rust-fold.vim

function! MakeRustFuncDefs()
    let b:RustFuncDefs = []

    let lnum = 1
    while lnum <= line('$')
        let current_line = getline(lnum)
        if match(current_line, '^ *\(pub \)\?fn') > -1
            call AddRustFunc(lnum)
        endif

        let lnum += 1
    endwhile
endfunction

function! AddRustFunc(lnum)
    let save_pos = getpos('.')
    call setpos('.', [0, a:lnum, 1, 0])

    call search('{')
    let start_lnum = line('.')

    let end_lnum = searchpair('{', '', '}', 'n')
    if end_lnum < 1
        call setpos('.', save_pos)
        return
    endif

    call add(b:RustFuncDefs, [start_lnum, end_lnum]);
    call setpos('.', save_pos)
endfunction

function! RustFold()
    if !exists("b:RustFuncDefs")
        call MakeRustFuncDefs()
    endif

    for [start_lnum, end_lnum] in b:RustFuncDefs
        if start_lnum > v:lnum
            return 0
        endif

        if v:lnum == start_lnum + 1
            return ">1"
        elseif v:lnum == end_lnum
            return "<1"
        elseif v:lnum > start_lnum && v:lnum < end_lnum
            return "="
        endif
    endfor
endfunction

autocmd FileType rust setlocal foldmethod=expr foldexpr=RustFold()