Create AttrTarget for attribute parsing

[mejrs]

I went back and forth on the design a bit, this is what I settled on.

r? @jdonszelmann

[rustbot]

jdonszelmann is not on the review rotation at the moment.
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[rustbot]

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann

Some changes occurred in compiler/rustc_attr_data_structures

cc @jdonszelmann

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

[bors]

☔ The latest upstream changes (presumably #143363) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #143645) made this pull request unmergeable. Please resolve the merge conflicts.

[jdonszelmann]

I did look at this version, and asked @JonathanBrouwer to come up with a version that'd combine this PR and my version which I pushed to my fork a while ago. I'll compare the two implementations. That version is #145085

[mejrs]

I did look at this version, and asked @JonathanBrouwer to come up with a version that'd combine this PR and my version which I pushed to my fork a while ago. I'll compare the two implementations. That version is #145085

I see. The primary motivation for my approach where the target enum references the ast of the target is that it lets us move a lot of logic from various passes (not only CheckAttr) into attribute parsing. That's what I'm missing from your and jonathan's versions.

[jdonszelmann]

hmm, I quite like reusing the old Target, and the current version is quite declarative which I liked a lot. I'm sorry, but I'm not sure I'd want to do in this direction.