fix(mesonlsp): improved root detection #3775

Current behavior:
root is set on first meson file user is opened

New behavior:
root is set to parent of meson.build with project function call

Author: SpikedPaladin

lsp/mesonlsp.lua

@@ -3,8 +3,35 @@
 --- https://github.com/JCWasmx86/mesonlsp
 ---
 --- An unofficial, unendorsed language server for meson written in C++
+
+---Checks if a given path contains a valid Meson project root file
+---Looks for 'meson.build' file which contains 'project()' function
+local meson_matcher = function(_, path)
+  local pattern = 'meson.build'
+  local f = vim.fn.glob(table.concat({ path, pattern }, '/'))
+  if f == '' then
+    return false
+  end
+  for line in io.lines(f) do
+    -- skip meson comments
+    if not line:match '^%s*#.*' then
+      local str = line:gsub('%s+', '')
+      if str ~= '' then
+        if str:match '^project%(' then
+          return true
+        else
+          break
+        end
+      end
+    end
+  end
+  return false
+end
+
 return {
   cmd = { 'mesonlsp', '--lsp' },
   filetypes = { 'meson' },
-  root_markers = { 'meson.build', 'meson_options.txt', 'meson.options', '.git' },
+  root_dir = function(bufnr, on_dir)
+    on_dir(vim.fs.root(bufnr, meson_matcher) or vim.fs.root(bufnr, '.git'))
+  end,
 }