --> $DIR/cast.rs:64:5
    |
 LL |     100 as f32;
    |     ^^^^^^^^^^ help: try: `100_f32`
 
 error: casting integer literal to f64 is unnecessary
-  --> $DIR/cast.rs:53:5
+  --> $DIR/cast.rs:65:5
    |
 LL |     100 as f64;
    |     ^^^^^^^^^^ help: try: `100_f64`
 
 error: casting integer literal to f64 is unnecessary
-  --> $DIR/cast.rs:54:5
+  --> $DIR/cast.rs:66:5
    |
 LL |     100_i32 as f64;
    |     ^^^^^^^^^^^^^^ help: try: `100_f64`