feat: add comments and ensure bevy_lint_driver is built

bevy_lint/tests/ui.rs

@@ -1,35 +1,52 @@
-use std::{ffi::OsString, path::PathBuf};
-use ui_test::{color_eyre, run_tests, CommandBuilder, Config};
+use std::path::{Path, PathBuf};
+use ui_test::{color_eyre::{self, eyre::ensure}, run_tests, CommandBuilder, Config};
 
 fn main() -> color_eyre::Result<()> {
-    let config = config();
+    let config = config()?;
     run_tests(config)
 }
 
-fn config() -> Config {
-    Config {
-        // Make this an empty string, because `bevy_lint_driver` does not currently support the
-        // `--version` flag, which is required to auto-discover the host.
+/// Generates a custom [`Config`] for `bevy_lint`'s UI tests.
+fn config() -> color_eyre::Result<Config> {
+    const DRIVER_PATH: &str = "../target/debug/bevy_lint_driver";
+
+    ensure!(Path::new(DRIVER_PATH).is_file());
+
+    let config = Config {
+        // When `host` is `None`, `ui_test` will attempt to auto-discover the host by calling
+        // `program -vV`. Unfortunately, `bevy_lint_driver` does not yet support the version flag,
+        // so we manually specify the host as an empty string. This means that, for now, host-
+        // specific configuration in UI tests will not work.
         host: Some(String::new()),
         program: CommandBuilder {
+            // We call `rustup run` to setup the proper environmental variables, so that
+            // `bevy_lint_driver` can link to `librustc_driver.so`.
             program: "rustup".into(),
             args: vec![
                 "run".into(),
+                // TODO: Use `build.rs` to change this dynamically.
                 "nightly-2024-08-21".into(),
-                "../target/debug/bevy_lint_driver".into(),
+                DRIVER_PATH.into(),
+                // `bevy_lint_driver` expects the first argument to be the path to `rustc`.
                 "rustc".into(),
+                // This is required so that `ui_test` can parse warnings and errors.
                 "--error-format=json".into(),
-                // This allows examples to `use bevy;`.
+                // These two lines tell `rustc` to search in `target/debug/deps` for dependencies.
+                // This is required for UI tests to import `bevy`.
                 "-L".into(),
                 "all=../target/debug/deps".into(),
+                // This lets UI tests write `use bevy::*;` without `extern bevy;` first.
                 "--extern=bevy".into(),
             ],
+
             out_dir_flag: Some("--out-dir".into()),
             input_file_flag: None,
             envs: Vec::new(),
             cfg_flag: Some("--print=cfg".into()),
         },
         out_dir: PathBuf::from("../target/ui"),
         ..Config::rustc("tests/ui")
-    }
+    };
+
+    Ok(config)
 }