Add documentation about the memory layout of `UnsafeCell<T>`
The documentation for `UnsafeCell<T>` currently does not make any promises about its memory layout. This PR adds this documentation, namely that the memory layout of `UnsafeCell<T>` is the same as the memory layout of its inner `T`.
# Use case
Without this layout promise, the following cast would not be legally possible:
```rust
fn example<T>(ptr: *mut T) -> *const UnsafeCell<T> {
ptr as *const UnsafeCell<T>
}
```
A use case where this can come up involves FFI. If Rust receives a pointer over a FFI boundary which provides shared read-write access (with some form of custom synchronization), and this pointer is managed by some Rust struct with lifetime `'a`, then it would greatly simplify its (internal) API and safety contract if a `&'a UnsafeCell<T>` can be created from a raw FFI pointer `*mut T`. A lot of safety checks can be done when receiving the pointer for the first time through FFI (non-nullness, alignment, initialize uninit bytes, etc.) and these properties can then be encoded into the `&UnsafeCell<T>` type. Without this documentation guarantee, this is not legal today outside of the standard library.
# Caveats
Casting in the opposite direction is still not valid, even with this documentation change:
```rust
fn example2<T>(ptr: &UnsafeCell<T>) -> &mut T {
let t = ptr as *const UnsafeCell<T> as *mut T;
unsafe { &mut *t }
}
```
This is because the only legal way to obtain a mutable pointer to the contents of the shared reference is through [`UnsafeCell::get`](https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html#method.get) and [`UnsafeCell::raw_get`](https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html#method.raw_get). Although there might be a desire to also make this legal at some point in the future, that part is outside the scope of this PR. Also see this relevant [Zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/136281-t-lang.2Fwg-unsafe-code-guidelines/topic/transmuting.20.26.20-.3E.20.26mut).
# Alternatives
Instead of adding a new documentation promise, it's also possible to add a new method to `UnsafeCell<T>` with signature `pub fn from_ptr_bikeshed(ptr: *mut T) -> *const UnsafeCell<T>` which indirectly only allows one-way casting to `*const UnsafeCell<T>`.
std: use `sync::Mutex` for internal statics
Since `sync::Mutex` is now `const`-constructible, it can be used for internal statics, removing the need for `sys_common::StaticMutex`. This adds some extra allocations on platforms which need to box their mutexes (currently SGX and some UNIX), but these will become unnecessary with the lock improvements tracked in #93740.
I changed the program argument implementation on Hermit, it does not need `Mutex` but can use atomics like some UNIX systems (ping `@mkroening` `@stlankes).`
Get rid of `rustc_query_description!`
**I am not entirely sure whether this is an improvement and would like to get your feedback on it.**
Helps with #96524.
Queries can provide an arbitrary expression for their description and their caching behavior. Before, these expressions where stored in a `rustc_query_description` macro emitted by the `rustc_queries` macro, and then used in `rustc_query_impl` to fill out the methods for the `QueryDescription` trait.
Instead, we now emit two new modules from `rustc_queries` containing the functions with the expressions. `rustc_query_impl` calls these functions now instead of invoking the macro.
Since we are now defining some of the functions in `rustc_middle::query`, we now need all the imports for the key types mthere as well.
r? `@cjgillot`
Rollup of 6 pull requests
Successful merges:
- #102773 (Use semaphores for thread parking on Apple platforms)
- #102884 (resolve: Some cleanup, asserts and tests for lifetime ribs)
- #102954 (Add missing checks for `doc(cfg_hide(...))`)
- #102998 (Drop temporaries created in a condition, even if it's a let chain)
- #103003 (Fix `suggest_floating_point_literal` ICE)
- #103041 (Update cargo)
Failed merges:
r? `@ghost`
`@rustbot` modify labels: rollup
Update cargo
12 commits in b8f30cb23c4e5f20854a4f683325782b7cff9837..b332991a57c9d055f1864de1eed93e2178d49440 2022-10-10 19:16:06 +0000 to 2022-10-13 22:05:28 +0000
- Differentiate the warning when an alias (built-in or user-defined) shadows an external subcommand (rust-lang/cargo#11170)
- chore: Update tests for latest clap (rust-lang/cargo#11235)
- feat(publish): Support 'publish.timeout' config behind '-Zpublish-timeout' (rust-lang/cargo#11230)
- Add missing edition (rust-lang/cargo#11231)
- doc(profiles): add module level doc (rust-lang/cargo#11219)
- refactor(publish): Clarify which SourceId is being used (rust-lang/cargo#11216)
- Add new SourceKind::SparseRegistry to differentiate sparse registries (rust-lang/cargo#11209)
- Fix deadlock when build scripts are waiting for input on stdin (rust-lang/cargo#11205)
- refactor: New variant `FeaturesFor::ArtifactDep` (rust-lang/cargo#11184)
- Fix rustdoc warning about unclosed HTML tag (rust-lang/cargo#11221)
- refactor(tests): Prepare for wait-for-publish test changes (rust-lang/cargo#11210)
- Add configuration option for controlling crates.io protocol (rust-lang/cargo#11215)
Drop temporaries created in a condition, even if it's a let chain
Fixes#100513.
During the lowering from AST to HIR we wrap expressions acting as conditions in a `DropTemps` expression so that any temporaries created in the condition are dropped after the condition is executed. Effectively this means we transform
```rust
if Some(1).is_some() { .. }
```
into (roughly)
```rust
if { let _t = Some(1).is_some(); _t } { .. }
```
so that if we create any temporaries, they're lifted into the new scope surrounding the condition, so for example something along the lines of
```rust
if { let temp = Some(1); let _t = temp.is_some(); _t }.
```
Before this PR, if the condition contained any let expressions we would not introduce that new scope, instead leaving the condition alone. This meant that in a let-chain like
```rust
if get_drop("first").is_some() && let None = get_drop("last") {
println!("second");
} else { .. }
```
the temporary created for `get_drop("first")` would be lifted into the _surrounding block_, which caused it to be dropped after the execution of the entire `if` expression.
After this PR, we wrap everything but the `let` expression in terminating scopes. The upside to this solution is that it's minimally invasive, but the downside is that in the worst case, an expression with `let` exprs interspersed like
```rust
if get_drop("first").is_some()
&& let Some(_a) = get_drop("fifth")
&& get_drop("second").is_some()
&& let Some(_b) = get_drop("fourth") { .. }
```
gets _multiple_ new scopes, roughly
```rust
if { let _t = get_drop("first").is_some(); _t }
&& let Some(_a) = get_drop("fifth")
&& { let _t = get_drop("second").is_some(); _t }
&& let Some(_b) = get_drop("fourth") { .. }
```
so instead of all of the temporaries being dropped at the end of the entire condition, they will be dropped right after they're evaluated (before the subsequent `let` expr). So while I'd say the drop behavior around let-chains is _less_ surprising after this PR, it still might not exactly match what people might expect.
For tests, I've just extended the drop order tests added in #100526. I'm not sure if that's the best way to go about it, though, so suggestions are welcome.
Add missing checks for `doc(cfg_hide(...))`
Part of #43781.
The `doc(cfg_hide(...))` attribute can only be used at the crate level and takes a list of attributes as argument.
r? ```@Manishearth```
Use semaphores for thread parking on Apple platforms
Currently we use a mutex-condvar pair for thread parking on Apple systems. Unfortunately, `pthread_cond_timedwait` uses the real-time clock for measuring time, which causes problems when the system time changes. The parking implementation in this PR uses a semaphore instead, which measures monotonic time by default, avoiding these issues. As a further benefit, this has the potential to improve performance a bit, since `unpark` does not need to wait for a lock to be released.
Since the Mach semaphores are poorly documented (I could not find availability or stability guarantees for instance), this uses a [dispatch semaphore](https://developer.apple.com/documentation/dispatch/dispatch_semaphore?language=objc) instead. While it adds a layer of indirection (it uses Mach semaphores internally), the overhead is probably negligible.
Tested on macOS 12.5.
r? ``````@thomcc``````
Make `dyn*` casts into a coercion, allow `dyn*` upcasting
I know that `dyn*` is likely not going to be a feature exposed to surface Rust, but this makes it slightly more ergonomic to write tests for these types anyways. ... and this was just fun to implement anyways.
1. Make `dyn*` into a coercion instead of a cast
2. Enable `dyn*` upcasting since we basically get it for free
3. Simplify some of the cast checking code since we're using the coercion path now
r? `@eholk` but feel free to reassign
cc `@nikomatsakis` and `@tmandry` who might care about making `dyn*` casts into a coercion
