Complex iterator chain is very slow to compile

[Nercury]

I thought this was a cool way to implement collection of values from a builder into an array, and stumbled upon this case:

use std::iter;

pub struct ConfigFilterRef {
    alpha_mask_size: Option<[i32; 2]>,
    alpha_size: Option<[i32; 2]>,
    bind_to_texture_rgb: Option<[i32; 2]>,
    bind_to_texture_rgba: Option<[i32; 2]>,
    blue_size: Option<[i32; 2]>,
    buffer_size: Option<[i32; 2]>,
    color_buffer_type: Option<[i32; 2]>,
    config_caveat: Option<[i32; 2]>,
    config_id: Option<[i32; 2]>,
    conformant: Option<[i32; 2]>,
}

impl ConfigFilterRef {
    pub fn new() -> ConfigFilterRef {
        ConfigFilterRef {
            alpha_mask_size: None,
            alpha_size: None,
            bind_to_texture_rgb: None,
            bind_to_texture_rgba: None,
            blue_size: None,
            buffer_size: None,
            color_buffer_type: None,
            config_caveat: None,
            config_id: None,
            conformant: None,
        }
    }

    pub fn attrs(self) -> Vec<i32> {
        iter::empty()
            // slow part start
            .chain(self.alpha_mask_size.iter().flat_map(|i| i))
            .chain(self.alpha_size.iter().flat_map(|i| i))
            .chain(self.bind_to_texture_rgb.iter().flat_map(|i| i))
            .chain(self.bind_to_texture_rgba.iter().flat_map(|i| i))
            .chain(self.blue_size.iter().flat_map(|i| i))
            .chain(self.buffer_size.iter().flat_map(|i| i))
            .chain(self.color_buffer_type.iter().flat_map(|i| i))
            .chain(self.config_caveat.iter().flat_map(|i| i))
            .chain(self.config_id.iter().flat_map(|i| i))
            .chain(self.conformant.iter().flat_map(|i| i))
            // slow part end
            .chain(&[0])
            .cloned()
            .collect()
    }
}

fn main() {
    ConfigFilterRef::new().attrs();
}

On stable:

   Compiling test v0.1.0 (file:///Users/nercury/webs/test)
cargo build  40.86s user 0.74s system 99% cpu 41.930 total

On nightly:

   Compiling test v0.1.0 (file:///Users/nercury/webs/test)
cargo build  46.43s user 0.73s system 99% cpu 47.174 total

On nightly-2016-02-28:

   Compiling test v0.1.0 (file:///Users/nercury/webs/test)
cargo build  44.31s user 0.77s system 99% cpu 45.082 total

Without "slow part":

   Compiling test v0.1.0 (file:///Users/nercury/webs/test)
cargo build  0.63s user 0.33s system 99% cpu 0.959 total

Feel free to close if this is a duplicate/won't fix.

[Nercury]

For the record, reducing type hierarchy helps, i.e.:

    pub fn attrs(self) -> Vec<i32> {
        [
            self.alpha_mask_size,
            self.alpha_size,
            self.bind_to_texture_rgb,
            self.bind_to_texture_rgba,
            self.blue_size,
            self.buffer_size,
            self.color_buffer_type,
            self.config_caveat,
            self.config_id,
            self.conformant,
        ]
            .into_iter()
            .flat_map(|i| i)
            .flat_map(|i| i)
            .chain(&[0])
            .cloned()
            .collect()
    }

   Compiling test v0.1.0 (file:///Users/nercury/webs/test)
cargo build  0.61s user 0.33s system 99% cpu 0.947 total

[bluss]

This is a bad bug, and there is already an issue open for it, see #22204.