Allow by-move binding for patterns with guards that don't reference the bindings

[kmcallister]

I have

match token {
    DoctypeToken(dt) if mode == Initial => ...,
    DoctypeToken(dt) => ...,

    TagToken(x) => ...,
    CommentToken(x) => ...,
    // other cases

This gives

error: cannot bind by-move into a pattern guard

In this situation it should be fine to defer the move until the guard has succeeded.

It's a bit frivolous because I could instead do

match token {
    DoctypeToken(dt) => if mode == Initial {
        ...
    } else {
        ...
    },

    TagToken(x) => ...,
    CommentToken(x) => ...,
    // other cases

But I like having all the branching in one construct, especially when I'm implementing a spec which is written that way. And I don't want to add mode to the match scrutinee because none of the other cases care about it.

See also #4649.

[steveklabnik]

/cc @rust-lang/lang

[durka]

Triage: still annoying.

edit: a more trivial (but quite related) enhancement is to not emit E0008 for pattern guards that don't move anything, let alone the captures. Example.

[huonw]

(Forward-)dupe of #15287, closing.