Disable incremental for std crates

[ehuss]

Incremental is not needed for std crates, so it should probably not be used.

I'm torn on exactly how to implement. Some options:

• Pass a flag to Profiles::get_profile
• Add an is_std flag to Unit, and check that when generating flags. This flag may be needed for other reasons, and would end up being the simplest change, but a bit of a hack.
• Mutate the std unit profiles after they have all been created (very hacky).

[alexcrichton]

Heh an excellent point! I think you're referring to this block, right? I think that this sort of logic (is_path()) is taken into account in at least one other place (passing --cap-lints allow vs not), but that may be the only other place that it's passed.

I wonder if the Package could be updated here? Perhaps something like a "can this package be edited" property which enables/disables incremental/warnings and is default to true for is_path, but for libstd deps we can go through and rewrite these fields to "no, you can't edit this"

[ehuss]

Sounds like a good idea to me.

[ehuss]

Closed by rust-lang/cargo#8177.