Auto merge of #34923 - eddyb:deny-fn-item-transmute, r=nikomatsakis
Auto merge of #34923 - eddyb:deny-fn-item-transmute, r=nikomatsakis

Deny (by default) transmuting from fn item types to pointer-sized types.

This sets the #19925 lint (transmute from zero-sized fn item type) to `deny` by default.
Technically a `[breaking-change]`, but will not affect dependent crates because of `--cap-lints`.

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