Add an AssertLinter command for tests

autoload/ale/assert.vim

@@ -0,0 +1,50 @@
+function! s:GetLinter() abort
+    let l:linters = ale#linter#GetLintersLoaded()
+    let l:filetype_linters = get(values(l:linters), 0, [])
+
+    if len(l:linters) is 0 || len(l:filetype_linters) is 0
+        throw 'No linters were loaded'
+    endif
+
+    if len(l:linters) > 1 || len(l:filetype_linters) > 1
+        throw 'More than one linter was loaded'
+    endif
+
+    return l:filetype_linters[0]
+endfunction
+
+" Load the currently loaded linter for a test case, and check that the command
+" matches the given string.
+function! ale#assert#Linter(expected_executable, expected_command) abort
+    let l:buffer = bufnr('')
+    let l:linter = s:GetLinter()
+    let l:executable = ale#linter#GetExecutable(l:buffer, l:linter)
+
+    if has_key(l:linter, 'command_chain')
+        let l:command = []
+
+        for l:chain_item in l:linter.command_chain
+            if empty(l:command)
+                call add(l:command, call(l:chain_item.callback, [l:buffer]))
+            else
+                call add(l:command, call(l:chain_item.callback, [l:buffer, []]))
+            endif
+        endfor
+    else
+        let l:command = ale#linter#GetCommand(l:buffer, l:linter)
+        " Replace %e with the escaped executable, so tests keep passing after
+        " linters are changed to use %e.
+        let l:command = substitute(l:command, '%e', '\=ale#Escape(l:executable)', 'g')
+    endif
+
+    AssertEqual
+    \   [a:expected_executable, a:expected_command],
+    \   [l:executable, l:command]
+endfunction
+
+command! -nargs=+ AssertLinter :call ale#assert#Linter(<args>)
+
+" A dummy function for making sure this module is loaded.
+function! ale#assert#Init() abort
+    call ale#linter#Reset()
+endfunction

autoload/ale/linter.vim

@@ -47,6 +47,13 @@ function! ale#linter#Reset() abort
     let s:linters = {}
 endfunction
 
+" Return a reference to the linters loaded.
+" This is only for tests.
+" Do not call this function.
+function! ale#linter#GetLintersLoaded() abort
+    return s:linters
+endfunction
+
 function! s:IsCallback(value) abort
     return type(a:value) == type('') || type(a:value) == type(function('type'))
 endfunction