stabilize `#![feature(min_const_generics)]` in 1.51
*A new Kind*
*A Sort long Prophesized*
*Once Fragile, now Eternal*
blocked on #79073.
# Stabilization report
This is the stabilization report for `#![feature(min_const_generics)]` (tracking issue #74878), a subset of `#![feature(const_generics)]` (tracking issue #44580), based on rust-lang/rfcs#2000.
The [version target](https://forge.rust-lang.org/#current-release-versions) is ~~1.50 (2020-12-31 => beta, 2021-02-11 => stable)~~ 1.51 (2021-02-111 => beta, 2021-03-25 => stable).
This report is a collaborative effort of `@varkor,` `@shepmaster` and `@lcnr.`
## Summary
It is currently possible to parameterize functions, type aliases, types, traits and implementations by types and lifetimes.
With `#![feature(min_const_generics)]`, it becomes possible, in addition, to parameterize these by constants.
This is done using the syntax `const IDENT: Type` in the parameter listing. Unlike full const generics, `min_const_generics` is limited to parameterization by integers, and constants of type `char` or `bool`.
We already use `#![feature(min_const_generics)]` on stable to implement many common traits for arrays. See [the documentation](https://doc.rust-lang.org/nightly/std/primitive.array.html) for specific examples.
Generic const arguments, for now, are not permitted to involve computations depending on generic parameters. This means that const parameters may only be instantiated using either:
1. const expressions that do not depend on any generic parameters, e.g. `{ foo() + 1 }`, where `foo` is a `const fn`
1. standalone const parameters, e.g. `{N}`
### Example
```rust
#![feature(min_const_generics)]
trait Foo<const N: usize> {
fn method<const M: usize>(&mut self, arr: [[u8; M]; N]);
}
struct Bar<T, const N: usize> {
inner: [T; N],
}
impl<const N: usize> Foo<N> for Bar<u8, N> {
fn method<const M: usize>(&mut self, arr: [[u8; M]; N]) {
for (elem, s) in self.inner.iter_mut().zip(arr.iter()) {
for &x in s {
*elem &= x;
}
}
}
}
fn function<const N: u16>() -> u16 {
// Const parameters can be used freely inside of functions.
(N + 1) / 2 * N
}
fn main() {
let mut bar = Bar { inner: [0xff; 3] };
// This infers the value of `M` from the type of the function argument.
bar.method([[0b11_00, 0b01_00], [0b00_11, 0b00_01], [0b11_00, 0b00_11]]);
assert_eq!(bar.inner, [0b01_00, 0b00_01, 0b00_00]);
// You can also explicitly specify the value of `N`.
assert_eq!(function::<17>(), 153);
}
```
## Motivation
Rust has the built-in array type, which is parametric over a constant. Without const generics, this type can be quite cumbersome to use as it is not possible to generically implement a trait for arrays of different lengths. For example, this meant that, for a long time, the standard library only contained trait implementations for arrays up to a length of 32. This restriction has since been lifted through the use of const generics.
Const parameters allow users to naturally specify variants of a generic type which are more naturally parameterized by values, rather than by types. For example, using const generics, many of the uses of the crate [typenum](https://crates.io/crates/typenum) may now be replaced with const parameters, improving compilation time as well as code readability and diagnostics.
The subset described by `min_const_generics` is self-contained, but extensive enough to help with the most frequent issues: implementing traits for arrays and using arbitrarily-sized arrays inside of other types. Furthermore, it extends naturally to full `const_generics` once the remaining design and implementation questions have been resolved.
## In-depth feature description
### Declaring const parameters
*Const parameters* are allowed in all places where types and lifetimes are supported. They use the syntax `const IDENT: Type`. Currently, const parameters must be declared after lifetime and type parameters. Their scope is equal to the scope of other generic parameters. They live in the value namespace.
`Type` must be one of `u8`, `u16`, `u32`, `u64`, `u128`, `usize`, `i8`, `i16`, `i32`, `i64`, `i128`, `isize`, `char` and `bool`. This restriction is implemented in two places:
1. during name resolution, where we forbid generic parameters
1. during well-formedness checking, where we only allow the types listed above
The updated syntax of parameter listings is:
```
GenericParams:
(OuterAttr* LifetimeParam),* (OuterAttr* TypeParam),* (OuterAttr* ConstParam),*
OuterAttr: '#[' ... ']'
LifetimeParam: ...
TypeParam: ...
ConstParam: 'const' IDENT ':' Type
```
Unlike type and lifetime parameters, const parameters of types can be used without being mentioned inside of a parameterized type because const parameters do not have issues concerning variance. This means that the following types are allowed:
```rust
struct Foo<const N: usize>;
enum Bar<const M: usize> { A, B }
```
### Const arguments
Const parameters are instantiated using *const arguments*. Any concrete const expression or const parameter as a standalone argument can be used. When applying an expression as const parameter, most expressions must be contained within a block, with two exceptions:
1. literals and single-segment path expressions
1. array lengths
This syntactic restriction is necessary to avoid ambiguity, or requiring infinite lookahead when parsing an expression as a generic argument.
In the cases where a generic argument could be resolved as either a type or const argument, we always interpret it as a type. This causes the following test to fail:
```rust
type N = u32;
struct Foo<const N: usize>;
fn foo<const N: usize>() -> Foo<N> { todo!() } // ERR
```
To circumvent this, the user may wrap the const parameter with braces, at which point it is unambiguously accepted.
```rust
type N = u32;
struct Foo<const N: usize>;
fn bar<const N: usize>() -> Foo<{ N }> { todo!() } // ok
```
Operations depending on generic parameters are **not** allowed, which is enforced during well-formedness checking. Allowing generic unevaluated constants would require a way to check if they would always evaluate successfully to prevent errors that are not caught at declaration time. This ability forms part of `#![feature(const_evaluatable_checked)]`, which is not yet being stabilised.
Since we are not yet stabilizing `#![feature(lazy_normalization_consts)]`, we must not supply the parent generics to anonymous constants except for repeat expressions. Doing so can cause cycle errors for arrays used in `where`-bounds. Not supplying the parent generics can however lead to ICEs occurring before well-formedness checking when trying to use a generic parameter. See #56445 for details.
Since we expect cases like this to occur more frequently once `min_const_generics` is stabilized, we have chosen to forbid generic parameters in anonymous constants during name resolution. While this changes the ICE in the situation above to an ordinary error, this is theoretically a breaking change, as early-bound lifetimes were previously permitted in repeat expressions but now are disallowed, causing the following snippet to break:
```rust
fn late_bound<'a>() {
let _ = [0; {
let _: &'a (); // ICE ==> ERR
3
}];
}
fn early_bound<'a>() where &'a (): Sized {
let _ = [0; {
let _: &'a (); // ok ==> ERR
3
}];
}
```
### Using const parameters
Const parameters can be used almost everywhere ordinary constants are allowed, except that they may not be used in the construction of consts, statics, functions, or types inside a function body and are subject to the generic argument restrictions mentioned above.
Expressions containing const parameters are eligible for promotion:
```rust
fn test<const N: usize>() -> &'static usize {
&(3 + N)
}
```
### Symbol mangling
See the [Rust symbol name mangling RFC](https://rust-lang.github.io/rfcs/2603-rust-symbol-name-mangling-v0.html) for an overview. Generic const parameters take the form `K[type][value]` when the value is known, or `Kp` where the value is not known, where:
- `[type]` is any integral type, `bool`, or `char`.
- `[value]` is the unsigned hex value for integers, preceded by `n` when negative; is `0` or `1` for `bool`; is the hex value for `char`.
### Exhaustiveness checking
We do not check the exhaustiveness of impls, meaning that the following example does **not** compile:
```rust
struct Foo<const B: bool>;
trait Bar {}
impl Bar for Foo<true> {}
impl Bar for Foo<false> {}
fn needs_bar(_: impl Bar) {}
fn generic<const B: bool>() {
let v = Foo::<B>;
needs_bar(v);
}
```
### Type inference
The value of const parameters can be inferred during typeck. One interesting case is the length of generic arrays, which can also be inferred from patterns (implemented in #70562). Practical usage of this can be seen in #76825.
### Equality of constants
`#![feature(min_const_generics)]` only permits generic parameters to be used as standalone generic arguments. We compare two parameters to be equal if they are literally the same generic parameter.
### Associated constants
Associated constants can use const parameters without restriction, see https://github.com/rust-lang/rust/pull/79135#issuecomment-748299774 for more details.
## Future work
As this is a limited subset of rust-lang/rfcs#2000, there are quite a few extensions we will be looking into next.
### Lazy normalization of constants
Stabilizing `#![feature(lazy_normalization_consts)]` (tracking issue #72219) will remove some special cases that are currently necessary for `min_const_generics`, and unblocks operations on const parameters.
### Relaxing ordering requirements between const and type parameters
We currently restrict the order of generic parameters so that types must come before consts. We could relax this, as is currently done with `const_generics`. Without this it is not possible to use both type defaults and const parameters at the same time.
Unrestricting the order will require us to improve some diagnostics that expect there to be a strict order between type and const parameters.
### Allowing more parameter types
We would like to support const parameters of more types, especially`&str` and user-defined types. Both are blocked on [valtrees]. There are also open questions regarding the design of `structural_match` concerning the latter. Supporting generic const parameter types such as `struct Foo<T, const N: T>` will be a lot harder and is unlikely to be implemented in the near future.
### Default values of const parameters
We do not yet support default values for const parameters. There is work in progress to enable this on nightly (see https://github.com/rust-lang/rust/pull/75384).
### Generic const operations
With `#![feature(min_const_generics)]`, only concrete const expressions and parameters as standalone arguments are allowed in types and repeat expressions. However, supporting generic const operations, such as `N + 1` or `std::mem::size_of::<T>()` is highly desirable. This feature is in early development under `#![feature(const_evaluatable_checked)]`.
## Implementation history
Many people have contributed to the design and implementation of const generics over the last three years. See https://github.com/rust-lang/rust/issues/44580#issuecomment-728913127 for a summary. Once again thank you to everybody who helped out here!
[valtrees]: https://github.com/rust-lang/rust/issues/72396
---
r? `@varkor`
Use `clone_from` from `hashbrown::{HashMap,HashSet}`.
This change updates the `std` hash collections to use `hashbrown`'s `clone_from`, which was itself added in #70052. Deriving `Clone` does not add a `clone_from` impl and uses the trait default, which calls `clone`.
Fixes#28481
lint-docs: Warn on missing lint when documenting.
In #79522, I missed converting one of the errors to a warning, in the situation where a lint is missing. This was unearthed by the renaming of overlapping-patterns (#78242). This will still be validated during the test phase.
Fix missing deny-by-default.md file.
I don't know why, but #80296 deleted this file. Add it back so that the docs can be viewed directly with mdbook without it auto-generating the file.
BTreeMap: clean up access to MaybeUninit arrays
Stop exposing and using immutable access to `MaybeUninit` slices when we need and have exclusive access to the tree.
r? `@Mark-Simulacrum`
Prevent caching normalization results with a cycle
When normalizing a projection which results in a cycle, we would cache the result of `project_type` without the nested obligations (because they're not needed for inference). This would result in the nested obligations only being handled once in fulfill, which would avoid the cycle error. `get_paranoid_cache_value_obligation` used to add an obligation that resulted in a cycle in this case previously, but was removed by #73905.
This PR makes the projection cache not cache the value of a projection if it was ever normalized in a cycle (except in a snapshot that's rolled back).
Fixes#79714.
r? `@nikomatsakis`
validate promoteds
Turn on const-value validation for promoteds. This is made possible now that https://github.com/rust-lang/rust/issues/67534 is resolved.
I don't think this is a breaking change. We don't promote any unsafe operation any more (since https://github.com/rust-lang/rust/pull/77526 landed). We *do* promote `const fn` calls under some circumstances (in `const`/`static` initializers), but union field access and similar operations are not allowed in `const fn`. So now is a perfect time to add this check. :D
r? `@oli-obk`
Fixes https://github.com/rust-lang/rust/issues/67465
Highlight edition-specific keywords correctly in code blocks, accounting for code block edition modifiers
Previously, edition-specific keywords (such as `async` and `await`) were not highlighted in code blocks, regardless of what edition was set. With this PR, this issue is fixed.
Now, the following behavior happens:
- When a code block is explicitly set to edition X, keywords from edition X are highlighted
- When a code block is explicitly set to a version that does not contain those keywords from edition X (e.g. edition Y), keywords from edition X are **not** highlighted
- When a code block has no explicit edition, keywords from the edition passed via `--edition` to rustdoc are highlighted
For example, a project set with `edition = "2015"` in its `Cargo.toml` would not highlight `async`/`await` unless the code block was set to `edition2018`. Additionally, a project set with `edition = "2018"` in its `Cargo.toml` *would* highlight `async`/`await` unless the code block was set to a version that did not contain those keywords (e.g. `edition2015`).
This PR fixes#80004.
r? `@jyn514`
Lazy compilation has the potential to significantly improve the startup
time of a program. While functions have to be codegened when called, it
is expected that a significant amount of all code is only required when
an error occurs or only when the program is used in certain ways.
The basic approach is to first codegen a shim for each function. This
shim calls the `__cg_clif_jit` function of cg_clif with a pointer to the
`Instance` corresponding to the function for which it is a shim.
`__cg_clif_jit` function then codegens this function and uses the hot
code swapping support of SimpleJIT to redirect future calls to the
function to the real version. Finally it calls the newly codegened
function.
Exclude unnecessary info from CodegenResults
`foreign_module` and `wasm_import_module` are not needed for linking, and hence can be removed from CodegenResults.
Fixes#77857
