remove ?Sized bounds from Index

[matklad]

I've noticed that we have an Idx: ?Sized bound on the index in the Index, which seems strange given that we accept index by value. My guess is that it was meant to be removed in #23601, but was overlooked.

If I remove this bound, ./x.py src/libstd/ src/libcore/ passes, which means at least that this is not covered by test.

I think there's three things we can do here:

• run crater with the bound removed to check if there are any regressions, and merge this, to be consistent with other operator traits
• run crater, get regressions, write a test for this with a note that "hey, we tried to fix it, its unfixable"
• decide, in the light of by-value DSTs, that this is a feature rather than a bug, and add a test

cc @rust-lang/libs

EDIT: the forth alternative is that there exist a genuine reason why this is the case, but I failed to see it :D

[rust-highfive]

r? @joshtriplett

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[Centril]

So this was not the PR I expected.

My guess is that it was meant to be removed in #23601, but was overlooked.

Perhaps. But if so, this is to me a happy accident.

run crater with the bound removed to check if there are any regressions, and merge this, to be consistent with other operator traits

I don't mind running crater, but I would not want to remove the bound when unsized_locals isn't stable and because its a future-compat hazard. Consistency is not a sufficient justification for that imo even if you find zero regressions.

decide, in the light of by-value DSTs, that this is a feature rather than a bug, and add a test

We can add a test now and later remove it if we change our minds.

[Centril]

@bors try

[bors]

⌛️ Trying commit a6b130e with merge 47f4f94...

[bors]

☀️ Try build successful - checks-travis
Build commit: 47f4f94

[Centril]

@craterbot run mode=check-only

[craterbot]

👌 Experiment pr-59527 created and queued.
🤖 Automatically detected try build 47f4f94
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-59527 is now running on agent aws-3-tmp.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[sfackler]

What concrete problem is this solving, exactly? It's not clear to me why we should invest crater/person time on removing this bound.

[Centril]

See discussion in https://internals.rust-lang.org/t/why-ops-index-idx-allows-unsized-idx/9738.

[matklad]

@sfackler I agree that this is basically a non-issue, except for the mere fact that we have a random untested bound in std’s public API.

[craterbot]

🎉 Experiment pr-59527 is completed!
📊 0 regressed and 0 fixed (50551 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the blacklist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more