Avoid monomorphization bloat in Visitor::visit_{i128,u128}()

[max-heller]

Instantiate 128-bit integer error reporting code once per de::Error type instead of once per de::Visitor instantiation.

Context

I work on a (unfortunately closed source) project with a lot of large, deeply nested types deriving Deserialize for which compilation of the monomorphized Deserialize impls alone takes multiple minutes using erased-serde. I've been looking into which methods in particular are blowing up compile times and binary sizes, and Visitor::visit_{i128,u128}() popped up as an easy first step: the default implementations aren't huge (construct and return an error), but with ~7500 copies they add up.

Testing

Created a sample application deserializing a complex type (pandoc_ast::Pandoc, which has previously been used as a test case (serde-rs/json#313)). This example also uses erased_serde, which has a lot of other monomorphization bloat that I'm looking into as well (though it seems harder to avoid).

// src/main.rs
fn main() {
    _ = erased_serde::deserialize::<pandoc_ast::Pandoc>(
        &mut <dyn erased_serde::Deserializer>::erase(
            &mut serde_json::Deserializer::from_slice(&[]),
        ),
    );
}

# Cargo.toml
[package]
name = "playground"
version = "0.1.0"
edition = "2024"

[dependencies]
erased-serde = "0.4.10"
pandoc_ast = "0.8.6"
serde_json = "1.0.149"

--release before/after (7.5% decrease in llvm-lines)

cargo llvm-lines --bin playground --release | head -n20
   Compiling playground v0.1.0 (/Users/max/Documents/playground)
    Finished [`release` profile [optimized]](https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles) target(s) in 2.56s
  Lines                 Copies              Function name
  -----                 ------              -------------
  284824                7055                (TOTAL)
   12566 (4.4%,  4.4%)    80 (1.1%,  1.1%)  serde_core::de::Visitor::visit_i128
   12566 (4.4%,  8.8%)    80 (1.1%,  2.3%)  serde_core::de::Visitor::visit_u128
    5967 (2.1%, 10.9%)    81 (1.1%,  3.4%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_byte_buf
    5967 (2.1%, 13.0%)    81 (1.1%,  4.6%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_string
    5945 (2.1%, 15.1%)    52 (0.7%,  5.3%)  serde::private::de::content::visit_content_seq
    4682 (1.6%, 16.7%)    71 (1.0%,  6.3%)  erased_serde::any::Any::take
    4445 (1.6%, 18.3%)    71 (1.0%,  7.3%)  erased_serde::any::Any::new
    3843 (1.3%, 19.7%)    27 (0.4%,  7.7%)  <serde_core::de::impls::<impl serde_core::de::Deserialize for alloc::vec::Vec<T>>::deserialize::VecVisitor<T> as serde_core::de::Visitor>::visit_seq
    3764 (1.3%, 21.0%)    50 (0.7%,  8.4%)  <serde::private::de::content::SeqDeserializer<E> as serde_core::de::SeqAccess>::next_element_seed
    3607 (1.3%, 22.2%)    54 (0.8%,  9.2%)  <&mut dyn erased_serde::de::SeqAccess as serde_core::de::SeqAccess>::next_element_seed
    3537 (1.2%, 23.5%)    81 (1.1%, 10.3%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_bool
    3537 (1.2%, 24.7%)    81 (1.1%, 11.5%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_borrowed_bytes
    3537 (1.2%, 26.0%)    81 (1.1%, 12.6%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_borrowed_str
    3537 (1.2%, 27.2%)    81 (1.1%, 13.8%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_bytes
    3537 (1.2%, 28.5%)    81 (1.1%, 14.9%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_char
    3537 (1.2%, 29.7%)    81 (1.1%, 16.1%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_enum
    3537 (1.2%, 30.9%)    81 (1.1%, 17.2%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_f32

cargo llvm-lines --bin playground --release | head -n20
   Compiling playground v0.1.0 (/Users/max/Documents/playground)
    Finished [`release` profile [optimized]](https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles) target(s) in 2.75s
  Lines                 Copies              Function name
  -----                 ------              -------------
  263176                7057                (TOTAL)
    5967 (2.3%,  2.3%)    81 (1.1%,  1.1%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_byte_buf
    5967 (2.3%,  4.5%)    81 (1.1%,  2.3%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_string
    5945 (2.3%,  6.8%)    52 (0.7%,  3.0%)  serde::private::de::content::visit_content_seq
    4682 (1.8%,  8.6%)    71 (1.0%,  4.0%)  erased_serde::any::Any::take
    4445 (1.7%, 10.3%)    71 (1.0%,  5.0%)  erased_serde::any::Any::new
    3843 (1.5%, 11.7%)    27 (0.4%,  5.4%)  <serde_core::de::impls::<impl serde_core::de::Deserialize for alloc::vec::Vec<T>>::deserialize::VecVisitor<T> as serde_core::de::Visitor>::visit_seq
    3764 (1.4%, 13.2%)    50 (0.7%,  6.1%)  <serde::private::de::content::SeqDeserializer<E> as serde_core::de::SeqAccess>::next_element_seed
    3607 (1.4%, 14.5%)    54 (0.8%,  6.9%)  <&mut dyn erased_serde::de::SeqAccess as serde_core::de::SeqAccess>::next_element_seed
    3537 (1.3%, 15.9%)    81 (1.1%,  8.0%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_bool
    3537 (1.3%, 17.2%)    81 (1.1%,  9.2%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_borrowed_bytes
    3537 (1.3%, 18.6%)    81 (1.1%, 10.3%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_borrowed_str
    3537 (1.3%, 19.9%)    81 (1.1%, 11.5%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_bytes
    3537 (1.3%, 21.2%)    81 (1.1%, 12.6%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_char
    3537 (1.3%, 22.6%)    81 (1.1%, 13.8%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_enum
    3537 (1.3%, 23.9%)    81 (1.1%, 14.9%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_f32
    3537 (1.3%, 25.3%)    81 (1.1%, 16.1%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_f64
    3537 (1.3%, 26.6%)    81 (1.1%, 17.2%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_i128

Unoptimized build before/after (2% decrease in llvm-lines)

cargo llvm-lines --bin playground | head -n20
   Compiling serde_core v1.0.228
   Compiling serde_json v1.0.149
   Compiling serde v1.0.228
   Compiling erased-serde v0.4.10
   Compiling pandoc_ast v0.8.6
   Compiling playground v0.1.0 (/Users/max/Documents/playground)
    Finished [`dev` profile [unoptimized + debuginfo]](https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles) target(s) in 2.58s
  Lines                 Copies              Function name
  -----                 ------              -------------
  190379                9049                (TOTAL)
    6681 (3.5%,  3.5%)    71 (0.8%,  0.8%)  erased_serde::any::Any::take
    5416 (2.8%,  6.4%)    52 (0.6%,  1.4%)  serde::private::de::content::visit_content_seq
    5256 (2.8%,  9.1%)    71 (0.8%,  2.1%)  erased_serde::any::Any::new
    4809 (2.5%, 11.6%)    50 (0.6%,  2.7%)  <serde::private::de::content::SeqDeserializer<E> as serde_core::de::SeqAccess>::next_element_seed
    3762 (2.0%, 13.6%)    80 (0.9%,  3.6%)  serde_core::de::Visitor::visit_i128
    3762 (2.0%, 15.6%)    80 (0.9%,  4.5%)  serde_core::de::Visitor::visit_u128
    3445 (1.8%, 17.4%)   133 (1.5%,  5.9%)  <core::result::Result<T,E> as erased_serde::map::ResultExt<T,E>>::unsafe_map
    3398 (1.8%, 19.2%)   140 (1.5%,  7.5%)  core::result::Result<T,E>::map
    3180 (1.7%, 20.9%)   228 (2.5%, 10.0%)  core::option::Option<T>::unwrap
    2832 (1.5%, 22.3%)    12 (0.1%, 10.1%)  <serde::private::de::content::TaggedContentVisitor<T> as serde_core::de::Visitor>::visit_map
    2771 (1.5%, 23.8%)    81 (0.9%, 11.0%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_byte_buf
    2771 (1.5%, 25.3%)    81 (0.9%, 11.9%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_string
    2711 (1.4%, 26.7%)    79 (0.9%, 12.8%)  serde_core::de::Visitor::visit_byte_buf
    2711 (1.4%, 28.1%)    79 (0.9%, 13.7%)  serde_core::de::Visitor::visit_string
    2698 (1.4%, 29.5%)    38 (0.4%, 14.1%)  <serde::private::de::content::ContentDeserializer<E> as serde_core::de::Deserializer>::deserialize_seq
    2665 (1.4%, 30.9%)    80 (0.9%, 15.0%)  serde_core::de::Visitor::visit_char
    2590 (1.4%, 32.3%)    70 (0.8%, 15.7%)  <alloc::boxed::Box<T,A> as core::ops::drop::Drop>::drop

cargo llvm-lines --bin playground | head -n20
   Compiling playground v0.1.0 (/Users/max/Documents/playground)
    Finished [`dev` profile [unoptimized + debuginfo]](https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles) target(s) in 1.18s
  Lines                 Copies              Function name
  -----                 ------              -------------
  186302                9043                (TOTAL)
    6681 (3.6%,  3.6%)    71 (0.8%,  0.8%)  erased_serde::any::Any::take
    5416 (2.9%,  6.5%)    52 (0.6%,  1.4%)  serde::private::de::content::visit_content_seq
    5256 (2.8%,  9.3%)    71 (0.8%,  2.1%)  erased_serde::any::Any::new
    4809 (2.6%, 11.9%)    50 (0.6%,  2.7%)  <serde::private::de::content::SeqDeserializer<E> as serde_core::de::SeqAccess>::next_element_seed
    3445 (1.8%, 13.7%)   133 (1.5%,  4.2%)  <core::result::Result<T,E> as erased_serde::map::ResultExt<T,E>>::unsafe_map
    3398 (1.8%, 15.6%)   140 (1.5%,  5.7%)  core::result::Result<T,E>::map
    3180 (1.7%, 17.3%)   228 (2.5%,  8.2%)  core::option::Option<T>::unwrap
    2832 (1.5%, 18.8%)    12 (0.1%,  8.4%)  <serde::private::de::content::TaggedContentVisitor<T> as serde_core::de::Visitor>::visit_map
    2771 (1.5%, 20.3%)    81 (0.9%,  9.3%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_byte_buf
    2771 (1.5%, 21.8%)    81 (0.9%, 10.2%)  <erased_serde::de::erase::Visitor<T> as erased_serde::de::Visitor>::erased_visit_string
    2711 (1.5%, 23.2%)    79 (0.9%, 11.0%)  serde_core::de::Visitor::visit_byte_buf
    2711 (1.5%, 24.7%)    79 (0.9%, 11.9%)  serde_core::de::Visitor::visit_string
    2698 (1.4%, 26.1%)    38 (0.4%, 12.3%)  <serde::private::de::content::ContentDeserializer<E> as serde_core::de::Deserializer>::deserialize_seq
    2665 (1.4%, 27.6%)    80 (0.9%, 13.2%)  serde_core::de::Visitor::visit_char
    2590 (1.4%, 28.9%)    70 (0.8%, 14.0%)  <alloc::boxed::Box<T,A> as core::ops::drop::Drop>::drop
    2456 (1.3%, 30.3%)     9 (0.1%, 14.1%)  <serde::private::de::content::ContentDeserializer<E> as serde_core::de::Deserializer>::deserialize_any
    2413 (1.3%, 31.6%)    85 (0.9%, 15.0%)  serde_core::de::Visitor::visit_newtype_struct

[Mingun]

The same thing, but better, implemented in #2781 in 8db73ad.

[max-heller]

The same thing, but better, implemented in #2781 in 8db73ad.

Regardless of the exact shape (new Unexpected public functions or not), it'd be nice to get in a surgical change like this or 8db73ad without blocking on the rest of that PR or that PR's dependencies.

[Mingun]

Yes, I just suggest to cherry-pick my commit if needed.