False Suggestion for const fn

[Demonstrandum]

cargo clippy -V
#=> clippy 0.0.212 (07f1736 2020-02-11)

warning: this could be a `const fn`
  --> src/vm/unmarshal.rs:23:1
   |
23 | / fn fix_slice_size<T, const N : usize>(slice : &[T]) -> &[T; N] {
24 | |     let ptr = slice.as_ptr() as *const [T; N];
25 | |     unsafe { &*ptr }
26 | | }
   | |_^
   |

This is a false suggestion, since const parameters are not permitted in const fn.

[flip1995]

cc @phansch

[phansch]

This seems easy to fix, if someone wants to take it. We should be able to handle this by copying the conditional from here:

https://github.com/rust-lang/rust/blob/183e893aaae581bd0ab499ba56b6c5e118557dc7/src/librustc_ast_passes/ast_validation.rs#L570-L573

        if let Const::Yes(const_span) = sig.header.constness {
            // Look for const generics and error if we find any.
            for param in &generics.params {
                if let GenericParamKind::Const { .. } = param.kind {

and return early instead of linting here:

rust-clippy/clippy_lints/src/missing_const_for_fn.rs

Lines 93 to 95 in 06f0ab0

	FnKind::ItemFn(_, _, header, ..) => {
	if already_const(header) {
	return;

[krishna-veerareddy]

I can work on this one.