rustfmt: actually fix the failing test

It only runs on nightly, but the next commits will show
why this is useful.

pkgs/development/compilers/rust/rustfmt-fix-self-tests.patch

@@ -0,0 +1,13 @@
+--- a/src/tools/rustfmt/src/ignore_path.rs
++++ b/src/tools/rustfmt/src/ignore_path.rs
+@@ -37,9 +37,9 @@
+     #[nightly_only_test]
+     #[test]
+     fn test_ignore_path_set() {
+-        use std::path::{Path, PathBuf};
+         use crate::config::{Config, FileName};
+         use crate::ignore_path::IgnorePathSet;
++        use std::path::{Path, PathBuf};
+
+         let config =
+             Config::from_toml(r#"ignore = ["foo.rs", "bar_dir/*"]"#, Path::new("")).unwrap();

pkgs/development/compilers/rust/rustfmt.nix

@@ -21,6 +21,9 @@ rustPlatform.buildRustPackage rec {
   CFG_RELEASE = rustPlatform.rust.rustc.version;
   CFG_RELEASE_CHANNEL = "stable";
 
+  # FIXME: seems fixed upstream, remove after the next update
+  patches = [ ./rustfmt-fix-self-tests.patch ];
+
   meta = with lib; {
     description = "A tool for formatting Rust code according to style guidelines";
     homepage = "https://github.com/rust-lang-nursery/rustfmt";