Rustfmt is extremely slow in large files when using nvim-lsp

[maddiemort]

• rust.vim version: 87c745d (latest master) installed through vim-plug
• OS: macOS 10.15.7
• Terminal: alacritty 0.7.2 (5ac8060)

Steps to reproduce:

• Install nvim from the tip of master (must be an 0.5 version)
• Install rustup
• Run rustup toolchain install nightly-2021-03-01
• Run rustup component add rustfmt --toolchain nightly-2021-03-01
• Install rust-analyzer (I'm on rust-analyzer 2021-02-15, installed via Nix unstable)

• Put the following minimal config in ~/.config/nvim/init.vim:

  ```vim set shell=/bin/bash " Install vim-plug if it isn't already if empty(glob('~/.local/share/nvim/site/autoload/plug.vim')) silent !curl -fLo ~/.local/share/nvim/site/autoload/plug.vim --create-dirs \ https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim au VimEnter * PlugInstall --sync | source $MYVIMRC endif call plug#begin('~/.local/share/nvim/plugged') Plug 'neovim/nvim-lspconfig' Plug 'rust-lang/rust.vim' call plug#end() lua <
• Make sure ~/.local/share/nvim/site/autoload and ~/.local/share/nvim/plugged are empty or do not exist
• Open nvim once to allow the plugins to install, then close it
• Create a crate that contains a very long Rust file (in my case, the file is over 3000 lines long - the problem appears with shorter files but is less noticeable), and open that long file
• Verify the language server is running by (e.g.) putting the cursor over a documented type name and running :lua vim.lsp.buf.hover() (documentation info should appear)
• Insert some extra blank lines in the file, so that rustfmt will remove them when run
• Run :w

Expected vs. actual behavior:

• Expected: the file saves quickly, with no noticeable delay, and the contents of the file are automatically formatted using rustfmt
• Actual: The file is formatted correctly, but takes more than 10 seconds to save, during which time the editor is completely frozen

Please note a few things:

• The issue also appears when running :RustFmt manually
• Saving the file becomes almost instantaneous if auto-formatting is disabled (but :RustFmt still hangs just as much)
• Saving the file and formatting it also becomes almost instantaneous (with auto-formatting enabled) if the language server is disabled (remove the Lua section in the config above), so it appears to be the interaction between the two things that's the problem.
• cargo +nightly-2021-03-01 fmt only takes about a second, at most, to run from outside nvim
• There has been some mention before of shells such as fish (which is my login shell) causing delays, and I do see additional delays if I remove set shell=/bin/bash from that config, but please note that that line means fish shouldn't be involved here. Changing my login shell to bash or zsh does not fix the problem.

I'm happy to do any debugging or profiling you'd like me to, but you'll have to let me know what to run in order to do that, because my knowledge of vim really isn't extensive enough for me to know how to do that myself. Thanks!

Paste debugging info from the Rust Vim plugin via one of the following commands: :RustInfo, :RustInfoToClipboard, or :RustInfoToFile <filename>.

``` rust.vim Global Variables: let g:ftplugin_rust_source_path = v:null let g:loaded_syntastic_rust_cargo_checker = v:null let g:loaded_syntastic_rust_filetype = v:null let g:loaded_syntastic_rust_rustc_checker = v:null let g:rust_bang_comment_leader = v:null let g:rust_cargo_avoid_whole_workspace = v:null let g:rust_clip_command = v:null let g:rust_conceal = v:null let g:rust_conceal_mod_path = v:null let g:rust_conceal_pub = v:null let g:rust_fold = v:null let g:rust_last_args = v:null let b:rust_last_args = [] let g:rust_last_rustc_args = v:null let b:rust_last_rustc_args = [] let g:rust_original_delimitMate_excluded_regions = v:null let g:rust_playpen_url = v:null let g:rust_prev_delimitMate_quotes = v:null let g:rust_recent_nearest_cargo_tol = v:null let g:rust_recent_root_cargo_toml = v:null let g:rust_recommended_style = v:null let g:rust_set_conceallevel = v:null let g:rust_set_conceallevel=1 = v:null let g:rust_set_foldmethod = v:null let g:rust_set_foldmethod=1 = v:null let g:rust_shortener_url = v:null let g:rustc_makeprg_no_percent = v:null let g:rustc_path = v:null let g:rustfmt_autosave = 1 let g:rustfmt_autosave_if_config_present = v:null let g:rustfmt_command = 'rustup run nightly-2021-03-01 rustfmt' let g:rustfmt_emit_files = 1 let g:rustfmt_fail_silently = 0 let g:rustfmt_options = '' let g:syntastic_extra_filetypes = ['rust'] let g:syntastic_rust_cargo_fname = v:null rustfmt 1.4.36-nightly (7de6968e 2021-02-07) rustc 1.51.0-nightly (1d0d76f8d 2021-01-24) cargo 1.51.0-nightly (783bc43c6 2021-01-20) NVIM v0.5.0-dev+c1fbc2ddf Build type: Release LuaJIT 2.1.0-beta3 Compilation: clang -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 -DNDEBUG -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -Wshadow -Wconversion -Wmissing-prototypes -Wimplicit-fallthrough -Wvla -fstack-protector-strong -fno-common -fdiagnostics-color=auto -DINCLUDE_GENERATED_DECLARATIONS -D_GNU_SOURCE -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -DMIN_LOG_LEVEL=3 -I/tmp/neovim-20210301-91403-1qxibkt/build/config -I/tmp/neovim-20210301-91403-1qxibkt/src -I/usr/local/include -I/usr/local/opt/luv/include -I/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include -I/usr/local/opt/gettext/include -I/tmp/neovim-20210301-91403-1qxibkt/build/src/nvim/auto -I/tmp/neovim-20210301-91403-1qxibkt/build/include Compiled by soren@Rigel.local Features: +acl +iconv +tui See ":help feature-compile" system vimrc file: "$VIM/sysinit.vim" fall-back for $VIM: "/usr/local/Cellar/neovim/HEAD-c1fbc2d_1/share/nvim" Run :checkhealth for more info ```

[rubdos]

FWIW, I'm experiencing the same delay with coc-nvim+coc-rust-analyzer.

[stsewd]

Hi, does this patch speed things up https://github.com/rust-lang/rust.vim/pull/448?

[amaush]

@stsewd Found your submitted patch and applied it to my vim-polyglot "rustfmt.vim" file and rustfmt autosave improved from 2.8 secs to 0.8 secs on a file with ~700 lines. Definitely a massive improvement. Thank you!

[maddiemort]

I haven't had the chance to fully test the performance impact of your patch yet @stsewd, but my first impression is that it triggers this issue: neovim/neovim#14645

[amaush]

I managed to reproduce the issue mentioned by @nerosnm by reverting 'nvim_buf_set_lines()' back to 'call_setline()'.

[gyscos]

I think this issue is still present - running :RustFmt here is terribly slow (~10 seconds), while running it separately from the CLI is instantaneous.

Looks like the "fix" PR mentioned above ended up being reverted, probably because it was causing #14645. Is there another workaround?

[awused]

It looks like https://github.com/neovim/neovim/issues/14645 is fixed. I've been manually applying https://github.com/rust-lang/rust.vim/pull/448 for years, but if the underlying neovim bug is fixed maybe it's time to re-merge it? I'll admit, I never noticed the https://github.com/neovim/neovim/issues/14645 bug before nvim 0.10.0, so I'm not the one to say if the behaviour that caused it to be rolled back is truly gone.