min_const_generics: ty param in const incorrect help

[lcnr]

#![feature(min_const_generics)]

fn test<T>() -> [u8; std::mem::size_of::<T>()] {
    todo!()
}

emits

error: generic parameters must not be used inside of non trivial constant values
 --> src/lib.rs:3:42
  |
3 | fn test<T>() -> [u8; std::mem::size_of::<T>()] {
  |                                          ^ non-trivial anonymous constants must not depend on the parameter `T`
  |
  = help: it is currently only allowed to use either `T` or `{ T }` as generic constants

The help is incorrect here, as type parameters in constants are not supported at all with min_const_generics.

The error is emitted here

rust/compiler/rustc_resolve/src/diagnostics.rs

Lines 469 to 485 in b5f55b7

	ResolutionError::ParamInNonTrivialAnonConst(name) => {
	let mut err = self.session.struct_span_err(
	span,
	"generic parameters must not be used inside of non trivial constant values",
	);
	err.span_label(
	span,
	&format!(
	"non-trivial anonymous constants must not depend on the parameter `{}`",
	name
	),
	);
	err.help(
	&format!("it is currently only allowed to use either `{0}` or `{{ {0} }}` as generic constants", name)
	);
	err
	}

We probably want to change ParamInNonTrivialAnonConst to instead contain an Option which is only Some for const parameters.

For type parameters we can instead emit a note saying

note: type parameters are currently not permitted in anonymous constants

If you need more help feel free to ask either here on on zulip.

[hameerabbasi]

@rustbot claim