Rollup merge of #75502 - ecstatic-morse:implicit-promotion-in-const-fn, r=RalfJung
Rollup merge of #75502 - ecstatic-morse:implicit-promotion-in-const-fn, r=RalfJung

Use implicit (not explicit) rules for promotability by default in `const fn`

For crater run. See https://github.com/rust-lang/const-eval/pull/54#discussion_r469995552.

cc #75586

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust