runtime(rust): Do not use rustfmt as 'formatprg' by default

This reverts commit 4ac995bf93.

This was added in #16807, with no explanation for why it was necessary beyond
"it's an example of an idea". It completely breaks `gq` for me—rustfmt doesn't
reflow comments so is not an appropriate tool here! Beyond that, formatting a
selection with rustfmt treats that selection as if it were an entire file,
throwing away any indentation.

For example, the commit causes `gq` to turn this:

```rust
pub fn foo() {
    // blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah
}
```

into this:

```rust
pub fn foo() {
// blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah
}

```

which is totally wrong. In contrast, if I clear `formatprg` then `gq` does the
right thing again:

```rust
pub fn foo() {
    // blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah
    // blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah
    // blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah
    // blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah
    // blah blah blah blah blah blah
}
```

related: #16967
related: #17055
closes: #18640

Signed-off-by: Aaron Jacobs <jacobsa@google.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/autoload/rustfmt.vim

@@ -1,8 +1,8 @@
 " Author: Stephen Sugden <stephen@stephensugden.com>
 " Last Modified: 2023-09-11
 " Last Change:
-" 2025 Mar 31 by Vim project (rename s:RustfmtConfigOptions())
-" 2025 Jul 14 by Vim project (don't parse rustfmt version automatically #17745)
+" 2025 Oct 27 by Vim project don't use rustfmt as 'formatprg' by default
+"
 "
 " Adapted from https://github.com/fatih/vim-go
 " For bugs, patches and license go to https://github.com/rust-lang/rust.vim
@@ -68,13 +68,7 @@ function! s:RustfmtWriteMode()
     endif
 endfunction
 
-function! rustfmt#RustfmtConfigOptions()
-    let default = '--edition 2018'
-
-    if !get(g:, 'rustfmt_find_toml', 0)
-        return default
-    endif
-
+function! s:RustfmtConfigOptions()
     let l:rustfmt_toml = findfile('rustfmt.toml', expand('%:p:h') . ';')
     if l:rustfmt_toml !=# ''
         return '--config-path '.shellescape(fnamemodify(l:rustfmt_toml, ":p"))
@@ -97,7 +91,7 @@ function! s:RustfmtCommandRange(filename, line1, line2)
 
     let l:arg = {"file": shellescape(a:filename), "range": [a:line1, a:line2]}
     let l:write_mode = s:RustfmtWriteMode()
-    let l:rustfmt_config = rustfmt#RustfmtConfigOptions()
+    let l:rustfmt_config = s:RustfmtConfigOptions()
 
     " FIXME: When --file-lines gets to be stable, add version range checking
     " accordingly.
@@ -112,7 +106,7 @@ endfunction
 
 function! s:RustfmtCommand()
     let write_mode = g:rustfmt_emit_files ? '--emit=stdout' : '--write-mode=display'
-    let config = rustfmt#RustfmtConfigOptions()
+    let config = s:RustfmtConfigOptions()
     return join([g:rustfmt_command, write_mode, config, g:rustfmt_options])
 endfunction
 

runtime/doc/ft_rust.txt

@@ -166,13 +166,6 @@ g:rustfmt_detect_version~
 	Disabled by default for performance reasons
 >
 	    let g:rustfmt_detect_version = 1
-<
-                                                       *g:rustfmt_find_toml*
-g:rustfmt_emit_files~
-	When set to 1, will try to find "rustfmt.toml" file by searching from
-	current path upwards.  Disabled by default for performance reasons
->
-	    let g:rustfmt_find_toml = 1
 <
                                                           *g:rust_playpen_url*
 g:rust_playpen_url~

runtime/ftplugin/rust.vim

@@ -3,7 +3,6 @@
 " Maintainer:	Chris Morgan <me@chrismorgan.info>
 " Last Change:	2024 Mar 17
 "		2024 May 23 by Riley Bruins <ribru17@gmail.com ('commentstring')
-"		2025 Mar 31 by Vim project (set 'formatprg' option)
 " For bugs, patches and license go to https://github.com/rust-lang/rust.vim
 
 if exists("b:did_ftplugin")
@@ -58,19 +57,6 @@ setlocal includeexpr=rust#IncludeExpr(v:fname)
 
 setlocal suffixesadd=.rs
 
-if executable(get(g:, 'rustfmt_command', 'rustfmt'))
-    if get(g:, "rustfmt_fail_silently", 0)
-        augroup rust.vim.FailSilently
-            autocmd! * <buffer>
-            autocmd ShellFilterPost <buffer> if v:shell_error | execute 'echom "shell filter returned error " . v:shell_error . ", undoing changes"' | undo | endif
-        augroup END
-    endif
-
-    let &l:formatprg = get(g:, 'rustfmt_command', 'rustfmt') . ' ' .
-                \ get(g:, 'rustfmt_options', '') . ' ' .
-                \ rustfmt#RustfmtConfigOptions()
-endif
-
 if exists("g:ftplugin_rust_source_path")
     let &l:path=g:ftplugin_rust_source_path . ',' . &l:path
 endif
@@ -163,7 +149,7 @@ endif
 
 let b:undo_ftplugin = "
             \ compiler make |
-            \ setlocal formatoptions< comments< commentstring< include< includeexpr< suffixesadd< formatprg<
+            \ setlocal formatoptions< comments< commentstring< include< includeexpr< suffixesadd<
             \|if exists('b:rust_set_style')
                 \|setlocal tabstop< shiftwidth< softtabstop< expandtab< textwidth<
                 \|endif