patch 9.0.0349: filetype of *.sil files not well detected

Problem:    Filetype of *.sil files not well detected.
Solution:   Inspect the file contents to guess the filetype.

runtime/autoload/dist/ft.vim

@@ -878,6 +878,23 @@ export def FTsig()
   endif
 enddef
 
+# This function checks the first 100 lines of files matching "*.sil" to
+# resolve detection between Swift Intermediate Language and SILE.
+export def FTsil()
+  for lnum in range(1, [line('$'), 100]->min())
+    var line: string = getline(lnum)
+    if line =~ '^\s*[\\%]'
+      setf sile
+      return
+    elseif line =~ '^\s*\S'
+      setf sil
+      return
+    endif
+  endfor
+  # no clue, default to "sil"
+  setf sil
+enddef
+
 export def FTsys()
   if exists("g:filetype_sys")
     exe "setf " .. g:filetype_sys

runtime/filetype.vim

@@ -1929,8 +1929,8 @@ au BufNewFile,BufRead *.cm			setf voscm
 au BufNewFile,BufRead *.swift			setf swift
 au BufNewFile,BufRead *.swift.gyb		setf swiftgyb
 
-" Swift Intermediate Language
-au BufNewFile,BufRead *.sil			setf sil
+" Swift Intermediate Language or SILE
+au BufNewFile,BufRead *.sil			call dist#ft#FTsil()
 
 " Sysctl
 au BufNewFile,BufRead */etc/sysctl.conf,*/etc/sysctl.d/*.conf	setf sysctl

src/testdir/test_filetype.vim

@@ -1827,6 +1827,44 @@ func Test_sig_file()
   filetype off
 endfunc
 
+" Test dist#ft#FTsil()
+func Test_sil_file()
+  filetype on
+
+  split Xfile.sil
+  call assert_equal('sil', &filetype)
+  bwipe!
+
+  let lines =<< trim END
+  // valid
+  let protoErasedPathA = \ABCProtocol.a
+
+  // also valid
+  let protoErasedPathA =
+          \ABCProtocol.a
+  END
+  call writefile(lines, 'Xfile.sil')
+
+  split Xfile.sil
+  call assert_equal('sil', &filetype)
+  bwipe!
+
+  " SILE
+
+  call writefile(['% some comment'], 'Xfile.sil')
+  split Xfile.sil
+  call assert_equal('sile', &filetype)
+  bwipe!
+
+  call writefile(['\begin[papersize=a6]{document}foo\end{document}'], 'Xfile.sil')
+  split Xfile.sil
+  call assert_equal('sile', &filetype)
+  bwipe!
+
+  call delete('Xfile.sil')
+  filetype off
+endfunc
+
 func Test_inc_file()
   filetype on
 

src/version.c

@@ -707,6 +707,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    349,
 /**/
     348,
 /**/