Who drops all the Arcs ?

[matklad]

Running cargo llvm-lines --lib --release -p ide_ssr | head -n 24 gives extremely curious results:

 Lines          Copies        Function name
  -----          ------        -------------
  533069 (100%)  28309 (100%)  (TOTAL)
   20349 (3.8%)    357 (1.3%)  alloc::raw_vec::RawVec<T,A>::current_memory
   18324 (3.4%)    332 (1.2%)  <alloc::sync::Weak<T> as core::ops::drop::Drop>::drop
   14024 (2.6%)    332 (1.2%)  alloc::sync::Weak<T>::inner
   11718 (2.2%)    378 (1.3%)  core::ptr::metadata::from_raw_parts_mut
   10710 (2.0%)    357 (1.3%)  <alloc::raw_vec::RawVec<T,A> as core::ops::drop::Drop>::drop
    7984 (1.5%)    332 (1.2%)  <alloc::sync::Arc<T> as core::ops::drop::Drop>::drop
    7968 (1.5%)    332 (1.2%)  core::alloc::layout::Layout::for_value_raw
    6790 (1.3%)     97 (0.3%)  hashbrown::raw::RawTable<T,A>::drop_elements
    6596 (1.2%)     97 (0.3%)  <hashbrown::raw::RawIterRange<T> as core::iter::traits::iterator::Iterator>::next
    6240 (1.2%)     60 (0.2%)  salsa::blocking_future::Promise<T>::transition
    6100 (1.1%)     48 (0.2%)  core::iter::traits::iterator::Iterator::try_fold
    5454 (1.0%)     31 (0.1%)  hashbrown::raw::RawTable<T,A>::resize
    5029 (0.9%)    332 (1.2%)  alloc::sync::Arc<T>::drop_slow
    4624 (0.9%)     34 (0.1%)  alloc::raw_vec::RawVec<T,A>::grow_amortized
    4122 (0.8%)    458 (1.6%)  core::mem::align_of
    3890 (0.7%)    357 (1.3%)  <alloc::vec::Vec<T,A> as core::ops::drop::Drop>::drop
    3852 (0.7%)     37 (0.1%)  hashbrown::raw::RawTable<T,A>::find
    3668 (0.7%)     89 (0.3%)  <core::result::Result<T,E> as core::ops::try_trait::Try>::branch
    3415 (0.6%)     17 (0.1%)  hashbrown::raw::RawTable<T,A>::rehash_in_place
    3408 (0.6%)     24 (0.1%)  alloc::raw_vec::RawVec<T,A>::allocate_in
    3402 (0.6%)    378 (1.3%)  core::ptr::slice_from_raw_parts_mut

Seems like something causes us to generate more than three hundreds (!!!) instantiations of Arc::drop.

[bjorn3]

Try compiling with -Zsymbol-mangling-version=v0. That should make the symbol names include the exact generic parameters they are instantiated with.

[matklad]

zulip stream: https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer/topic/Dropping.20arcs