Add support for syntax_tree fixer (#4268)

This is the library that now powers prettier/plugin-ruby but is also
stands on its own: https://github.com/ruby-syntax-tree/syntax_tree

autoload/ale/fix/registry.vim

@@ -221,6 +221,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['swift'],
 \       'description': 'Apply SwiftFormat to a file.',
 \   },
+\   'syntax_tree': {
+\       'function': 'ale#fixers#syntax_tree#Fix',
+\       'suggested_filetypes': ['ruby'],
+\       'description': 'Fix ruby files with stree write',
+\   },
 \   'apple-swift-format': {
 \       'function': 'ale#fixers#appleswiftformat#Fix',
 \       'suggested_filetypes': ['swift'],

autoload/ale/fixers/syntax_tree.vim

@@ -0,0 +1,19 @@
+call ale#Set('ruby_syntax_tree_options', '')
+call ale#Set('ruby_syntax_tree_executable', 'stree')
+
+function! ale#fixers#syntax_tree#GetCommand(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'ruby_syntax_tree_executable')
+    let l:options = ale#Var(a:buffer, 'ruby_syntax_tree_options')
+
+    return ale#ruby#EscapeExecutable(l:executable, 'stree')
+    \   . ' write'
+    \   . (!empty(l:options) ? ' ' . l:options : '')
+    \   . ' %t'
+endfunction
+
+function! ale#fixers#syntax_tree#Fix(buffer) abort
+    return {
+    \   'command': ale#fixers#syntax_tree#GetCommand(a:buffer),
+    \   'read_temporary_file': 1,
+    \}
+endfunction