Add support for nvim's virtualtext on cursor

- Add g:ale_virtualtext_cursor boolean to enable/disable it
- Add g:ale_virtualtext_prefix to configure what prefix to use (default:
'> ')
- Requires neovim 0.3.2's unreleased API `nvim_buf_set_virtual_text`

autoload/ale/cursor.vim

@@ -52,6 +52,29 @@ function! ale#cursor#TruncatedEcho(original_message) abort
     endtry
 endfunction
 
+function! ale#cursor#ClearVirtualText() abort
+    if !has('nvim-0.3.2')
+        return
+    endif
+
+    let l:buffer = bufnr('')
+
+    call nvim_buf_clear_highlight(l:buffer, 1000, 0, -1)
+endfunction
+
+function! ale#cursor#ShowVirtualText(message, hl_group) abort
+    if !has('nvim-0.3.2')
+        return
+    endif
+
+    let l:cursor_position = getcurpos()
+    let l:line = line('.')
+    let l:buffer = bufnr('')
+    let l:prefix = get(g:, 'ale_virtualtext_prefix', '> ')
+
+    call nvim_buf_set_virtual_text(l:buffer, 1000, l:line-1, [[l:prefix.a:message, a:hl_group]], {})
+endfunction
+
 function! s:FindItemAtCursor(buffer) abort
     let l:info = get(g:ale_buffer_info, a:buffer, {})
     let l:loclist = get(l:info, 'loclist', [])
@@ -72,7 +95,7 @@ endfunction
 function! ale#cursor#EchoCursorWarning(...) abort
     let l:buffer = bufnr('')
 
-    if !g:ale_echo_cursor && !g:ale_cursor_detail
+    if !g:ale_echo_cursor && !g:ale_cursor_detail && !g:ale_virtualtext_cursor
         return
     endif
 
@@ -108,12 +131,30 @@ function! ale#cursor#EchoCursorWarning(...) abort
             call ale#preview#CloseIfTypeMatches('ale-preview')
         endif
     endif
+
+    if g:ale_virtualtext_cursor
+        call ale#cursor#ClearVirtualText()
+
+        if !empty(l:loc)
+            let l:msg = get(l:loc, 'detail', l:loc.text)
+            let l:hl_group = 'ALEInfo'
+            let l:type = get(l:loc, 'type', 'E')
+
+            if l:type is# 'E'
+                let l:hl_group = 'ALEError'
+            elseif l:type is# 'I'
+                let l:hl_group = 'ALEWarning'
+            endif
+
+            call ale#cursor#ShowVirtualText(l:msg, l:hl_group)
+        endif
+    endif
 endfunction
 
 function! ale#cursor#EchoCursorWarningWithDelay() abort
     let l:buffer = bufnr('')
 
-    if !g:ale_echo_cursor && !g:ale_cursor_detail
+    if !g:ale_echo_cursor && !g:ale_cursor_detail && !g:ale_virtualtext_cursor
         return
     endif
 

autoload/ale/debugging.vim

@@ -22,14 +22,14 @@ let s:global_variable_list = [
 \    'ale_lint_delay',
 \    'ale_lint_on_enter',
 \    'ale_lint_on_filetype_changed',
+\    'ale_lint_on_insert_leave',
 \    'ale_lint_on_save',
 \    'ale_lint_on_text_changed',
-\    'ale_lint_on_insert_leave',
 \    'ale_linter_aliases',
 \    'ale_linters',
 \    'ale_linters_explicit',
-\    'ale_list_window_size',
 \    'ale_list_vertical',
+\    'ale_list_window_size',
 \    'ale_loclist_msg_format',
 \    'ale_max_buffer_history_size',
 \    'ale_max_signs',
@@ -52,6 +52,8 @@ let s:global_variable_list = [
 \    'ale_statusline_format',
 \    'ale_type_map',
 \    'ale_use_global_executables',
+\    'ale_virtualtext_cursor',
+\    'ale_virtualtext_prefix',
 \    'ale_warn_about_trailing_blank_lines',
 \    'ale_warn_about_trailing_whitespace',
 \]

autoload/ale/engine.vim

@@ -298,7 +298,7 @@ function! ale#engine#SetResults(buffer, loclist) abort
     endif
 
     if l:linting_is_done
-        if g:ale_echo_cursor
+        if g:ale_echo_cursor || g:ale_virtualtext_cursor
             " Try and echo the warning now.
             " This will only do something meaningful if we're in normal mode.
             call ale#cursor#EchoCursorWarning()

autoload/ale/events.vim

@@ -131,7 +131,7 @@ function! ale#events#Init() abort
                 autocmd InsertLeave * call ale#Queue(0)
             endif
 
-            if g:ale_echo_cursor || g:ale_cursor_detail
+            if g:ale_echo_cursor || g:ale_cursor_detail || g:ale_virtualtext_cursor
                 autocmd CursorMoved,CursorHold * if exists('*ale#engine#Cleanup') | call ale#cursor#EchoCursorWarningWithDelay() | endif
                 " Look for a warning to echo as soon as we leave Insert mode.
                 " The script's position variable used when moving the cursor will