Rollup merge of rust-lang#129111 - Zalathar:python-sysroot, r=jieyouxu

Port the `sysroot-crates-are-unstable` Python script to rmake

New version of rust-lang#126231, and a follow-up to rust-lang#129071.

One major difference is that the new version no longer tries to report *all* accidentally-stable crates, because the `run_make_support` helpers tend to halt the test as soon as something goes wrong. That's unfortunate, but I think it won't matter much in practice, and preserving the old behaviour doesn't seem worth the extra effort.

---

Part of rust-lang#110479 (Python purge), with this being one of the non-trivial Python scripts that actually seems feasible and worthwhile to remove.

This is *not* part of rust-lang#121876 (Makefile purge), because the underlying test is already using rmake; this PR just modifies the existing rmake recipe to do all the work itself instead of delegating to Python. So there's no particular urgency here.

r? ```@jieyouxu```

try-job: aarch64-gnu
try-job: aarch64-apple
try-job: test-various
try-job: armhf-gnu
try-job: x86_64-msvc
try-job: i686-mingw

tests/run-make/sysroot-crates-are-unstable/rmake.rs

@@ -1,5 +1,102 @@
-use run_make_support::python_command;
+// Check that crates in the sysroot are treated as unstable, unless they are
+// on a list of known-stable sysroot crates.
+
+use std::path::{Path, PathBuf};
+use std::str;
+
+use run_make_support::{rfs, rustc, target};
+
+fn is_stable_crate(name: &str) -> bool {
+    matches!(name, "std" | "alloc" | "core" | "proc_macro")
+}
 
 fn main() {
-    python_command().arg("test.py").run();
+    for cr in get_unstable_sysroot_crates() {
+        check_crate_is_unstable(&cr);
+    }
+    println!("Done");
+}
+
+#[derive(Debug)]
+struct Crate {
+    name: String,
+    path: PathBuf,
+}
+
+fn check_crate_is_unstable(cr: &Crate) {
+    let Crate { name, path } = cr;
+
+    print!("- Verifying that sysroot crate '{name}' is an unstable crate ...");
+
+    // Trying to use this crate from a user program should fail.
+    let output = rustc()
+        .crate_type("rlib")
+        .target(target())
+        .extern_(name, path)
+        .input("-")
+        .stdin(format!("extern crate {name};"))
+        .run_fail();
+
+    // Make sure it failed for the intended reason, not some other reason.
+    // (The actual feature required varies between crates.)
+    output.assert_stderr_contains("use of unstable library feature");
+
+    println!(" OK");
+}
+
+fn get_unstable_sysroot_crates() -> Vec<Crate> {
+    let sysroot = PathBuf::from(rustc().print("sysroot").run().stdout_utf8().trim());
+    let sysroot_libs_dir = sysroot.join("lib").join("rustlib").join(target()).join("lib");
+    println!("Sysroot libs dir: {sysroot_libs_dir:?}");
+
+    // Generate a list of all library crates in the sysroot.
+    let sysroot_crates = get_all_crates_in_dir(&sysroot_libs_dir);
+    println!(
+        "Found {} sysroot crates: {:?}",
+        sysroot_crates.len(),
+        sysroot_crates.iter().map(|cr| &cr.name).collect::<Vec<_>>()
+    );
+
+    // Self-check: If we didn't find `core`, we probably checked the wrong directory.
+    assert!(
+        sysroot_crates.iter().any(|cr| cr.name == "core"),
+        "Couldn't find `core` in {sysroot_libs_dir:?}"
+    );
+
+    let unstable_sysroot_crates =
+        sysroot_crates.into_iter().filter(|cr| !is_stable_crate(&cr.name)).collect::<Vec<_>>();
+    // Self-check: There should be at least one unstable crate in the directory.
+    assert!(
+        !unstable_sysroot_crates.is_empty(),
+        "Couldn't find any unstable crates in {sysroot_libs_dir:?}"
+    );
+    unstable_sysroot_crates
+}
+
+fn get_all_crates_in_dir(libs_dir: &Path) -> Vec<Crate> {
+    let mut libs = vec![];
+    rfs::read_dir_entries(libs_dir, |path| {
+        if !path.is_file() {
+            return;
+        }
+        if let Some(name) = crate_name_from_path(path) {
+            libs.push(Crate { name, path: path.to_owned() });
+        }
+    });
+    libs.sort_by(|a, b| a.name.cmp(&b.name));
+    libs
+}
+
+/// Treat a file as a crate if its name begins with `lib` and ends with `.rlib`.
+/// The crate name is the part before the first hyphen (if any).
+fn crate_name_from_path(path: &Path) -> Option<String> {
+    let name = path
+        .file_name()?
+        .to_str()?
+        .strip_prefix("lib")?
+        .strip_suffix(".rlib")?
+        .split('-')
+        .next()
+        .expect("split always yields at least one string");
+    Some(name.to_owned())
 }