Update const_eval.md for feature stabilization #1166
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We are in the process of stabilizing `const_fn_fn_ptr_basics` and `const_fn_trait_bound`. This edit updates const_eval.md to reflect the new capabilities after stabilizing these features.
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I removed the line about trait bounds. I didn't see much in the text that said one way or another about function pointers, so I didn't directly mention them yet. |
This was referenced Feb 10, 2022
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oli-obk
reviewed
Feb 28, 2022
oli-obk
approved these changes
Feb 28, 2022
|
LGTM. But, since when do we allow comparing raw pointers in const contexts? This sure looks like it is always rejected. |
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Looks like that note goes all the way back, but I don't think we ever allowed ptr-to-int casts or raw ptr comparisons in const contexts? At least we never allowed actually executing them. |
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I went ahead and remove "comparing raw pointers" too since those aren't allowed in const contexts either. |
matthiaskrgr
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Mar 7, 2022
…r=wesleywiser
Stabilize const_fn_fn_ptr_basics, const_fn_trait_bound, and const_impl_trait
# Stabilization Report
This PR serves as a request for stabilization for three const evaluation features:
1. `const_fn_fn_ptr_basics`
2. `const_fn_trait_bound`
3. `const_impl_trait`
These are being stabilized together because they are relatively minor and related updates to existing functionality.
## `const_fn_fn_ptr_basics`
Allows creating, passing, and casting function pointers in a `const fn`.
The following is an example of what is now allowed:
```rust
const fn get_function() -> fn() {
fn foo() {
println!("Hello, World!");
}
foo
}
```
Casts between function pointer types are allowed, as well as transmuting from integers:
```rust
const fn get_function() -> fn() {
unsafe {
std::mem::transmute(0x1234usize)
}
}
```
However, casting from a function pointer to an integer is not allowed:
```rust
const fn fn_to_usize(f: fn()) -> usize {
f as usize //~ pointers cannot be cast to integers during const eval
}
```
Calling function pointers is also not allowed.
```rust
const fn call_fn_ptr(f: fn()) {
f() //~ function pointers are not allowed in const fn
}
```
### Test Coverage
The following tests include code that exercises this feature:
- `src/test/ui/consts/issue-37550.rs`
- `src/test/ui/consts/issue-46553.rs`
- `src/test/ui/consts/issue-56164.rs`
- `src/test/ui/consts/min_const_fn/allow_const_fn_ptr_run_pass.rs`
- `src/test/ui/consts/min_const_fn/cast_fn.rs`
- `src/test/ui/consts/min_const_fn/cmp_fn_pointers.rs`
## `const_fn_trait_bound`
Allows trait bounds in `const fn`. Additionally, this feature allows creating and passing `dyn Trait` objects.
Examples such as the following are allowed by this feature:
```rust
const fn do_thing<T: Foo>(_x: &T) {
// ...
}
```
Previously only `Sized` was allowed as a trait bound.
There is no way to call methods from the trait because trait methods cannot currently be marked as const. Allowing trait bounds in const functions does allow the const function to use the trait's associated types and constants.
This feature also allowes `dyn Trait` types. These work equivalently to non-const code. Similar to other pointers in const code, the value of a `dyn Trait` pointer cannot be observed.
Note that due to rust-lang#90912, it was already possible to do the example above as follows:
```rust
const fn do_thing<T>(_x: &T) where (T,): Foo {
// ...
}
```
### Test Coverage
The following tests include code that exercises `const_fn_trait_bound`:
- `src/test/ui/consts/const-fn.rs`
- `src/test/ui/consts/issue-88071.rs`
- `src/test/ui/consts/min_const_fn/min_const_fn.rs`
- `src/test/ui/consts/min_const_fn/min_const_fn_dyn.rs`
- `src/test/ui/nll/issue-55825-const-fn.rs`
- Many of the tests in `src/test/ui/rfc-2632-const-trait-impl/` also exercise this feature.
## `const_impl_trait`
Allows argument and return position `impl Trait` in a `const fn`, such as in the following example:
```rust
const fn do_thing(x: impl Foo) -> impl Foo {
x
}
```
Similar to generic parameters and function pointers, this allows the creation of such opaque types, but not doing anything with them beyond accessing associated types and constants.
### Test Coverage
The following tests exercise this feature:
- `src/test/ui/type-alias-impl-trait/issue-53096.rs`
- `src/test/ui/type-alias-impl-trait/issue-53678-generator-and-const-fn.rs`
## Documentation
These features are documented along with the other const evaluation features in the Rust Reference at https://doc.rust-lang.org/stable/reference/const_eval.html.
There is a PR that updates this documentation to reflect the capabilities enabled by these features at rust-lang/reference#1166.
Tracking issues: rust-lang#57563, rust-lang#63997, rust-lang#93706
ehuss
approved these changes
Mar 8, 2022
Closed
|
Thanks! |
bors
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Mar 16, 2022
Update books ## reference 8 commits in 9d289c05fce7254b99c6a0d354d84abb7fd7a032..0a2fe6651fbccc6416c5110fdf5b93fb3cb29247 2022-02-23 08:58:20 -0800 to 2022-03-15 09:32:25 -0700 - Documentation PR for cfg_panic (rust-lang/reference#1157) - Document aarch64 `target_feature` options (rust-lang/reference#1102) - Try to clarify destructor not being run scenario. (rust-lang/reference#1107) - Add undocumented Punctuation token Tilde `~` (rust-lang/reference#1149) - update UB list for safe target_feature (rust-lang/reference#1050) - Update const_eval.md for feature stabilization (rust-lang/reference#1166) - Remove `.intel_syntax`/`.att_syntax` support entirely. - Fix `.intel_syntax` directive ## book 3 commits in 3f255ed40b8c82a0434088568fbed270dc31bf00..036e88a4f135365de85358febe5324976a56030a 2022-02-27 21:26:12 -0500 to 2022-03-04 21:53:33 -0500 - Fix some links and small wordings - Snapshot of chapter 19 for nostarch - Clarify fully-qualified syntax explanation ## rust-by-example 2 commits in 2a928483a20bb306a7399c0468234db90d89afb5..d504324f1e7dc7edb918ac39baae69f1f1513b8e 2022-02-28 11:36:59 -0300 to 2022-03-07 09:26:32 -0300 - Fixed extra indentation at line 43 in Phantom Testcase example. (rust-lang/rust-by-example#1515) - Typo fixed in description of inline ASM cpuid function (rust-lang/rust-by-example#1514) ## rustc-dev-guide 3 commits in 32f2a5b..0e4b961 2022-03-01 10:45:24 -0600 to 2022-03-14 08:40:37 -0700 - update winget install instructions to ensure proper packages are installed (-e for --exact, and full package names to ensure arbitrary packages from the msstore source aren't installed) - Add missing rustdoc tests explanations - Fix incorrectly escaped backtick
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We are in the process of stabilizing
const_fn_fn_ptr_basicsandconst_fn_trait_bound(see rust-lang/rust#93827). This edit updates const_eval.md to reflect the new capabilities after stabilizing these features.