Use a const ParamEnv when in default_method_body_is_const

[fee1-dead]

r? @oli-obk

This PR fixes the param_env function to return constness: Const correctly for trait methods marked with #[default_method_body_is_const]. The snippet below is erroneously accepted by the compiler and has been fixed by this change. (Playground)

#![feature(const_fn_trait_bound)]
#![feature(const_trait_impl)]

trait Tr {}
impl Tr for () {}

const fn foo<T>() where T: ~const Tr {}

pub trait Foo {
    #[default_method_body_is_const]
    fn foo() {
        foo::<()>();
    }
}

[oli-obk]

@bors r+ rollup

[bors]

📌 Commit a1f91aa has been approved by oli-obk