fix(gc): guard dladdr symbolication behind cfg(unix) — perry-runtime does not build on Windows

changelog.d/8316-native-stack-scan-windows.md

@@ -0,0 +1,20 @@
+### Fixed
+
+- **`perry-runtime` failed to compile for `*-pc-windows-msvc`.**
+  `gc::native_stack_scan::resolve_symbol` reached `libc::dladdr` and
+  `libc::Dl_info` unconditionally. Both are POSIX-only, so the crate did not
+  build on Windows at all — which is why the `native-roots-rs4gc
+  (windows-latest, x86-64, PE)` arm of `gc-native-roots` failed in its **build**
+  step rather than in a probe, taking `gc-native-roots-complete` red with it.
+
+  The two `pthread_get_stack*_np` calls in the same file were already inside a
+  `#[cfg(target_os = "macos")]` block; `resolve_symbol` was the only unguarded
+  POSIX use. It is now `#[cfg(unix)]`, with a `#[cfg(not(unix))]` arm that
+  reports the bare address. The scan is a debug-only diagnostic
+  (`PERRY_GC_SCAN_NATIVE_STACK=1`), so the non-POSIX arm degrades symbolication
+  instead of pulling in a platform symbolizer.
+
+  `resolve_symbol`'s callers live in the ungated `run_native_stack_scan`, so the
+  new arm is reachable on Windows and does not trip `dead_code` under
+  `-D warnings` — the failure mode of #8306, where a fix applied to only one
+  host's `cfg` arm could not be seen by the macOS lint run.

crates/perry-runtime/src/gc/native_stack_scan.rs

@@ -285,6 +285,14 @@ fn walk_frame_pointers() -> Vec<FrameInfo> {
 }
 
 /// Resolve a code address to a symbol name using `dladdr`.
+///
+/// `dladdr`/`Dl_info` are POSIX-only. They were reached unconditionally, so
+/// `perry-runtime` failed to COMPILE for `*-pc-windows-msvc` — which is why the
+/// `native-roots-rs4gc (windows-latest)` arm of `gc-native-roots` died in its
+/// build step rather than in a probe. This diagnostic is debug-only
+/// (`PERRY_GC_SCAN_NATIVE_STACK=1`), so the non-POSIX arm degrades to the bare
+/// address rather than pulling in a platform symbolizer.
+#[cfg(unix)]
 fn resolve_symbol(addr: usize) -> String {
     unsafe {
         let mut info: libc::Dl_info = std::mem::zeroed();
@@ -311,6 +319,12 @@ fn resolve_symbol(addr: usize) -> String {
     }
 }
 
+/// No `dladdr` off POSIX: report the bare address.
+#[cfg(not(unix))]
+fn resolve_symbol(addr: usize) -> String {
+    format!("0x{addr:x} (symbolication unavailable on this target)")
+}
+
 #[derive(Clone, Copy)]
 struct StaleStackSlot {
     stack_addr: usize,