Auto merge of rust-lang#104087 - nbdd0121:const, r=scottmcm

Stabilise inline_const

# Stabilisation Report

## Summary

This PR will stabilise `inline_const` feature in expression position. `inline_const_pat` is still unstable and will *not* be stabilised.

The feature will allow code like this:
```rust
foo(const { 1 + 1 })
```
which is roughly desugared into
```rust
struct Foo;
impl Foo {
    const FOO: i32 = 1 + 1;
}
foo(Foo::FOO)
```

This feature is from rust-lang/rfcs#2920 and is tracked in rust-lang#76001 (the tracking issue should *not* be closed as it needs to track inline const in pattern position). The initial implementation is done in rust-lang#77124.

## Difference from RFC

There are two major differences (enhancements) as implemented from the RFC. First thing is that the RFC says that the type of an inline const block inferred from the content *within* it, but we currently can infer the type using the information from outside the const block as well. This is a frequently requested feature to the initial implementation (e.g. rust-lang#89964). The inference is implemented in rust-lang#89561 and is done by treating inline const similar to a closure and therefore share inference context with its parent body.

This allows code like:
```rust
let v: Vec<i32> = const { Vec::new() };
```

Another enhancement that differs from the RFC is that we currently allow inline consts to reference generic parameters. This is implemented in rust-lang#96557.

This allows code like:
```rust
fn create_none_array<T, const N: usize>() -> [Option<T>; N] {
    [const { None::<T> }; N]
}
```

This enhancement also makes inline const usable as static asserts:

```rust
fn require_zst<T>() {
    const { assert!(std::mem::size_of::<T>() == 0) }
}
```

## Documentation

Reference: rust-lang/reference#1295

## Unresolved issues

We still have a few issues that are not resolved, but I don't think it necessarily has to block stabilisation:
* expr fragment specifier issue: rust-lang#86730
* ~~`const {}` behaves similar to `async {}` but not to `{}` and `unsafe {}` (they are treated as `ExpressionWithoutBlock` rather than `ExpressionWithBlock`): https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/const.20blocks.20differ.20from.20normal.20and.20from.20unsafe.20blocks/near/290229453~~

## Tests

There are a few tests in https://github.com/rust-lang/rust/tree/master/src/test/ui/inline-const

Author: bors

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -556,7 +556,6 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
         half_open_range_patterns_in_slices,
         "half-open range patterns in slices are unstable"
     );
-    gate_all!(inline_const, "inline-const is experimental");
     gate_all!(inline_const_pat, "inline-const in pattern position is experimental");
     gate_all!(associated_const_equality, "associated const equality is incomplete");
     gate_all!(yeet_expr, "`do yeet` expression is experimental");

compiler/rustc_feature/src/accepted.rs

@@ -211,6 +211,8 @@ declare_features! (
     (accepted, inclusive_range_syntax, "1.26.0", Some(28237)),
     /// Allows inferring outlives requirements (RFC 2093).
     (accepted, infer_outlives_requirements, "1.30.0", Some(44493)),
+    /// Allow anonymous constants from an inline `const` block
+    (accepted, inline_const, "CURRENT_RUSTC_VERSION", Some(76001)),
     /// Allows irrefutable patterns in `if let` and `while let` statements (RFC 2086).
     (accepted, irrefutable_let_patterns, "1.33.0", Some(44495)),
     /// Allows `#[instruction_set(_)]` attribute.