compiler and language documentation disagree on lifetime extension for array expressions

[dianne]

I'm not sure whether this is a compiler or language docs issue, so I'll report it here and cc @rust-lang/lang-docs

The Reference and FLS specify that the operands of extending array expressions are extending. Clicking through to see the definition of array expressions, both specifications include [expr; N] repetition expressions. Thus from these specifications I would expect

let x = [&temp(); 1];

to extend temp() to live in the same scope as x. This is not the case (playground):

fn temp() {}

fn main() {
    let extended = [&temp()];
    extended; // this is okay

    let not_extended = [&temp(); 1];
    not_extended;
    //~^ ERROR: temporary value dropped while borrowed
}

On the compiler side, the implementation of extending expressions only includes normal array expressions, not repetition expressions, which are represented as hir::ExprKind::Repeat rather than hir:ExprKind::Array.

@rustbot label +T-compiler +T-lang +T-lang-docs +A-docs +A-temporary-lifetime-extension +A-array

[joshtriplett]

We discussed this in today's @rust-lang/lang meeting. We felt that the current lang and reference descriptions are the behavior we would expect, and the current implementation is not the correct behavior. We'd like to see a proposed implementation change to match the specification, and extend array expressions whether or not they have a repetition count. (That implementation change will likely need an FCP.)