chore: rework nix_flake_fmt to use the new nix formatter subcommand

Core `nix` accepted [my PR to add a new `nix formatter build`
command](NixOS/nix#13063)!

That means we can (eventually) get rid of this huge mess of code. I'm
excited to try out the new feature, so I (unfortunately) made things
even more complicated by adding a "new" and and "old" codepath.
I've structured things so somebody with no knowledge of this code could
remove the legacy codepath in the future.

Author: jfly

lua/null-ls/builtins/formatting/nix_flake_fmt.lua

@@ -4,26 +4,8 @@ local log = require("null-ls.logger")
 local client = require("null-ls.client")
 
 local FORMATTING = methods.internal.FORMATTING
-local NOTIFICATION_TITLE = "discovering `nix fmt` entrypoint"
-local NOTIFICATION_TOKEN = "nix-flake-fmt-discovery"
-
---- Asynchronously computes the command that `nix fmt` would run, or nil if
---- we're not in a flake with a formatter, or if we fail to discover the
---- formatter somehow. When finished, it invokes the `done` callback with a
---- single string|nil parameter identifier the `nix fmt` entrypoint if found.
----
---- The formatter must follow treefmt's [formatter
---- spec](https://github.com/numtide/treefmt/blob/main/docs/formatter-spec.md).
----
---- This basically re-implements the "entrypoint discovery" that `nix fmt` does.
---- So why are we doing this ourselves rather than just invoking `nix fmt`?
---- Unfortunately, it can take a few moments to evaluate all your nix code to
---- figure out the formatter entrypoint. It can even be slow enough to exceed
---- Neovim's default LSP timeout.
---- By doing this ourselves, we can cache the result.
-local find_nix_fmt = function(opts, done)
-    done = vim.schedule_wrap(done)
 
+local run_job = function(opts)
     local async = require("plenary.async")
     local Job = require("plenary.job")
 
@@ -35,12 +17,30 @@ local find_nix_fmt = function(opts, done)
         Job:new(_opts):start()
     end, 2)
 
-    local tmpname = async.wrap(function(_done)
+    return run_job(opts)
+end
+
+local tmpname = function()
+    local async = require("plenary.async")
+
+    local mktemp = async.wrap(function(_done)
         vim.defer_fn(function()
             _done(vim.fn.tempname())
         end, 0)
     end, 1)
+    return mktemp()
+end
 
+--- Asynchronously build and return the formatter for the flake located at {root},
+--- If {root} is not a flake, or does not have a formatter, or we cannot build the formatter, return `nil`.
+--- This legacy codepath is quite complicated, and unnecessary now that `nix` has core support for
+--- returning the fromatter command.
+--- TODO: remove after the `nix formatter` subcommand has been released for a while.
+--- The command was introduced in https://github.com/NixOS/nix/commit/d155bb901241441149c701b9efc92f5785c2e1c3
+---
+--- @param root string
+--- @return string|nil
+local legacy_find_nix_fmt = function(root)
     local get_current_system = function()
         local status, stdout_lines, stderr_lines = run_job({
             command = "nix",
@@ -141,12 +141,6 @@ local find_nix_fmt = function(opts, done)
             builtins.toJSON result
         ]]
 
-        client.send_progress_notification(NOTIFICATION_TOKEN, {
-            kind = "report",
-            title = NOTIFICATION_TITLE,
-            message = "evaluating",
-        })
-
         local status, stdout_lines, stderr_lines = run_job({
             command = "nix",
             args = {
@@ -218,34 +212,97 @@ local find_nix_fmt = function(opts, done)
         return true
     end
 
+    local drv_path, nix_fmt_path = evaluate_flake_formatter(root)
+    if drv_path == nil then
+        return nil
+    end
+
+    -- Build the derivation. This ensures that `nix_fmt_path` exists.
+    if not build_derivation({ drv = drv_path, out_link = tmpname() }) then
+        return nil
+    end
+
+    return nix_fmt_path
+end
+
+local nix_has_formatter_subcommand = function()
+    local status, _, _ = run_job({
+        command = "nix",
+        args = {
+            "--extra-experimental-features",
+            "nix-command flakes",
+            "formatter",
+            "--help",
+        },
+    })
+
+    return status == 0
+end
+
+--- Asynchronously computes the command that `nix fmt` would run, or nil if
+--- we're not in a flake with a formatter, or if we fail to discover the
+--- formatter somehow. When finished, it invokes the `done` callback with a
+--- single string|nil parameter identifier the `nix fmt` entrypoint if found.
+---
+--- The formatter must follow treefmt's [formatter
+--- spec](https://github.com/numtide/treefmt/blob/main/docs/formatter-spec.md).
+---
+--- This basically re-implements the "entrypoint discovery" that `nix fmt` does.
+--- So why are we doing this ourselves rather than just invoking `nix fmt`?
+--- Unfortunately, it can take a few moments to evaluate all your nix code to
+--- figure out the formatter entrypoint. It can even be slow enough to exceed
+--- Neovim's default LSP timeout.
+--- By doing this ourselves, we can cache the result.
+local find_nix_fmt = function(opts, done)
+    done = vim.schedule_wrap(done)
+
+    local async = require("plenary.async")
+
+    local notification_title = "discovering `nix fmt` entrypoint"
+    local notification_token = "nix-flake-fmt-discovery"
+
     async.run(function()
-        client.send_progress_notification(NOTIFICATION_TOKEN, {
+        client.send_progress_notification(notification_token, {
             kind = "begin",
-            title = NOTIFICATION_TITLE,
+            title = notification_title,
         })
 
         local _done = function(result)
             done(result)
-            client.send_progress_notification(NOTIFICATION_TOKEN, {
+            client.send_progress_notification(notification_token, {
                 kind = "end",
-                title = NOTIFICATION_TITLE,
+                title = notification_title,
                 message = "done",
             })
         end
 
-        local drv_path, nix_fmt_path = evaluate_flake_formatter(opts.root)
-        if drv_path == nil then
-            return _done(nil)
-        end
+        local nix_fmt_path ---@type string|nil
+        local is_legacy = not nix_has_formatter_subcommand()
+        if is_legacy then
+            nix_fmt_path = legacy_find_nix_fmt()
+        else
+            local status, stdout_lines, stderr_lines = run_job({
+                command = "nix",
+                args = {
+                    "--extra-experimental-features",
+                    "nix-command",
+                    "formatter",
+                    "build",
+                    "--out-link",
+                    tmpname(),
+                },
+            })
 
-        -- Build the derivation. This ensures that `nix_fmt_path` exists.
-        client.send_progress_notification(NOTIFICATION_TOKEN, {
-            kind = "report",
-            title = NOTIFICATION_TITLE,
-            message = "building",
-        })
-        if not build_derivation({ drv = drv_path, out_link = tmpname() }) then
-            return _done(nil)
+            if status ~= 0 then
+                local stderr = table.concat(stderr_lines, "\n")
+                vim.defer_fn(function()
+                    log:warn(string.format("unable to build 'nix fmt' entrypoint. stderr: %s", stderr))
+                end, 0)
+                return false
+            end
+
+            local stdout = table.concat(stdout_lines, "\n")
+            nix_fmt_path = stdout
         end
 
         return _done(nix_fmt_path)