Use Set instead of Vec in transitive_relation
Helps with #103195. It doesn't fix the underlying quadraticness but it makes it _a lot_ faster to an extent where even doubling the amount of nested references still takes less than two seconds (50s on nightly).
I want to see whether this causes regressions (because the vec was usually quite small) or improvements (as lookup for bigger sets is now much faster) in real code.
Remove "execute" bit from lock file permissions
Previously, flock would set the "execute" bit on Rust lock files. That makes no sense.
This patch clears the "execute" bit on Rust lock files.
See issue #102531.
Remove `-Ztime`
Because it has a lot of overlap with `-Ztime-passes` but is generally less useful. Plus some related cleanups.
Best reviewed one commit at a time.
r? `@davidtwco`
`print_time_passes_entry` unconditionally prints data about a pass. The
most commonly used call site, in `VerboseTimingGuard::drop`, guards it
with a `should_print_passes` test. But there are a couple of other call
sites that don't do that test.
This commit moves the `should_print_passes` test within
`print_time_passes_entry` so that all passes are treated equally.
The compiler currently has `-Ztime` and `-Ztime-passes`. I've used
`-Ztime-passes` for years but only recently learned about `-Ztime`.
What's the difference? Let's look at the `-Zhelp` output:
```
-Z time=val -- measure time of rustc processes (default: no)
-Z time-passes=val -- measure time of each rustc pass (default: no)
```
The `-Ztime-passes` description is clear, but the `-Ztime` one is less so.
Sounds like it measures the time for the entire process?
No. The real difference is that `-Ztime-passes` prints out info about passes,
and `-Ztime` does the same, but only for a subset of those passes. More
specifically, there is a distinction in the profiling code between a "verbose
generic activity" and an "extra verbose generic activity". `-Ztime-passes`
prints both kinds, while `-Ztime` only prints the first one. (It took me
a close reading of the source code to determine this difference.)
In practice this distinction has low value. Perhaps in the past the "extra
verbose" output was more voluminous, but now that we only print stats for a
pass if it exceeds 5ms or alters the RSS, `-Ztime-passes` is less spammy. Also,
a lot of the "extra verbose" cases are for individual lint passes, and you need
to also use `-Zno-interleave-lints` to see those anyway.
Therefore, this commit removes `-Ztime` and the associated machinery. One thing
to note is that the existing "extra verbose" activities all have an extra
string argument, so the commit adds the ability to accept an extra argument to
the "verbose" activities.
Make cycle errors recoverable
In particular, this allows rustdoc to recover from cycle errors when normalizing associated types for documentation.
In the past, ```@jackh726``` has said we need to be careful about overflow errors: https://github.com/rust-lang/rust/pull/91430#issuecomment-983997013
> Off the top of my head, we definitely should be careful about treating overflow errors the same as
"not implemented for some reason" errors. Otherwise, you could end up with behavior that is
different depending on recursion depth. But, that might be context-dependent.
But cycle errors should be safe to unconditionally report; they don't depend on the recursion depth, they will always be an error whenever they're encountered.
Helps with https://github.com/rust-lang/rust/issues/81091.
r? ```@lcnr``` cc ```@matthewjasper```
In particular, this allows rustdoc to recover from cycle errors when normalizing associated types for documentation.
In the past, `@jackh726` has said we need to be careful about overflow errors:
> Off the top of my head, we definitely should be careful about treating overflow errors the same as
"not implemented for some reason" errors. Otherwise, you could end up with behavior that is
different depending on recursion depth. But, that might be context-dependent.
But cycle errors should be safe to unconditionally report; they don't depend on the recursion depth, they will always be an error whenever they're encountered.
On later stages, the feature is already stable.
Result of running:
rg -l "feature.let_else" compiler/ src/librustdoc/ library/ | xargs sed -s -i "s#\\[feature.let_else#\\[cfg_attr\\(bootstrap, feature\\(let_else\\)#"
Replace `rustc_data_structures::thin_vec::ThinVec` with `thin_vec::ThinVec`
`rustc_data_structures::thin_vec::ThinVec` looks like this:
```
pub struct ThinVec<T>(Option<Box<Vec<T>>>);
```
It's just a zero word if the vector is empty, but requires two
allocations if it is non-empty. So it's only usable in cases where the
vector is empty most of the time.
This commit removes it in favour of `thin_vec::ThinVec`, which is also
word-sized, but stores the length and capacity in the same allocation as
the elements. It's good in a wider variety of situation, e.g. in enum
variants where the vector is usually/always non-empty.
The commit also:
- Sorts some `Cargo.toml` dependency lists, to make additions easier.
- Sorts some `use` item lists, to make additions easier.
- Changes `clean_trait_ref_with_bindings` to take a
`ThinVec<TypeBinding>` rather than a `&[TypeBinding]`, because this
avoid some unnecessary allocations.
r? `@spastorino`
Fix a bunch of typo
This PR will fix some typos detected by [typos].
I only picked the ones I was sure were spelling errors to fix, mostly in
the comments.
[typos]: https://github.com/crate-ci/typos
This PR will fix some typos detected by [typos].
I only picked the ones I was sure were spelling errors to fix, mostly in
the comments.
[typos]: https://github.com/crate-ci/typos
`rustc_data_structures::thin_vec::ThinVec` looks like this:
```
pub struct ThinVec<T>(Option<Box<Vec<T>>>);
```
It's just a zero word if the vector is empty, but requires two
allocations if it is non-empty. So it's only usable in cases where the
vector is empty most of the time.
This commit removes it in favour of `thin_vec::ThinVec`, which is also
word-sized, but stores the length and capacity in the same allocation as
the elements. It's good in a wider variety of situation, e.g. in enum
variants where the vector is usually/always non-empty.
The commit also:
- Sorts some `Cargo.toml` dependency lists, to make additions easier.
- Sorts some `use` item lists, to make additions easier.
- Changes `clean_trait_ref_with_bindings` to take a
`ThinVec<TypeBinding>` rather than a `&[TypeBinding]`, because this
avoid some unnecessary allocations.
In some places we use `Vec<Attribute>` and some places we use
`ThinVec<Attribute>` (a.k.a. `AttrVec`). This results in various points
where we have to convert between `Vec` and `ThinVec`.
This commit changes the places that use `Vec<Attribute>` to use
`AttrVec`. A lot of this is mechanical and boring, but there are
some interesting parts:
- It adds a few new methods to `ThinVec`.
- It implements `MapInPlace` for `ThinVec`, and introduces a macro to
avoid the repetition of this trait for `Vec`, `SmallVec`, and
`ThinVec`.
Overall, it makes the code a little nicer, and has little effect on
performance. But it is a precursor to removing
`rustc_data_structures::thin_vec::ThinVec` and replacing it with
`thin_vec::ThinVec`, which is implemented more efficiently.
Upgrade indexmap and thorin-dwp to use hashbrown 0.12
This removes the last dependencies on hashbrown 0.11.
This also upgrades to hashbrown 0.12.3 to fix a double-free (#99372).
Stop keeping metadata in memory before writing it to disk
Fixes#96358
I created this PR according with the instruction given in the issue except for the following points:
- While the issue says "Write metadata into the temporary file in `encode_and_write_metadata` even if `!need_metadata_file`", I could not do that. That is because though I tried to do that and run `x.py test`, I got a lot of test failures as follows.
<details>
<summary>List of failed tests</summary>
<pre>
<code>
failures:
[ui] src/test/ui/json-multiple.rs
[ui] src/test/ui/json-options.rs
[ui] src/test/ui/rmeta/rmeta-rpass.rs
[ui] src/test/ui/save-analysis/emit-notifications.rs
[ui] src/test/ui/svh/changing-crates.rs
[ui] src/test/ui/svh/svh-change-lit.rs
[ui] src/test/ui/svh/svh-change-significant-cfg.rs
[ui] src/test/ui/svh/svh-change-trait-bound.rs
[ui] src/test/ui/svh/svh-change-type-arg.rs
[ui] src/test/ui/svh/svh-change-type-ret.rs
[ui] src/test/ui/svh/svh-change-type-static.rs
[ui] src/test/ui/svh/svh-use-trait.rs
test result: FAILED. 12915 passed; 12 failed; 100 ignored; 0 measured; 0 filtered out; finished in 71.41s
Some tests failed in compiletest suite=ui mode=ui host=x86_64-unknown-linux-gnu target=x86_64-unknown-linux-gnu
Build completed unsuccessfully in 0:01:58
</code>
</pre>
</details>
- I could not resolve the extra tasks about `create_rmeta_file` and `create_compressed_metadata_file` for my lack of ability.
Fix RSS reporting on macOS
> NOTE: This is a duplicate of #98164, which I closed because I borked my rustc fork
Currently, `rustc_data_structures::profiling::get_resident_set_size()` always returns `None` on macOS. This is because
macOS does not implement procfs used in the unix version of the function:
```rust
...
else if #[cfg(unix)] {
pub fn get_resident_set_size() -> Option<usize> {
let field = 1;
let contents = fs::read("/proc/self/statm").ok()?;
let contents = String::from_utf8(contents).ok()?;
let s = contents.split_whitespace().nth(field)?;
let npages = s.parse::<usize>().ok()?;
Some(npages * 4096)
}
...
```
The proposed solution uses libproc, and more specifically `proc_pidinfo`, which has been available on macOS since 10.5 if the function signature inside libproc.h is to be believed:
```c
int proc_pidinfo(int pid, int flavor, uint64_t arg, void *buffer, int buffersize) __OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);
```
Rename rustc_serialize::opaque::Encoder as MemEncoder.
This avoids the name clash with `rustc_serialize::Encoder` (a trait),
and allows lots qualifiers to be removed and imports to be simplified
(e.g. fewer `as` imports).
(This was previously merged as commit 5 in #94732 and then was reverted
in #97905 because of a perf regression caused by commit 4 in #94732.)
r? ```@bjorn3```
This avoids the name clash with `rustc_serialize::Encoder` (a trait),
and allows lots qualifiers to be removed and imports to be simplified
(e.g. fewer `as` imports).
(This was previously merged as commit 5 in #94732 and then was reverted
in #97905 because of a perf regression caused by commit 4 in #94732.)
This avoids the name clash with `rustc_serialize::Encoder` (a trait),
and allows lots qualifiers to be removed and imports to be simplified
(e.g. fewer `as` imports).
There are two impls of the `Encoder` trait: `opaque::Encoder` and
`opaque::FileEncoder`. The former encodes into memory and is infallible, the
latter writes to file and is fallible.
Currently, standard `Result`/`?`/`unwrap` error handling is used, but this is a
bit verbose and has non-trivial cost, which is annoying given how rare failures
are (especially in the infallible `opaque::Encoder` case).
This commit changes how `Encoder` fallibility is handled. All the `emit_*`
methods are now infallible. `opaque::Encoder` requires no great changes for
this. `opaque::FileEncoder` now implements a delayed error handling strategy.
If a failure occurs, it records this via the `res` field, and all subsequent
encoding operations are skipped if `res` indicates an error has occurred. Once
encoding is complete, the new `finish` method is called, which returns a
`Result`. In other words, there is now a single `Result`-producing method
instead of many of them.
This has very little effect on how any file errors are reported if
`opaque::FileEncoder` has any failures.
Much of this commit is boring mechanical changes, removing `Result` return
values and `?` or `unwrap` from expressions. The more interesting parts are as
follows.
- serialize.rs: The `Encoder` trait gains an `Ok` associated type. The
`into_inner` method is changed into `finish`, which returns
`Result<Vec<u8>, !>`.
- opaque.rs: The `FileEncoder` adopts the delayed error handling
strategy. Its `Ok` type is a `usize`, returning the number of bytes
written, replacing previous uses of `FileEncoder::position`.
- Various methods that take an encoder now consume it, rather than being
passed a mutable reference, e.g. `serialize_query_result_cache`.
Because it really has two halves:
- A read-only part that checks if further work is needed.
- The further work part, which is much less hot.
This makes things a bit clearer and nicer.
It creates the src pointer first, which is then invalidated by a
unique borrow of the destination pointer. Swap the borrows around
to fix this. Found with miri.
Some tests took too long and owning_ref is fundamentally flawed,
so don't run these tests or run them with a shorter N. This makes
miri with `-Zmiri-strict-provenance` usable to find UB.
Begin fixing all the broken doctests in `compiler/`
Begins to fix#95994.
All of them pass now but 24 of them I've marked with `ignore HELP (<explanation>)` (asking for help) as I'm unsure how to get them to work / if we should leave them as they are.
There are also a few that I marked `ignore` that could maybe be made to work but seem less important.
Each `ignore` has a rough "reason" for ignoring after it parentheses, with
- `(pseudo-rust)` meaning "mostly rust-like but contains foreign syntax"
- `(illustrative)` a somewhat catchall for either a fragment of rust that doesn't stand on its own (like a lone type), or abbreviated rust with ellipses and undeclared types that would get too cluttered if made compile-worthy.
- `(not-rust)` stuff that isn't rust but benefits from the syntax highlighting, like MIR.
- `(internal)` uses `rustc_*` code which would be difficult to make work with the testing setup.
Those reason notes are a bit inconsistently applied and messy though. If that's important I can go through them again and try a more principled approach. When I run `rg '```ignore \(' .` on the repo, there look to be lots of different conventions other people have used for this sort of thing. I could try unifying them all if that would be helpful.
I'm not sure if there was a better existing way to do this but I wrote my own script to help me run all the doctests and wade through the output. If that would be useful to anyone else, I put it here: https://github.com/Elliot-Roberts/rust_doctest_fixing_tool
Fixing #95444 by only displaying passes that take more than 5 millise…
As discussed in #95444, I have added the code to test and only display prints that are greater than 5 milliseconds.
r? `@jyn514`
Add a dedicated length-prefixing method to `Hasher`
This accomplishes two main goals:
- Make it clear who is responsible for prefix-freedom, including how they should do it
- Make it feasible for a `Hasher` that *doesn't* care about Hash-DoS resistance to get better performance by not hashing lengths
This does not change rustc-hash, since that's in an external crate, but that could potentially use it in future.
Fixes#94026
r? rust-lang/libs
---
The core of this change is the following two new methods on `Hasher`:
```rust
pub trait Hasher {
/// Writes a length prefix into this hasher, as part of being prefix-free.
///
/// If you're implementing [`Hash`] for a custom collection, call this before
/// writing its contents to this `Hasher`. That way
/// `(collection![1, 2, 3], collection![4, 5])` and
/// `(collection![1, 2], collection![3, 4, 5])` will provide different
/// sequences of values to the `Hasher`
///
/// The `impl<T> Hash for [T]` includes a call to this method, so if you're
/// hashing a slice (or array or vector) via its `Hash::hash` method,
/// you should **not** call this yourself.
///
/// This method is only for providing domain separation. If you want to
/// hash a `usize` that represents part of the *data*, then it's important
/// that you pass it to [`Hasher::write_usize`] instead of to this method.
///
/// # Examples
///
/// ```
/// #![feature(hasher_prefixfree_extras)]
/// # // Stubs to make the `impl` below pass the compiler
/// # struct MyCollection<T>(Option<T>);
/// # impl<T> MyCollection<T> {
/// # fn len(&self) -> usize { todo!() }
/// # }
/// # impl<'a, T> IntoIterator for &'a MyCollection<T> {
/// # type Item = T;
/// # type IntoIter = std::iter::Empty<T>;
/// # fn into_iter(self) -> Self::IntoIter { todo!() }
/// # }
///
/// use std:#️⃣:{Hash, Hasher};
/// impl<T: Hash> Hash for MyCollection<T> {
/// fn hash<H: Hasher>(&self, state: &mut H) {
/// state.write_length_prefix(self.len());
/// for elt in self {
/// elt.hash(state);
/// }
/// }
/// }
/// ```
///
/// # Note to Implementers
///
/// If you've decided that your `Hasher` is willing to be susceptible to
/// Hash-DoS attacks, then you might consider skipping hashing some or all
/// of the `len` provided in the name of increased performance.
#[inline]
#[unstable(feature = "hasher_prefixfree_extras", issue = "88888888")]
fn write_length_prefix(&mut self, len: usize) {
self.write_usize(len);
}
/// Writes a single `str` into this hasher.
///
/// If you're implementing [`Hash`], you generally do not need to call this,
/// as the `impl Hash for str` does, so you can just use that.
///
/// This includes the domain separator for prefix-freedom, so you should
/// **not** call `Self::write_length_prefix` before calling this.
///
/// # Note to Implementers
///
/// The default implementation of this method includes a call to
/// [`Self::write_length_prefix`], so if your implementation of `Hasher`
/// doesn't care about prefix-freedom and you've thus overridden
/// that method to do nothing, there's no need to override this one.
///
/// This method is available to be overridden separately from the others
/// as `str` being UTF-8 means that it never contains `0xFF` bytes, which
/// can be used to provide prefix-freedom cheaper than hashing a length.
///
/// For example, if your `Hasher` works byte-by-byte (perhaps by accumulating
/// them into a buffer), then you can hash the bytes of the `str` followed
/// by a single `0xFF` byte.
///
/// If your `Hasher` works in chunks, you can also do this by being careful
/// about how you pad partial chunks. If the chunks are padded with `0x00`
/// bytes then just hashing an extra `0xFF` byte doesn't necessarily
/// provide prefix-freedom, as `"ab"` and `"ab\u{0}"` would likely hash
/// the same sequence of chunks. But if you pad with `0xFF` bytes instead,
/// ensuring at least one padding byte, then it can often provide
/// prefix-freedom cheaper than hashing the length would.
#[inline]
#[unstable(feature = "hasher_prefixfree_extras", issue = "88888888")]
fn write_str(&mut self, s: &str) {
self.write_length_prefix(s.len());
self.write(s.as_bytes());
}
}
```
With updates to the `Hash` implementations for slices and containers to call `write_length_prefix` instead of `write_usize`.
`write_str` defaults to using `write_length_prefix` since, as was pointed out in the issue, the `write_u8(0xFF)` approach is insufficient for hashers that work in chunks, as those would hash `"a\u{0}"` and `"a"` to the same thing. But since `SipHash` works byte-wise (there's an internal buffer to accumulate bytes until a full chunk is available) it overrides `write_str` to continue to use the add-non-UTF-8-byte approach.
---
Compatibility:
Because the default implementation of `write_length_prefix` calls `write_usize`, the changed hash implementation for slices will do the same thing the old one did on existing `Hasher`s.
This accomplishes two main goals:
- Make it clear who is responsible for prefix-freedom, including how they should do it
- Make it feasible for a `Hasher` that *doesn't* care about Hash-DoS resistance to get better performance by not hashing lengths
This does not change rustc-hash, since that's in an external crate, but that could potentially use it in future.
95444: Adding passes that include memory increase
Fix95444: Change the substraction with the abs_diff() method
Fix95444: Change the substraction with abs_diff() method
rustc_metadata: Do not encode unnecessary module children
This should remove the syntax context shift and the special case for `ExternCrate` in decoder in https://github.com/rust-lang/rust/pull/95880.
This PR also shifts some work from decoding to encoding, which is typically useful for performance (but probably not much in this case).
r? `@cjgillot`
Allow self-profiler to only record potentially costly arguments when argument recording is turned on
As discussed [on zulip](https://rust-lang.zulipchat.com/#narrow/stream/247081-t-compiler.2Fperformance/topic/Identifying.20proc-macro.20slowdowns/near/277304909) with `@wesleywiser,` I'd like to record proc-macro expansions in the self-profiler, with some detailed data (per-expansion spans for example, to follow #95473).
At the same time, I'd also like to avoid doing expensive things when tracking a generic activity's arguments, if they were not specifically opted into the event filter mask, to allow the self-profiler to be used in hotter contexts.
This PR tries to offer:
- a way to ensure a closure to record arguments will only be called in that situation, so that potentially costly arguments can still be recorded when needed. With the additional requirement that, if possible, it would offer a way to record non-owned data without adding many `generic_activity_with_arg_{...}`-style methods. This lead to the `generic_activity_with_arg_recorder` single entry-point, and the closure parameter would offer the new methods, able to be executed in a context where costly argument could be created without disturbing the profiled piece of code.
- some facilities/patterns allowing to record more rustc specific data in this situation, without making `rustc_data_structures` where the self-profiler is defined, depend on other rustc crates (causing circular dependencies): in particular, spans. They are quite tricky to turn into strings (if the default `Debug` impl output does not match the context one needs them for), and since I'd also like to avoid the allocation there when arg recording is turned off today, that has turned into another flexibility requirement for the API in this PR (separating the span-specific recording into an extension trait). **edit**: I've removed this from the PR so that it's easier to review, and opened https://github.com/rust-lang/rust/pull/95739.
- allow for extensibility in the future: other ways to record arguments, or additional data attached to them could be added in the future (e.g. recording the argument's name as well as its data).
Some areas where I'd love feedback:
- the API and names: the `EventArgRecorder` and its method for example. As well as the verbosity that comes from the increased flexibility.
- if I should convert the existing `generic_activity_with_arg{s}` to just forward to `generic_activity_with_arg_recorder` + `recorder.record_arg` (or remove them altogether ? Probably not): I've used the new API in the simple case I could find of allocating for an arg that may not be recorded, and the rest don't seem costly.
- [x] whether this API should panic if no arguments were recorded by the user-provided closure (like this PR currently does: it seems like an error to use an API dedicated to record arguments but not call the methods to then do so) or if this should just record a generic activity without arguments ?
- whether the `record_arg` function should be `#[inline(always)]`, like the `generic_activity_*` functions ?
As mentioned, r? `@wesleywiser` following our recent discussion.
make unaligned_references lint deny-by-default
This lint has been warn-by-default for a year now (since https://github.com/rust-lang/rust/pull/82525), so I think it is time to crank it up a bit. Code that triggers the lint causes UB (without `unsafe`) when executed, so we really don't want people to write code like this.
Cached stable hash cleanups
r? `@nnethercote`
Add a sanity assertion in debug mode to check that the cached hashes are actually the ones we get if we compute the hash each time.
Add a new data structure that bundles all the hash-caching work to make it easier to re-use it for different interned data structures
Enforce well formedness for type alias impl trait's hidden type
fixes#84657
This was not an issue with return-position-impl-trait because the generic bounds of the function are the same as those of the opaque type, and the hidden type must already be well formed within the function.
With type-alias-impl-trait the hidden type could be defined in a function that has *more* lifetime bounds than the type alias. This is fine, but the hidden type must still be well formed without those additional bounds.
This allows profiling costly arguments to be recorded only when `-Zself-profile-events=args` is on: using a closure that takes an `EventArgRecorder` and call its `record_arg` or `record_args` methods.
Add debug assertions to some unsafe functions
As suggested by https://github.com/rust-lang/rust/issues/51713
~~Some similar code calls `abort()` instead of `panic!()` but aborting doesn't work in a `const fn`, and the intrinsic for doing dispatch based on whether execution is in a const is unstable.~~
This picked up some invalid uses of `get_unchecked` in the compiler, and fixes them.
I can confirm that they do in fact pick up invalid uses of `get_unchecked` in the wild, though the user experience is less-than-awesome:
```
Running unittests (target/x86_64-unknown-linux-gnu/debug/deps/rle_decode_fast-04b7918da2001b50)
running 6 tests
error: test failed, to rerun pass '--lib'
Caused by:
process didn't exit successfully: `/home/ben/rle-decode-helper/target/x86_64-unknown-linux-gnu/debug/deps/rle_decode_fast-04b7918da2001b50` (signal: 4, SIGILL: illegal instruction)
```
~~As best I can tell these changes produce a 6% regression in the runtime of `./x.py test` when `[rust] debug = true` is set.~~
Latest commit (6894d559bd) brings the additional overhead from this PR down to 0.5%, while also adding a few more assertions. I think this actually covers all the places in `core` that it is reasonable to check for safety requirements at runtime.
Thoughts?
Rollup of 5 pull requests
Successful merges:
- #95294 (Document Linux kernel handoff in std::io::copy and std::fs::copy)
- #95443 (Clarify how `src/tools/x` searches for python)
- #95452 (fix since field version for termination stabilization)
- #95460 (Spellchecking compiler code)
- #95461 (Spellchecking some comments)
Failed merges:
r? `@ghost`
`@rustbot` modify labels: rollup
Lazy type-alias-impl-trait take two
### user visible change 1: RPIT inference from recursive call sites
Lazy TAIT has an insta-stable change. The following snippet now compiles, because opaque types can now have their hidden type set from wherever the opaque type is mentioned.
```rust
fn bar(b: bool) -> impl std::fmt::Debug {
if b {
return 42
}
let x: u32 = bar(false); // this errors on stable
99
}
```
The return type of `bar` stays opaque, you can't do `bar(false) + 42`, you need to actually mention the hidden type.
### user visible change 2: divergence between RPIT and TAIT in return statements
Note that `return` statements and the trailing return expression are special with RPIT (but not TAIT). So
```rust
#![feature(type_alias_impl_trait)]
type Foo = impl std::fmt::Debug;
fn foo(b: bool) -> Foo {
if b {
return vec![42];
}
std::iter::empty().collect() //~ ERROR `Foo` cannot be built from an iterator
}
fn bar(b: bool) -> impl std::fmt::Debug {
if b {
return vec![42]
}
std::iter::empty().collect() // Works, magic (accidentally stabilized, not intended)
}
```
But when we are working with the return value of a recursive call, the behavior of RPIT and TAIT is the same:
```rust
type Foo = impl std::fmt::Debug;
fn foo(b: bool) -> Foo {
if b {
return vec![];
}
let mut x = foo(false);
x = std::iter::empty().collect(); //~ ERROR `Foo` cannot be built from an iterator
vec![]
}
fn bar(b: bool) -> impl std::fmt::Debug {
if b {
return vec![];
}
let mut x = bar(false);
x = std::iter::empty().collect(); //~ ERROR `impl Debug` cannot be built from an iterator
vec![]
}
```
### user visible change 3: TAIT does not merge types across branches
In contrast to RPIT, TAIT does not merge types across branches, so the following does not compile.
```rust
type Foo = impl std::fmt::Debug;
fn foo(b: bool) -> Foo {
if b {
vec![42_i32]
} else {
std::iter::empty().collect()
//~^ ERROR `Foo` cannot be built from an iterator over elements of type `_`
}
}
```
It is easy to support, but we should make an explicit decision to include the additional complexity in the implementation (it's not much, see a721052457cf513487fb4266e3ade65c29b272d2 which needs to be reverted to enable this).
### PR formalities
previous attempt: #92007
This PR also includes #92306 and #93783, as they were reverted along with #92007 in #93893fixes#93411fixes#88236fixes#89312fixes#87340fixes#86800fixes#86719fixes#84073fixes#83919fixes#82139fixes#77987fixes#74282fixes#67830fixes#62742fixes#54895
These debug assertions are all implemented only at runtime using
`const_eval_select`, and in the error path they execute
`intrinsics::abort` instead of being a normal debug assertion to
minimize the impact of these assertions on code size, when enabled.
Of all these changes, the bounds checks for unchecked indexing are
expected to be most impactful (case in point, they found a problem in
rustc).
