Auto merge of #3700 - phansch:would_you_like_some_help_with_this_const_fn, r=oli-obk

Prevent incorrect cast_lossless suggestion in const_fn

`::from` is not a const fn, so applying the suggestion of
`cast_lossless` would fail to compile. The fix is to skip the lint if
the cast is found inside a const fn.

Fixes #3656

clippy_lints/src/utils/mod.rs

@@ -82,6 +82,10 @@ pub fn in_constant(cx: &LateContext<'_, '_>, id: NodeId) -> bool {
             node: ItemKind::Static(..),
             ..
         }) => true,
+        Node::Item(&Item {
+            node: ItemKind::Fn(_, header, ..),
+            ..
+        }) => header.constness == Constness::Const,
         _ => false,
     }
 }