ci(test): enforce dylint warnings as errors

Pass RUSTFLAGS="-D warnings" to cargo dylint so any rule violation now
fails the script, matching what cargo clippy already does. Suppress the
two known macro_trailing_comma sites in tests/tree_builder.rs and
tests/visualizer.rs via crate-level cfg_attr-gated attributes:

* Gate everything on cfg(dylint_lib = "perfectionist") so the
  attributes only apply during dylint runs (the cfg is set by dylint's
  RUSTC wrapper). This keeps stable cargo check untouched and matches
  the user preference for narrow handling of unknown_tool, since
  perfectionist is registered as a tool only when dylint is active.
* register_tool(perfectionist) under #![feature(register_tool)] makes
  rustc accept the perfectionist:: lint prefix on the nightly toolchain
  that dylint uses, avoiding E0710.
* Use expect rather than allow for macro_trailing_comma per project
  preference, with an accompanying allow(unfulfilled_lint_expectations)
  because perfectionist appears to short-circuit when its lint is
  expected at crate scope, leaving the expect unfulfilled despite
  actual violations.
* allow(unexpected_cfgs) covers the dylint_lib cfg name itself, which
  rustc does not learn from --check-cfg.

Author: claude

test.sh

@@ -75,7 +75,8 @@ unit() (
 run_if "${FMT:-true}" cargo fmt -- --check
 # Dylint inspects the source under all feature gates in a single pass, so it is
 # run once with `--all-features` rather than across every feature combination.
-run_if "${DYLINT:-false}" cargo dylint --all -- --all-features --all-targets
+run_if "${DYLINT:-false}" env RUSTFLAGS="-D warnings ${RUSTFLAGS:-}" \
+  cargo dylint --all -- --all-features --all-targets
 unit "$@"
 unit --no-default-features "$@"
 unit --all-features "$@"

tests/tree_builder.rs

@@ -1,3 +1,24 @@
+#![cfg_attr(dylint_lib = "perfectionist", feature(register_tool))]
+#![cfg_attr(dylint_lib = "perfectionist", register_tool(perfectionist))]
+#![cfg_attr(
+    dylint_lib = "perfectionist",
+    expect(
+        perfectionist::macro_trailing_comma,
+        reason = "rustfmt collapses single-element multi-line vec! and removes the trailing comma; see https://github.com/KSXGitHub/perfectionist/issues"
+    )
+)]
+#![cfg_attr(
+    dylint_lib = "perfectionist",
+    allow(
+        unfulfilled_lint_expectations,
+        reason = "perfectionist's macro_trailing_comma short-circuits when its lint is allowed/expected at crate scope, so #[expect] cannot be fulfilled even though violations exist"
+    )
+)]
+#![allow(
+    unexpected_cfgs,
+    reason = "`dylint_lib` is set by dylint's compiler wrapper and is not in --check-cfg"
+)]
+
 use build_fs_tree::{FileSystemTree, dir, file};
 use derive_more::From;
 use parallel_disk_usage::{

tests/visualizer.rs

@@ -1,3 +1,24 @@
+#![cfg_attr(dylint_lib = "perfectionist", feature(register_tool))]
+#![cfg_attr(dylint_lib = "perfectionist", register_tool(perfectionist))]
+#![cfg_attr(
+    dylint_lib = "perfectionist",
+    expect(
+        perfectionist::macro_trailing_comma,
+        reason = "rustfmt collapses single-element multi-line vec! and removes the trailing comma; see https://github.com/KSXGitHub/perfectionist/issues"
+    )
+)]
+#![cfg_attr(
+    dylint_lib = "perfectionist",
+    allow(
+        unfulfilled_lint_expectations,
+        reason = "perfectionist's macro_trailing_comma short-circuits when its lint is allowed/expected at crate scope, so #[expect] cannot be fulfilled even though violations exist"
+    )
+)]
+#![allow(
+    unexpected_cfgs,
+    reason = "`dylint_lib` is set by dylint's compiler wrapper and is not in --check-cfg"
+)]
+
 use parallel_disk_usage::{
     bytes_format::BytesFormat::*,
     data_tree::DataTree,