Run pass tests against real rustc as well

[RalfJung]

Some of our pass-dep tests currently do not pass when they are run against real rustc, e.g. tests/pass-dep/libc/libc-epoll.rs. That's quite strange, but I guess it's not surprising given that we are not even running these tests against real rustc in CI.

[RalfJung]

Turns out this is a lot harder than I thought, for two reasons:

• Many of our tests have -Zmiri flags which rustc of course chokes on. We should ideally ignore those flags when building with rustc.
• When building and running the tests with rustc, the output from the program happens at runtime, not at "build" time. So for ui-test, the stdout/stderr files we have do not match, since it expects that output to be generated at build time -- after all, ui-test thinks it is just building the program when Miri actually runs it.

We'll also want some way to have a test opt-out from this, some of our tests fundamentally can only work in Miri.

So this will probably require some pretty advanced ui_test surgery or so, not quite sure how to best do this.

Here's my WIP patch, I will not work further on this:

diff --git a/tests/ui.rs b/tests/ui.rs
index 9cbcf6e42..f523130b3 100644
--- a/tests/ui.rs
+++ b/tests/ui.rs
@@ -99,14 +99,14 @@ fn miri_config(target: &str, path: &str, mode: Mode, with_dependencies: bool) ->
     config
 }
 
-fn run_tests(
+fn run_tests_miri(
     mode: Mode,
     path: &str,
     target: &str,
-    with_dependencies: bool,
+    with_dependencies: Dependencies,
     tmpdir: &Path,
 ) -> Result<()> {
-    let mut config = miri_config(target, path, mode, with_dependencies);
+    let mut config = miri_config(target, path, mode, matches!(with_dependencies, WithDependencies));
 
     // Add a test env var to do environment communication tests.
     config.program.envs.push(("MIRI_ENV_VAR_TEST".into(), Some("0".into())));
@@ -176,6 +176,73 @@ fn run_tests(
     )
 }
 
+fn run_tests_rustc(
+    mode: Mode,
+    path: &str,
+    target: &str,
+    with_dependencies: Dependencies,
+    tmpdir: &Path,
+) -> Result<()> {
+    let mut config = Config {
+        target: Some(target.to_owned()),
+        stderr_filters: stderr_filters().into(),
+        stdout_filters: stdout_filters().into(),
+        mode,
+        program: CommandBuilder::rustc(),
+        out_dir: PathBuf::from(std::env::var_os("CARGO_TARGET_DIR").unwrap()).join("miri_ui_rustc"),
+        edition: Some("2021".into()), // keep in sync with `./miri run`
+        threads: std::env::var("MIRI_TEST_THREADS")
+            .ok()
+            .map(|threads| NonZero::new(threads.parse().unwrap()).unwrap()),
+        ..Config::rustc(path)
+    };
+
+    if matches!(with_dependencies, WithDependencies) {
+        config.dependencies_crate_manifest_path =
+            Some(Path::new("test_dependencies").join("Cargo.toml"));
+        // Reset `RUSTFLAGS` to work around <https://github.com/rust-lang/rust/pull/119574#issuecomment-1876878344>.
+        config.dependency_builder.envs.push(("RUSTFLAGS".into(), None));
+    }
+
+    // Add a test env var to do environment communication tests.
+    config.program.envs.push(("MIRI_ENV_VAR_TEST".into(), Some("0".into())));
+    // Let the tests know where to store temp files (they might run for a different target, which can make this hard to find).
+    config.program.envs.push(("MIRI_TEMP".into(), Some(tmpdir.to_owned().into())));
+    // If a test ICEs, we want to see a backtrace.
+    config.program.envs.push(("RUST_BACKTRACE".into(), Some("1".into())));
+
+    // Add some flags we always want.
+    config.program.args.push("-Dwarnings".into());
+    config.program.args.push("-Dunused".into());
+    config.program.args.push("-Ainternal_features".into());
+    config.program.args.push("-Zui-testing".into());
+    config.program.args.push("--target".into());
+    config.program.args.push(target.into());
+
+    // Handle command-line arguments.
+    let args = ui_test::Args::test()?;
+    config.with_args(&args, /*default_bless*/ false);
+    eprintln!("   Compiler: {}", config.program.display());
+    ui_test::run_tests_generic(
+        // Only run one test suite. In the future we can add all test suites to one `Vec` and run
+        // them all at once, making best use of systems with high parallelism.
+        vec![config],
+        // The files we're actually interested in (all `.rs` files).
+        ui_test::default_file_filter,
+        // This could be used to overwrite the `Config` on a per-test basis.
+        |_, _, _| {},
+        (
+            match args.format {
+                Format::Terse => status_emitter::Text::quiet(),
+                Format::Pretty => status_emitter::Text::verbose(),
+            },
+            status_emitter::Gha::</* GHA Actions groups*/ false> {
+                name: format!("{mode:?} {path} ({target})"),
+            },
+        ),
+    )
+}
+
 macro_rules! regexes {
     ($name:ident: $($regex:expr => $replacement:expr,)*) => {
         fn $name() -> &'static [(Match, &'static [u8])] {
@@ -239,7 +306,7 @@ enum Dependencies {
 
 use Dependencies::*;
 
-fn ui(
+fn ui_miri(
     mode: Mode,
     path: &str,
     target: &str,
@@ -249,14 +316,24 @@ fn ui(
     let msg = format!("## Running ui tests in {path} for {target}");
     eprintln!("{}", msg.green().bold());
 
-    let with_dependencies = match with_dependencies {
-        WithDependencies => true,
-        WithoutDependencies => false,
-    };
-    run_tests(mode, path, target, with_dependencies, tmpdir)
+    run_tests_miri(mode, path, target, with_dependencies, tmpdir)
         .with_context(|| format!("ui tests in {path} for {target} failed"))
 }
 
+fn ui_rustc(
+    mode: Mode,
+    path: &str,
+    target: &str,
+    with_dependencies: Dependencies,
+    tmpdir: &Path,
+) -> Result<()> {
+    let msg = format!("## Running ui tests in {path} with rustc for {target}");
+    eprintln!("{}", msg.green().bold());
+
+    run_tests_rustc(mode, path, target, with_dependencies, tmpdir)
+        .with_context(|| format!("ui tests in {path} with rustc for {target} failed"))
+}
+
 fn get_target() -> String {
     env::var("MIRI_TEST_TARGET").ok().unwrap_or_else(get_host)
 }
@@ -276,17 +353,29 @@ fn main() -> Result<()> {
         }
     }
 
-    ui(Mode::Pass, "tests/pass", &target, WithoutDependencies, tmpdir.path())?;
-    ui(Mode::Pass, "tests/pass-dep", &target, WithDependencies, tmpdir.path())?;
-    ui(Mode::Panic, "tests/panic", &target, WithDependencies, tmpdir.path())?;
-    ui(
+    // If requested, build and run the tests with rustc instead.
+    if env::var("MIRI_TEST_WITH_RUSTC").is_ok_and(|v| v != "0") {
+        ui_rustc(
+            Mode::Run { exit_code: 0 },
+            "tests/pass-dep",
+            &target,
+            WithDependencies,
+            tmpdir.path(),
+        )?;
+        return Ok(());
+    }
+
+    ui_miri(Mode::Pass, "tests/pass", &target, WithoutDependencies, tmpdir.path())?;
+    ui_miri(Mode::Pass, "tests/pass-dep", &target, WithDependencies, tmpdir.path())?;
+    ui_miri(Mode::Panic, "tests/panic", &target, WithDependencies, tmpdir.path())?;
+    ui_miri(
         Mode::Fail { require_patterns: true, rustfix: RustfixMode::Disabled },
         "tests/fail",
         &target,
         WithoutDependencies,
         tmpdir.path(),
     )?;
-    ui(
+    ui_miri(
         Mode::Fail { require_patterns: true, rustfix: RustfixMode::Disabled },
         "tests/fail-dep",
         &target,
@@ -294,8 +383,8 @@ fn main() -> Result<()> {
         tmpdir.path(),
     )?;
     if cfg!(target_os = "linux") {
-        ui(Mode::Pass, "tests/native-lib/pass", &target, WithoutDependencies, tmpdir.path())?;
-        ui(
+        ui_miri(Mode::Pass, "tests/native-lib/pass", &target, WithoutDependencies, tmpdir.path())?;
+        ui_miri(
             Mode::Fail { require_patterns: true, rustfix: RustfixMode::Disabled },
             "tests/native-lib/fail",
             &target,