feat: Add "nightly" option to rustfmt (#811)

Author: beeender

doc/formatter_options.md

@@ -97,6 +97,8 @@ options = {
 options = {
   -- The default edition of Rust to use when no Cargo.toml file is found
   default_edition = "2021",
+  -- Set to true to use the nightly version of rustfmt
+  nightly = false,
 }
 ```
 

lua/conform/formatters/rustfmt.lua

@@ -10,12 +10,18 @@ return {
   options = {
     -- The default edition of Rust to use when no Cargo.toml file is found
     default_edition = "2021",
+    -- Set to true to use the nightly version of rustfmt
+    nightly = false,
   },
   args = function(self, ctx)
     local args = { "--emit=stdout" }
     local edition = util.parse_rust_edition(ctx.dirname) or self.options.default_edition
     table.insert(args, "--edition=" .. edition)
 
+    if self.options.nightly then
+      -- "+nightly" must be the first argument
+      table.insert(args, 1, "+nightly")
+    end
     return args
   end,
   cwd = util.root_file({