Generate fewer monomorphizations in Fold

[dtolnay]

cargo llvm-lines | head

use std::mem::{self, size_of};
use syn::fold::Fold;
use syn::{File, Ident};

struct MyFold;

impl Fold for MyFold {
    fn fold_ident(&mut self, i: Ident) -> Ident {
        println!("{}", i);
        i
    }
}

fn main() {
    let file: File = unsafe { mem::transmute([1usize; size_of::<File>() / size_of::<usize>()]) };
    MyFold.fold_file(file);
}

Before:

  Lines                 Copies              Function name
  -----                 ------              -------------
  101619                2821                (TOTAL)
   25088 (24.7%, 24.7%)   98 (3.5%,  3.5%)  alloc::vec::in_place_collect::from_iter_in_place
   11858 (11.7%, 36.4%)   98 (3.5%,  6.9%)  core::iter::traits::iterator::Iterator::try_fold
    6370 (6.3%, 42.6%)    98 (3.5%, 10.4%)  core::iter::adapters::map::map_try_fold::{{closure}}
    4658 (4.6%, 47.2%)    34 (1.2%, 11.6%)  syn::punctuated::do_extend
    4116 (4.1%, 51.3%)    98 (3.5%, 15.1%)  <I as alloc::vec::in_place_collect::SpecInPlaceCollect<T,I>>::collect_in_place
    3136 (3.1%, 54.3%)    98 (3.5%, 18.6%)  <alloc::vec::Vec<T> as syn::gen::helper::fold::FoldHelper>::lift
    2352 (2.3%, 56.7%)   103 (3.7%, 22.2%)  core::option::Option<T>::map

After:

  Lines                Copies              Function name
  -----                ------              -------------
  43627                1366                (TOTAL)
   2352 (5.4%,  5.4%)   103 (7.5%,  7.5%)  core::option::Option<T>::map
   2329 (5.3%, 10.7%)    17 (1.2%,  8.8%)  syn::punctuated::do_extend
   1800 (4.1%, 14.9%)    20 (1.5%, 10.2%)  <alloc::vec::Vec<T,A> as core::iter::traits::collect::IntoIterator>::into_iter
   1792 (4.1%, 19.0%)     7 (0.5%, 10.8%)  alloc::vec::in_place_collect::from_iter_in_place
   1659 (3.8%, 22.8%)    21 (1.5%, 12.3%)  <alloc::vec::into_iter::IntoIter<T,A> as core::iter::traits::iterator::Iterator>::next
   1173 (2.7%, 25.5%)    17 (1.2%, 13.5%)  <syn::punctuated::Punctuated<T,P> as syn::gen::helper::fold::FoldHelper>::lift::{{closure}}
    960 (2.2%, 27.7%)    20 (1.5%, 15.0%)  <alloc::vec::into_iter::IntoIter<T,A> as core::ops::drop::Drop>::drop