remove overzealous Box<ZeroSizeType> optimization

[qmx]

No description provided.

[rust-highfive]

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[eddyb]

r? @arielb1

[arielb1]

@bors r+

Let's see whether this is important in practice.

[bors]

📌 Commit 916ccc5 has been approved by arielb1

[bors]

💡 This pull request was already approved, no need to approve it again.

• There's another pull request that is currently being tested, blocking this pull request: Remove deprecated lang items #44526

[bors]

📌 Commit 916ccc5 has been approved by arielb1

[bstrie]

What bug is this fixing? "Overzealous" how?

[eddyb]

@bstrie This was leftover cruft from before @arielb1 moved drop glue to MIR shims. All it does is skip very little code, for a very special case. It was annoying because of two reasons:

• it was used from monomorphize::resolve which @qmx was trying to move to librustc and generalize (to use in MIR inlining), and if you have a type parameter you can't check its size
• it's a Box special case, the fewer of those, the better