Fix error loading "text" syntax for hover (#4574)

rust-analyzer sometimes returns a hover result with language being
"text", but there's no syntax/text.vim, so this would fail with:

    Error detected while processing function <SNR>150_VimOutputCallback[6]..<lambda>8[1]..ale#lsp#HandleMessage[30]..ale#hover#HandleLSPResponse[42]..ale#floating_preview#Show[13]..<SNR>161_VimShow:
    line   13:
    E484: Cannot open file syntax/text.vim

Only including the file when it actually exists fixes this.

autoload/ale/hover.vim

@@ -70,6 +70,18 @@ function! s:ConvertLanguageName(language) abort
     return a:language
 endfunction
 
+" Cache syntax file (non-)existence to avoid calling globpath repeatedly.
+let s:syntax_file_exists_cache = {}
+
+function! s:SyntaxFileExists(syntax_file) abort
+    if !has_key(s:syntax_file_exists_cache, a:syntax_file)
+        let s:syntax_file_exists_cache[a:syntax_file] =
+        \   !empty(globpath(&runtimepath, a:syntax_file))
+    endif
+
+    return s:syntax_file_exists_cache[a:syntax_file]
+endfunction
+
 function! ale#hover#ParseLSPResult(contents) abort
     let l:includes = {}
     let l:highlights = []
@@ -160,10 +172,11 @@ function! ale#hover#ParseLSPResult(contents) abort
                 let l:language = s:ConvertLanguageName(l:language)
 
                 if !empty(l:language)
-                    let l:includes[l:language] = printf(
-                    \   'syntax/%s.vim',
-                    \   l:language,
-                    \)
+                    let l:syntax_file = printf('syntax/%s.vim', l:language)
+
+                    if s:SyntaxFileExists(l:syntax_file)
+                        let l:includes[l:language] = l:syntax_file
+                    endif
 
                     let l:start = len(l:lines) + 1
                     let l:end = l:start + len(l:marked_lines)

test/test_hover.vader

@@ -168,7 +168,7 @@ Execute(LSP hover response with lists of strings should be handled):
 
 Execute(LSP hover response with lists of strings and marked strings should be handled):
   call HandleValidLSPResult({'contents': [
-  \  {'language': 'rust', 'value': 'foo'},
+  \  {'language': 'python', 'value': 'foo'},
   \  "bar\n",
   \]})
 
@@ -178,8 +178,8 @@ Execute(LSP hover response with lists of strings and marked strings should be ha
   \   {
   \     'commands': [
   \       'unlet! b:current_syntax',
-  \       'syntax include @ALE_hover_rust syntax/rust.vim',
-  \       'syntax region ALE_hover_1 start=/\%1l/ end=/\%2l/ contains=@ALE_hover_rust',
+  \       'syntax include @ALE_hover_python syntax/python.vim',
+  \       'syntax region ALE_hover_1 start=/\%1l/ end=/\%2l/ contains=@ALE_hover_python',
   \     ],
   \   },
   \ ],

test/test_hover_parsing.vader

@@ -35,12 +35,12 @@ Execute(A string with a code fence should be handled):
   \ [
   \   [
   \     'unlet! b:current_syntax',
-  \     'syntax include @ALE_hover_python syntax/python.vim',
+  \     'syntax include @ALE_hover_javascript syntax/javascript.vim',
   \     'unlet! b:current_syntax',
-  \     'syntax include @ALE_hover_typescript syntax/typescript.vim',
+  \     'syntax include @ALE_hover_python syntax/python.vim',
   \     'syntax region ALE_hover_1 start=/\%1l/ end=/\%3l/ contains=@ALE_hover_python',
   \     'syntax region ALE_hover_2 start=/\%5l/ end=/\%8l/ contains=@ALE_hover_python',
-  \     'syntax region ALE_hover_3 start=/\%8l/ end=/\%10l/ contains=@ALE_hover_typescript',
+  \     'syntax region ALE_hover_3 start=/\%8l/ end=/\%10l/ contains=@ALE_hover_javascript',
   \   ],
   \   [
   \     'def foo():',
@@ -64,7 +64,7 @@ Execute(A string with a code fence should be handled):
   \   'def bar():',
   \   '    pass',
   \   '```',
-  \   '```typescript',
+  \   '```javascript',
   \   'const baz = () => undefined',
   \   '```',
   \ ], "\n"))
@@ -74,12 +74,12 @@ Execute(Multiple strings with fences should be handled):
   \ [
   \   [
   \     'unlet! b:current_syntax',
-  \     'syntax include @ALE_hover_python syntax/python.vim',
+  \     'syntax include @ALE_hover_javascript syntax/javascript.vim',
   \     'unlet! b:current_syntax',
-  \     'syntax include @ALE_hover_typescript syntax/typescript.vim',
+  \     'syntax include @ALE_hover_python syntax/python.vim',
   \     'syntax region ALE_hover_1 start=/\%1l/ end=/\%3l/ contains=@ALE_hover_python',
   \     'syntax region ALE_hover_2 start=/\%5l/ end=/\%8l/ contains=@ALE_hover_python',
-  \     'syntax region ALE_hover_3 start=/\%8l/ end=/\%10l/ contains=@ALE_hover_typescript',
+  \     'syntax region ALE_hover_3 start=/\%8l/ end=/\%10l/ contains=@ALE_hover_javascript',
   \   ],
   \   [
   \     'def foo():',
@@ -106,7 +106,7 @@ Execute(Multiple strings with fences should be handled):
   \     'def bar():',
   \     '    pass',
   \     '```',
-  \     '```typescript',
+  \     '```javascript',
   \     'const baz = () => undefined',
   \     '```',
   \   ], "\n"),
@@ -124,7 +124,7 @@ Execute(Objects with kinds should be handled):
   \     'def foo():',
   \     '    pass',
   \     '',
-  \     '```typescript',
+  \     '```javascript',
   \     'const baz = () => undefined',
   \     '```',
   \   ],
@@ -142,7 +142,7 @@ Execute(Objects with kinds should be handled):
   \   {
   \     'kind': 'plaintext',
   \     'value': join([
-  \       '```typescript',
+  \       '```javascript',
   \       'const baz = () => undefined',
   \       '```',
   \     ], "\n"),
@@ -171,3 +171,19 @@ Execute(Simple markdown formatting should be handled):
   \   '```',
   \   'formatted \_ line \_',
   \ ], "\n"))
+
+Execute(Non-existent syntax files shouldn't be loaded):
+  AssertEqual
+  \ [
+  \   [
+  \     'syntax region ALE_hover_1 start=/\%1l/ end=/\%2l/ contains=@ALE_hover_text',
+  \   ],
+  \   [
+  \     'hello',
+  \   ],
+  \ ],
+  \ ale#hover#ParseLSPResult(join([
+  \   '```text',
+  \   'hello',
+  \   '```',
+  \ ], "\n"))