diagnostics: support sending ALE output to Neovim's diagnostics API (#4345)

Support replacing ALE's display of problems with sending problems to the Neovim diagnostics API.

:help g:ale_use_neovim_diagnostics_api

Co-authored-by: David Balatero <dbalatero@users.noreply.github.com>
Co-authored-by: Georgi Angelchev <angelchev@live.co.uk>
Co-authored-by: w0rp <devw0rp@gmail.com>

autoload/ale/debugging.vim

@@ -55,6 +55,7 @@ let s:global_variable_list = [
 \    'ale_sign_highlight_linenrs',
 \    'ale_statusline_format',
 \    'ale_type_map',
+\    'ale_use_neovim_diagnostics_api',
 \    'ale_use_global_executables',
 \    'ale_virtualtext_cursor',
 \    'ale_warn_about_trailing_blank_lines',

autoload/ale/engine.vim

@@ -184,9 +184,13 @@ endfunction
 function! ale#engine#SetResults(buffer, loclist) abort
     let l:linting_is_done = !ale#engine#IsCheckingBuffer(a:buffer)
 
+    if g:ale_use_neovim_diagnostics_api
+        call ale#engine#SendResultsToNeovimDiagnostics(a:buffer, a:loclist)
+    endif
+
     " Set signs first. This could potentially fix some line numbers.
     " The List could be sorted again here by SetSigns.
-    if g:ale_set_signs
+    if !g:ale_use_neovim_diagnostics_api && g:ale_set_signs
         call ale#sign#SetSigns(a:buffer, a:loclist)
     endif
 
@@ -199,11 +203,12 @@ function! ale#engine#SetResults(buffer, loclist) abort
         call ale#statusline#Update(a:buffer, a:loclist)
     endif
 
-    if g:ale_set_highlights
+    if !g:ale_use_neovim_diagnostics_api && g:ale_set_highlights
         call ale#highlight#SetHighlights(a:buffer, a:loclist)
     endif
 
-    if g:ale_virtualtext_cursor is# 'all' || g:ale_virtualtext_cursor == 2
+    if !g:ale_use_neovim_diagnostics_api
+    \&& (g:ale_virtualtext_cursor is# 'all' || g:ale_virtualtext_cursor == 2)
         call ale#virtualtext#SetTexts(a:buffer, a:loclist)
     endif
 
@@ -214,7 +219,8 @@ function! ale#engine#SetResults(buffer, loclist) abort
             call ale#cursor#EchoCursorWarning()
         endif
 
-        if g:ale_virtualtext_cursor is# 'current' || g:ale_virtualtext_cursor == 1
+        if !g:ale_use_neovim_diagnostics_api
+        \&& (g:ale_virtualtext_cursor is# 'current' || g:ale_virtualtext_cursor == 1)
             " Try and show the warning now.
             " This will only do something meaningful if we're in normal mode.
             call ale#virtualtext#ShowCursorWarning()
@@ -238,6 +244,19 @@ function! ale#engine#SetResults(buffer, loclist) abort
     endif
 endfunction
 
+function! ale#engine#SendResultsToNeovimDiagnostics(buffer, loclist) abort
+    if !has('nvim-0.6')
+        " We will warn the user on startup as well if they try to set
+        " g:ale_use_neovim_diagnostics_api outside of a Neovim context.
+        return
+    endif
+
+    " Keep the Lua surface area really small in the VimL part of ALE,
+    " and just require the diagnostics.lua module on demand.
+    let l:SendDiagnostics = luaeval('require("diagnostics").sendAleResultsToDiagnostics')
+    call l:SendDiagnostics(a:buffer, a:loclist)
+endfunction
+
 function! s:RemapItemTypes(type_map, loclist) abort
     for l:item in a:loclist
         let l:key = l:item.type

autoload/ale/virtualtext.vim

@@ -187,10 +187,8 @@ function! ale#virtualtext#ShowCursorWarning(...) abort
     let l:buffer = bufnr('')
 
     if mode(1) isnot# 'n'
-        return
-    endif
-
-    if ale#ShouldDoNothing(l:buffer)
+    \|| g:ale_use_neovim_diagnostics_api
+    \|| ale#ShouldDoNothing(l:buffer)
         return
     endif
 
@@ -210,12 +208,13 @@ function! ale#virtualtext#ShowCursorWarningWithDelay() abort
         return
     endif
 
+    call s:StopCursorTimer()
+
     if mode(1) isnot# 'n'
+    \|| g:ale_use_neovim_diagnostics_api
         return
     endif
 
-    call s:StopCursorTimer()
-
     let l:pos = getpos('.')[0:2]
 
     " Check the current buffer, line, and column number against the last