Idea: Edition visibility for the standard library

[Ixrec]

@binomial0

This is good to hear. I think it would be a good idea to explicitly state that “rustfixability” is a hard requirement for any such edition removals. Removing an item without a working rustfix fix is a breaking change, and should be treated as such.

For what it's worth, we more or less agreed on that point about a year ago in Committing to a Rustfix Tool for All Epochs/Checkpoints Changes, and this section of the epochs RFC explicitly states that consensus in far more precise terms.