Provide more explanation for Deref in String docs
While working on a different project I encountered a point of confusion where using `&String` to dereference a `String` into `&str` did not compile. I found the explanation of [String Deref](https://doc.rust-lang.org/std/string/struct.String.html#deref), thought that it matched what I was trying to do, and was confused as to why my program did not compile when the docs stated that it would work with 'any function which takes a `&str`'. At the bottom it is mentioned that this will 'generally' work, unless `String` is needed, but I found this statement confusing based on the previous claim of 'any'. Looking further into the docs I was able to find the function `as_str()` that works instead.
I thought it might be helpful to mention here deref coercion, an instance in which using `&String` does not work, to explain why it does not work, then direct users to a different option that should work in this instance. A user casually skimming the page will likely come to this explanation first, then find `as_str()` later, but be no the wiser as to what potentially went wrong.
r? @steveklabnik
Fixed mutable vars being marked used when they weren't
#### NB : bootstrapping is slow on my machine, even with `keep-stage` - fixes for occurances in the current codebase are <s>in the pipeline</s> done. This PR is being put up for review of the fix of the issue.
Fixes#43526, Fixes#30280, Fixes#25049
### Issue
Whenever the compiler detected a mutable deref being used mutably, it marked an associated value as being used mutably as well. In the case of derefencing local variables which were mutable references, this incorrectly marked the reference itself being used mutably, instead of its contents - with the consequence of making the following code emit no warnings
```
fn do_thing<T>(mut arg : &mut T) {
... // don't touch arg - just deref it to access the T
}
```
### Fix
Make dereferences not be counted as a mutable use, but only when they're on borrows on local variables.
#### Why not on things other than local variables?
* Whenever you capture a variable in a closure, it gets turned into a hidden reference - when you use it in the closure, it gets dereferenced. If the closure uses the variable mutably, that is actually a mutable use of the thing being dereffed to, so it has to be counted.
* If you deref a mutable `Box` to access the contents mutably, you are using the `Box` mutably - so it has to be counted.
Add precondition to `Layout` that the `align` fit in a u32.
Add precondition to `Layout` that the `align` not exceed 2^31.
This precondition takes the form of a behavorial change in `Layout::from_size_align` (so it returns `None` if the input `align` is too large) and a new requirement for safe usage of `Layout::from_size_align_unchecked`.
Fix#30170.
Stabilize more APIs for the 1.20.0 release
In addition to the few stabilizations that have already landed, this cleans up the remaining APIs that are in `final-comment-period` right now to be stable by the 1.20.0 release
add a note to Vec's Extend<&T> impl about its slice specialization
From the regular documentation view, it's not at all apparent that [this specialization](5669c9988f/src/liballoc/vec.rs (L1879-L1891)) exists for `slice::Iter`. This adds a documentation blurb to the Extend impl itself to note that this optimization exists.
Make the "main" constructors of NonZero/Shared/Unique return Option
Per discussion in https://github.com/rust-lang/rust/issues/27730#issuecomment-303939441.
This is a breaking change to unstable APIs.
The old behavior is still available under the name `new_unchecked`. Note that only that one can be `const fn`, since `if` is currently not allowed in constant contexts.
In the case of `NonZero` this requires adding a new `is_zero` method to the `Zeroable` trait. I mildly dislike this, but it’s not much worse than having a `Zeroable` trait in the first place. `Zeroable` and `NonZero` are both unstable, this can be reworked later.
Clarify that sort_unstable is deterministic
@frankmcsherry complained that the documentation said "it is randomized but deterministic", which is a contradictory statement.
This PR uses a different and clearer wording.
Fix in weak_count in Arc in the case the weak count is locked.
In the case the weak count was locked, the weak_count function could
return usize::MAX. We need to test this condition manually.
This precondition takes the form of a behavorial change in
`Layout::from_size_align` (so it returns `None` if the `align` is too
large) and a new requirement for safe usage of
`Layout::from_size_align_unchecked`.
Fix#30170.
rustc: Implement the #[global_allocator] attribute
This PR is an implementation of [RFC 1974] which specifies a new method of
defining a global allocator for a program. This obsoletes the old
`#![allocator]` attribute and also removes support for it.
[RFC 1974]: https://github.com/rust-lang/rfcs/pull/1974
The new `#[global_allocator]` attribute solves many issues encountered with the
`#![allocator]` attribute such as composition and restrictions on the crate
graph itself. The compiler now has much more control over the ABI of the
allocator and how it's implemented, allowing much more freedom in terms of how
this feature is implemented.
cc #27389
This PR is an implementation of [RFC 1974] which specifies a new method of
defining a global allocator for a program. This obsoletes the old
`#![allocator]` attribute and also removes support for it.
[RFC 1974]: https://github.com/rust-lang/rfcs/pull/197
The new `#[global_allocator]` attribute solves many issues encountered with the
`#![allocator]` attribute such as composition and restrictions on the crate
graph itself. The compiler now has much more control over the ABI of the
allocator and how it's implemented, allowing much more freedom in terms of how
this feature is implemented.
cc #27389
Document unintuitive argument order for Vec::dedup_by relation
When trying to use `dedup_by` to merge some auxiliary information from removed elements into kept elements, I was surprised to observe that `vec.dedup_by(same_bucket)` calls `same_bucket(a, b)` where `b` appears before `a` in the vector, and discards `a` when true is returned. This argument order is probably a bug, but since it has already been stabilized, I guess we should document it as a feature and move on.
(`Vec::dedup` also uses `==` with this unexpected argument order, but I figure that’s not important since `==` is expected to be symmetric with no side effects.)
Delete deprecated & unstable range-specific `step_by`
Using the new one is annoying while this one exists, since the inherent method hides the one on iterator.
Tracking issue: #27741
Replacement: #41439
Deprecation: #42310 for 1.19
Fixes#41477
When trying to use dedup_by to merge some auxiliary information from
removed elements into kept elements, I was surprised to observe that
vec.dedup_by(same_bucket) calls same_bucket(a, b) where b appears
before a in the vector, and discards a when true is returned. This
argument order is probably a bug, but since it has already been
stabilized, I guess we should document it as a feature and move on.
(Vec::dedup also uses == with this unexpected argument order, but I
figure that’s not important since == is expected to be symmetric with
no side effects.)
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
