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Vec drop and truncate: drop using raw slice *mut [T] #71148

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merged 2 commits into from May 1, 2020

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@bluss bluss commented Apr 14, 2020

By creating a *mut [T] directly (without going through &mut [T]), avoid
questions of validity of the contents of the slice.

Consider the following risky code:

unsafe {
    let mut v = Vec::<bool>::with_capacity(16);
    v.set_len(16);
}

The intention is that with this change, we avoid one of the soundness
questions about the above snippet, because Vec::drop no longer
produces a mutable slice of the vector's contents.

r? @RalfJung

By creating a *mut [T] directly (without going through &mut [T]), avoid
questions of validity of the contents of the slice.

Consider the following risky code:

```rust
unsafe {
    let mut v = Vec::<bool>::with_capacity(16);
    v.set_len(16);
}
```

The intention is that with this change, the above snippet will be
sound because Vec::drop does no longer produces a mutable slice of
the vector's contents.
@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 14, 2020
@RalfJung
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The intention is that with this change, the above snippet will be
sound because Vec::drop no longer produces a mutable slice of
the vector's contents.

I am not sure if this change actually achieves that -- that depends on whether drop_in_place may be called on uninitialized data for types that have no drop glue.

@bluss bluss added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 14, 2020
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bluss commented Apr 14, 2020

I am not sure if this change actually achieves that -- that depends on whether drop_in_place may be called on uninitialized data for types that have no drop glue.

This just seems like the best choice; it should be a no-op if the type does not have a drop glue.
But it does conflict(?) with the current documentation for the drop_in_place intrinsic - but there is no reason the pointer should actually be read from, for a Copy type.

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"Valid for reads" just means that you may read the memory (the pointer doesn't dangle and is still Stacked-Borrows-valid), it doesn't say anything about the data stored there.

I am okay landing this just based on the principle of using the weakest sufficient type. I am just saying to achieve what you stated in the OP, you should get official blessing that drop_in_place works like that (and I cannot grant that blessing, that's lang team territory).

@@ -2379,7 +2379,7 @@ unsafe impl<#[may_dangle] T> Drop for Vec<T> {
fn drop(&mut self) {
unsafe {
// use drop for [T]
ptr::drop_in_place(&mut self[..]);
ptr::drop_in_place(ptr::slice_from_raw_parts_mut(self.as_mut_ptr(), self.len))
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Might be worth adding a brief comment for why we use this complicated operation here instead of the simple one.

Really, already for #70558, what we should really have is a way to get a raw slice for the entire Vec, and then subslicing on raw slices. Then we would not have to call slice_from_raw_parts for any of the methods changed in that PR or here.

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Comment added, though it's very vague since I don't dare say so much about what it actually allows

@joelpalmer joelpalmer added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 23, 2020
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@bluss this has some minor outstanding review comments.

Update Vec drop with a comment to explain why we want to use a raw
slice, and extend this pattern to also include the Vec's IntoIter.
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bluss commented Apr 28, 2020

Updated. PR description updated to weaken the promises made. I found a workaround for my original problem in ndarray; just making drop conditional for when we have needs_drop::<T>().

That could be a way to sidestep the question even inside Vec::drop too - (the question being, are we allowed to drop_in_place(*mut [T]) when the values are uninit, but without drop glue?). In the past it has just been decided that using needs_drop here is just noise - calling drop_in_place would have been equivalent in effect.

@bluss bluss added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 29, 2020
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Looking good, thanks!
@bors r+ rollup

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bors commented Apr 30, 2020

📌 Commit f654daf has been approved by RalfJung

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 30, 2020
bors added a commit to rust-lang-ci/rust that referenced this pull request Apr 30, 2020
Rollup of 8 pull requests

Successful merges:

 - rust-lang#71148 (Vec drop and truncate: drop using raw slice *mut [T])
 - rust-lang#71465 (Add a convenience method on `TyCtxt` for checking for thread locals)
 - rust-lang#71567 (Handle build completion message from Cargo)
 - rust-lang#71590 (MIR dump: print pointers consistently with Miri output)
 - rust-lang#71682 (Bump pulldown-cmark)
 - rust-lang#71688 (Allow `Downcast` projections unconditionally in const-checking)
 - rust-lang#71691 (Allow `Unreachable` terminators unconditionally in const-checking)
 - rust-lang#71719 (Update backtrace-sys)

Failed merges:

r? @ghost
@bors bors merged commit 4adebb9 into rust-lang:master May 1, 2020
@bluss bluss deleted the vec-drop-raw-slice branch May 1, 2020 06:24
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5 participants