Announcing Rust 1.79.0

const { ... } blocks are now stable in expression position, permitting explicitly entering a const context without requiring extra declarations (e.g., defining const items or associated constants on a trait).

Unlike const items (const ITEM: ... = ...), inline consts are able to make use of in-scope generics, and have their type inferred rather than written explicitly, making them particularly useful for inline code snippets. For example, a pattern like:

const EMPTY: Option<Vec<u8>> = None;
let foo = [EMPTY; 100];

can now be written like this:

let foo = [const { None }; 100];

Notably, this is also true of generic contexts, where previously a verbose trait declaration with an associated constant would be required:

fn create_none_array<T, const N: usize>() -> [Option<T>; N] {
    [const { None::<T> }; N]
}

This makes this code much more succinct and easier to read.

See the reference documentation for details.

Rust 1.79 stabilizes the associated item bounds syntax, which allows us to put bounds in associated type position within other bounds, i.e. T: Trait<Assoc: Bounds...>. This avoids the need to provide an extra, explicit generic type just to constrain the associated type.

This feature allows specifying bounds in a few places that previously either were not possible or imposed extra, unnecessary constraints on usage:

• where clauses - in this position, this is equivalent to breaking up the bound into two (or more) where clauses. For example, where T: Trait<Assoc: Bound> is equivalent to where T: Trait, <T as Trait>::Assoc: Bound.
• Supertraits - a bound specified via the new syntax is implied when the trait is used, unlike where clauses. Sample syntax: trait CopyIterator: Iterator<Item: Copy> {}.
• Associated type item bounds - This allows constraining the nested rigid projections that are associated with a trait’s associated types. e.g. trait Trait { type Assoc: Trait2<Assoc2: Copy>; }.
• opaque type bounds (RPIT, TAIT) - This allows constraining associated types that are associated with the opaque type without having to name the opaque type. For example, impl Iterator<Item: Copy> defines an iterator whose item is Copy without having to actually name that item bound.

See the stabilization report for more details.

Temporaries which are immediately referenced in construction are now automatically lifetime extended in match and if constructs. This has the same behavior as lifetime extension for temporaries in block constructs.

For example:

let a = if true {
    ..;
    &temp() // used to error, but now gets lifetime extended
} else {
    ..;
    &temp() // used to error, but now gets lifetime extended
};

and

let a = match () {
    _ => {
        ..;
        &temp() // used to error, but now gets lifetime extended
    }
};

are now consistent with prior behavior:

let a = {
    ..;
    &temp() // lifetime is extended
};

This behavior is backwards compatible since these programs used to fail compilation.

The standard library distributed by the Rust project is now compiled with -Cforce-frame-pointers=yes, enabling downstream users to more easily profile their programs. Note that the standard library also continues to come up with line-level debug info (e.g., DWARF), though that is stripped by default in Cargo’s release profiles.