fix(rust-analyzer): skip stderr for cargo metadata (neovim#1235)

Vim's `system` output includes both STDOUT and STDERR, which means it
may contain all sorts of debug output, especially if the user is using
custom rustup toolchains. This change ignores STDERR output for parsing
the metadata output to fix rust_analyzer in those contexts.

This change also moves to `jobstart`/`jobwait` over `system`, which
avoids pitfalls related to shell argument parsing and escaping, and
allows finer-grained control over output redirection.

lua/lspconfig/rust_analyzer.lua

@@ -18,14 +18,36 @@ configs.rust_analyzer = {
     filetypes = { 'rust' },
     root_dir = function(fname)
       local cargo_crate_dir = util.root_pattern 'Cargo.toml'(fname)
-      local cmd = 'cargo metadata --no-deps --format-version 1'
+      local cmd = { 'cargo', 'metadata', '--no-deps', '--format-version', '1' }
       if cargo_crate_dir ~= nil then
-        cmd = cmd .. ' --manifest-path ' .. util.path.join(cargo_crate_dir, 'Cargo.toml')
+        cmd[#cmd + 1] = '--manifest-path'
+        cmd[#cmd + 1] = util.path.join(cargo_crate_dir, 'Cargo.toml')
+      end
+      local cargo_metadata = ''
+      local cargo_metadata_err = ''
+      local cm = vim.fn.jobstart(cmd, {
+        on_stdout = function(j, d, e)
+          cargo_metadata = table.concat(d, '\n')
+        end,
+        on_stderr = function(j, d, e)
+          cargo_metadata_err = table.concat(d, '\n')
+        end,
+        stdout_buffered = true,
+        stderr_buffered = true,
+      })
+      if cm > 0 then
+        cm = vim.fn.jobwait({ cm })[1]
+      else
+        cm = -1
       end
-      local cargo_metadata = vim.fn.system(cmd)
       local cargo_workspace_dir = nil
-      if vim.v.shell_error == 0 then
+      if cm == 0 then
         cargo_workspace_dir = vim.fn.json_decode(cargo_metadata)['workspace_root']
+      else
+        vim.notify(
+          string.format('cmd [%q] failed:\n%s', table.concat(cmd, ' '), cargo_metadata_err),
+          vim.log.levels.Warning
+        )
       end
       return cargo_workspace_dir
         or cargo_crate_dir