analysis-stats hangs with fixed-point combinator

[atomicky]

rust-analyzer (installed by VS Code plugin) hangs and rustfmt takes too long (infinite?) time if I add the following code:

#[inline]
pub fn fix<A, B, F>(f: &F) -> impl '_ + Fn(A) -> B
where
    F: Fn(&dyn Fn(A) -> B, A) -> B,
{
    #[inline]
    move |x| f(&fix(f), x)
}

#[inline]
pub fn get_fact() -> impl 'static + Fn(u16) -> u64 {
    fix(&|f, n| {
        if n < 2 {
            1u64
        } else {
            n as u64 * f(n - 1)
        }
    })
}

There is no problem with rust-analyzer (except for #8917) if I comment out this code and re-open VS Code.

In this code, I define fixed-point combinator and use it to implement a closure that calculate factorial. The code compiles and no problem occur at execution (see https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=82edb19f2e437ab6ff5a8045c4ab0593). Therefore, the code is not in itself a bad one. Maybe cyclic definition problem or something (not with rustc but only with rust-analyzer) happens, but I'm not sure.

The hanging process name was rust-analyzer-x86_64-unknown-linux-gnu. I also checked that ~/.config/Code/User/globalStorage/matklad.rust-analyzer/rust-analyzer-x86_64-unknown-linux-gnu analysis-stats PATH/TO/project_dir also hangs while processing get_fact:

$ ~/.config/Code/User/globalStorage/matklad.rust-analyzer/rust-analyzer-x86_64-unknown-linux-gnu analysis-stats .
Failed to create perf counter: Operation not permitted (os error 1)
Database loaded:     500.93ms
Failed to create perf counter: Operation not permitted (os error 1)
  crates: 1, mods: 1, decls: 3, fns: 3
Item Collection:     1.26s
Failed to create perf counter: Operation not permitted (os error 1)
1/3 33% processing: get_fact^C

I use following version with VS Code plugin on Kubuntu 20.04:

$ ls ~/.config/Code/User/globalStorage/matklad.rust-analyzer
rust-analyzer-linux  rust-analyzer-x86_64-unknown-linux-gnu
$ ~/.config/Code/User/globalStorage/matklad.rust-analyzer/rust-analyzer-x86_64-unknown-linux-gnu --version
rust-analyzer df0936b4a 2021-08-02 stable
$ ~/.config/Code/User/globalStorage/matklad.rust-analyzer/rust-analyzer-linux --version
rust-analyzer ac30710

[flodiebold]

Looks like it might be another case of rust-lang/chalk#688.

[atomicky]

You are right. It turns out that this issue is almost same one as #7680. All problems I mentioned above also happen with the following code:

fn f<R>(_: impl Fn(&dyn Fn() -> R) -> R) {}

fn main() {
    f(|_| ());
}

If I use the original code mentioned at that issue (use Box<dyn Fn() -> R> instead of &dyn Fn() -> R), some problems improve:

• rust-analyzer VS Code plugin and some operation (e.g. format) seems responsive.
• rust-analyzer process will be terminated when VS Code is closed.

However, analysis-stats command remain hanging and rust-analyzer plugin consume CPU usage with Box<dyn Fn() -> R>.