While let chain causing rust-analyzer to complain about the feature being unstable in Rust 1.66, wasn't this just merged into stable recently?

Question

    while let Some(peek_ch) = chars.peek() && peek_ch.is_whitespace() {
      chars.next();
    }

The above code is causing rust to complain with

`let` expressions in this position are unstable
see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information

My understanding was if-let and while-let chaining was stabilized? Also, from this error and github issue I can't determine what unstable feature to enable to allow this, how do you normally determine that?

Answer 1

The problem is that you're not allowed to use && with while let yet because unfortunately the merge of the stabilization had to be reverted

If you use a nightly compiler it will tell you what feature you have to enable to make it work:

> cargo +nightly run
...
error[E0658]: `let` expressions in this position are unstable
 --> src/main.rs:4:11
  |
4 |     while let Some(peek_ch) = chars.peek() && peek_ch.is_whitespace() {
  |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
  = help: add `#![feature(let_chains)]` to the crate attributes to enable

So this works with nightly rust:

#![feature(let_chains)]
fn main() {
    let mut chars = "hello".chars().peekable();
    while let Some(peek_ch) = chars.peek() && peek_ch.is_whitespace() {
        chars.next();
    }
}

Playground link

Or you can work around it with one of PeterHalls stable solutions.

Answer 2

The problem is the &&, not the while let. As written, you are trying to match the result of the expression chars.peek() && peek_ch.is_whitespace(), but this doesn't make sense because peek returns an Option<T> while is_whitespace returns a bool.

The error message is a little bit misleading because it thinks you are trying to use if let chains, an unstable feature which allows you to match on multiple patterns.

You can rewrite this with:

while let Some(peek_ch) = chars.peek() {
    if peek_ch.is_whitespace() {
        chars.next();
    } else {
        break;
    }
}

Or

while let Some(peek_ch) = chars.peek().filter(|c| c.is_whitespace()) {
    chars.next();
}