Rust: Slow compilation caused by check_trait_item

Created on 26 Aug 2020 · 3Comments · Source: rust-lang/rust

I'm seeing weirdly high compilation times with specifically tailored traits.

If I define a trait like so (full-gist):

macro_rules! assoc {
    ($e : ident, $f :ident) => {
        type $e: Send + Sync + std::fmt::Debug + std::fmt::Display;
        fn $f ();
    }
}


trait FooTrait {
    assoc!(A, get_a);
    assoc!(B, get_b);
    assoc!(C, get_c);
    // ... 
    assoc!(Z, get_z);
}

I get very high compilation times that are seemingly very sensitive to the number of bounds on the associated types and the number of pairs of fn+type.

At ~50 fn+type pairs and 4 type bounds it takes just over 1min to build with rustc 1.47.0-nightly (e5e33ebd2 2020-08-11).

Summarize output (full-gist) shows that most of the time spent is in check_trait_item_well_formed.

| Item                                             | Self time | % of total time | Time     | Item count |
+--------------------------------------------------+-----------+-----------------+----------+------------+
| check_trait_item_well_formed                     | 63.07s    | 97.565          | 63.10s   | 104        |
+--------------------------------------------------+-----------+-----------------+----------+------------+
| check_item_well_formed                           | 1.25s     | 1.938           | 1.26s    | 4          |
+--------------------------------------------------+-----------+-----------------+----------+------------+
| run_linker                                       | 194.61ms  | 0.301           | 194.61ms | 1          |

Weirdly enough, if we modify the macro to just generate the associated types or just generate the methods, we don't get the high compilation times.

Originally found this with a pattern where each fn returns one of the associated types: fn get_a() -> Self::A, which is how I stumbled upon the problem. Though it doesn't seem like the fns need to reference the associated types to trigger it.

A-traits C-enhancement I-compiletime T-compiler

Source