Add gometalinter for go files

Linter is disabled by default (see g:ale_go_gometalinter_enabled) as it
conflicts with a number of established ALE linters (golint, govet,
gosimple, staticcheck, etc).

ale_linters/go/gometalinter.vim

@@ -0,0 +1,61 @@
+" Author: Ben Reedy <https://github.com/breed808>
+" Description: Adds support for the gometalinter suite for Go files
+
+" Disable gometalinter by default
+if !exists('g:ale_go_gometalinter_enabled')
+    let g:ale_go_gometalinter_enabled = 0
+endif
+
+if !exists('g:ale_go_gometalinter_options')
+    let g:ale_go_gometalinter_options = ''
+endif
+
+function! ale_linters#go#gometalinter#GetCommand(buffer) abort
+    if g:ale_go_gometalinter_enabled == 1
+        return 'gometalinter ' . g:ale_go_gometalinter_options . ' ' . fnameescape(fnamemodify(bufname(a:buffer), ':p:h'))
+    endif
+    return ''
+endfunction
+
+function! ale_linters#go#gometalinter#Handler(buffer, lines) abort
+    " Matches patterns line the following:
+    "
+    " file.go:27: missing argument for Printf("%s"): format reads arg 2, have only 1 args
+    " file.go:53:10: if block ends with a return statement, so drop this else and outdent its block (move short variable declaration to its own line if necessary)
+    " file.go:5:2: expected declaration, found 'STRING' "log"
+
+    " gometalinter returns relative paths so use tail of filename as part of pattern matcher
+    let l:filename = fnamemodify(bufname(a:buffer), ':t')
+    let l:path_pattern = '[a-zA-Z]\?\\\?:\?[[:alnum:]/\.\-_]\+'
+    let l:pattern = '^' . l:path_pattern . ':\(\d\+\):\?\(\d\+\)\?:\?:\?\(warning\|error\):\?\s\*\?\(.\+\)$'
+    let l:output = []
+
+    for l:line in a:lines
+        let l:match = matchlist(l:line, l:pattern)
+
+        " Omit errors from files other than the one currently open
+        if len(l:match) == 0 || l:line !~ l:filename
+            continue
+        endif
+
+        " vcol is Needed to indicate that the column is a character.
+        call add(l:output, {
+        \   'bufnr': a:buffer,
+        \   'lnum': l:match[1] + 0,
+        \   'vcol': 0,
+        \   'col': l:match[2] + 0,
+        \   'text': l:match[4],
+        \   'type': tolower(l:match[3]) ==# 'warning' ? 'W' : 'E',
+        \   'nr': -1,
+        \})
+    endfor
+    return l:output
+endfunction
+
+call ale#linter#Define('go', {
+\   'name': 'gometalinter',
+\   'executable': 'gometalinter',
+\   'command_callback': 'ale_linters#go#gometalinter#GetCommand',
+\   'callback': 'ale_linters#go#gometalinter#Handler',
+\   'lint_file': 1,
+\})