feat(fs): add vim.fs.root (#28477)

vim.fs.root() is a function for finding a project root relative to a
buffer using one or more "root markers". This is useful for LSP and
could be useful for other "projects" designs, as well as for any plugins
which work with a "projects" concept.

runtime/lua/vim/fs.lua

@@ -197,13 +197,6 @@ end
 --- Examples:
 ---
 --- ```lua
---- -- location of Cargo.toml from the current buffer's path
---- local cargo = vim.fs.find('Cargo.toml', {
----   upward = true,
----   stop = vim.uv.os_homedir(),
----   path = vim.fs.dirname(vim.api.nvim_buf_get_name(0)),
---- })
----
 --- -- list all test directories under the runtime directory
 --- local test_dirs = vim.fs.find(
 ---   {'test', 'tst', 'testdir'},
@@ -334,6 +327,56 @@ function M.find(names, opts)
   return matches
 end
 
+--- Find the first parent directory containing a specific "marker", relative to a buffer's
+--- directory.
+---
+--- Example:
+---
+--- ```lua
+--- -- Find the root of a Python project, starting from file 'main.py'
+--- vim.fs.root(vim.fs.joinpath(vim.env.PWD, 'main.py'), {'pyproject.toml', 'setup.py' })
+---
+--- -- Find the root of a git repository
+--- vim.fs.root(0, '.git')
+---
+--- -- Find the parent directory containing any file with a .csproj extension
+--- vim.fs.root(0, function(name, path)
+---   return name:match('%.csproj$') ~= nil
+--- end)
+--- ```
+---
+--- @param source integer|string Buffer number (0 for current buffer) or file path to begin the
+---               search from.
+--- @param marker (string|string[]|fun(name: string, path: string): boolean) A marker, or list
+---               of markers, to search for. If a function, the function is called for each
+---               evaluated item and should return true if {name} and {path} are a match.
+--- @return string? # Directory path containing one of the given markers, or nil if no directory was
+---         found.
+function M.root(source, marker)
+  assert(source, 'missing required argument: source')
+  assert(marker, 'missing required argument: marker')
+
+  local path ---@type string
+  if type(source) == 'string' then
+    path = source
+  elseif type(source) == 'number' then
+    path = vim.api.nvim_buf_get_name(source)
+  else
+    error('invalid type for argument "source": expected string or buffer number')
+  end
+
+  local paths = M.find(marker, {
+    upward = true,
+    path = path,
+  })
+
+  if #paths == 0 then
+    return nil
+  end
+
+  return vim.fs.dirname(paths[1])
+end
+
 --- Split a Windows path into a prefix and a body, such that the body can be processed like a POSIX
 --- path. The path must use forward slashes as path separator.
 ---

runtime/lua/vim/lsp.lua

@@ -210,7 +210,7 @@ end
 --- vim.lsp.start({
 ---    name = 'my-server-name',
 ---    cmd = {'name-of-language-server-executable'},
----    root_dir = vim.fs.dirname(vim.fs.find({'pyproject.toml', 'setup.py'}, { upward = true })[1]),
+---    root_dir = vim.fs.root(0, {'pyproject.toml', 'setup.py'}),
 --- })
 --- ```
 ---
@@ -219,9 +219,9 @@ end
 --- - `name` arbitrary name for the LSP client. Should be unique per language server.
 --- - `cmd` command string[] or function, described at |vim.lsp.start_client()|.
 --- - `root_dir` path to the project root. By default this is used to decide if an existing client
----   should be re-used. The example above uses |vim.fs.find()| and |vim.fs.dirname()| to detect the
----   root by traversing the file system upwards starting from the current directory until either
----   a `pyproject.toml` or `setup.py` file is found.
+---   should be re-used. The example above uses |vim.fs.root()| and |vim.fs.dirname()| to detect
+---   the root by traversing the file system upwards starting from the current directory until
+---   either a `pyproject.toml` or `setup.py` file is found.
 --- - `workspace_folders` list of `{ uri:string, name: string }` tables specifying the project root
 ---   folders used by the language server. If `nil` the property is derived from `root_dir` for
 ---   convenience.