Clippy considers HeaderName to be "interior mutable"

[rawler]

When augmenting the header constants defined in http::header with my own custom header, clippy warns me that HeaderName is "interior mutable".

warning: a const item should never be interior mutable --> src/my_source.rs:39:1	39	const X_REQUEST_ID_HEADER: HeaderName = HeaderName::from_static("x-request-id");	-----^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
make this a static item (maybe with lazy_static)

= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#declare_interior_mutable_const = note: #[warn(clippy::declare_interior_mutable_const)] on by default

[rawler]

Haven't had time to dig into whether HeaderName really is interior-mutable yet, or if clippy is just misdetecting. If it really is correct, I'm guessing the same interior mutability could be detect by compiler when not used as const, preventing some optimizations at compile-time?

Regardess, this right now gets in the way from defining custom HeaderName as const.

[seanmonstar]

See #599.

[rawler]

Ugh. I thought I search for duplicates.