Add "GC Zeal" infrastructure for additional, aggressive GC assertions

This initial commit just sets up the initial GC Zeal infrastructure, it doesn't
actually start using it anywhere yet.

* Adds support for `cfg(gc_zeal)` to `crates/wasmtime/build.rs` and
  `crates/cranelift/build.rs`
* Defines `gc_assert!` macro
* Defines `wasmtime_environ::gc::POISON` constant for GC heap poisoning
* Adds debug assert that `POISON` doesn't overlap any valid `VMGcKind`
  discriminant
* Adds CI job to run GC-related tests with `cfg(gc_zeal)`

Author: fitzgen

.github/workflows/main.yml

@@ -663,6 +663,35 @@ jobs:
     - run: cargo fuzz check --dev --fuzz-dir ./cranelift/assembler-x64/fuzz
     - run: cargo fuzz check --dev --fuzz-dir ./crates/wizer/fuzz
 
+  # Run GC-related tests with `cfg(gc_zeal)` set
+  gc_zeal:
+    needs: determine
+    name: GC Zeal Tests
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v6
+      with:
+        submodules: true
+    - uses: ./.github/actions/install-rust
+    - name: Run GC tests with cfg(gc_zeal) enabled
+      env:
+        GC_KEYWORDS: "gc drc any eq struct array i31 extern exn ref rec"
+        RUSTFLAGS: "--cfg gc_zeal"
+      run: |
+        set -e
+
+        cargo test --test all -- $GC_KEYWORDS
+
+        # The wast test runner doesn't support multiple test filters. Instead,
+        # make a list of all matches, and then run each individually.
+        tmp=$(mktemp)
+        for kw in $GC_KEYWORDS; do
+          cargo test --test wast -- --list $kw  | cut -d ':' -f 1 >> $tmp
+        done
+        for f in $(cat $tmp | sort -u); do
+          cargo test --test wast -- $f
+        done
+
   # Perform all tests of the c-api
   test_capi:
     needs: determine

crates/cranelift/build.rs

@@ -0,0 +1,7 @@
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    // When enabled, `cfg(gc_zeal)` activates aggressive GC debugging
+    // assertions.
+    println!("cargo:rustc-check-cfg=cfg(gc_zeal)");
+}

crates/environ/src/gc.rs

@@ -22,6 +22,25 @@ use crate::{
 use alloc::sync::Arc;
 use core::alloc::Layout;
 
+/// Poison byte written over unallocated GC heap memory when `cfg(gc_zeal)` is
+/// enabled.
+pub const POISON: u8 = 0xFF;
+
+/// Assert a condition, but only when `gc_zeal` is enabled.
+#[macro_export]
+macro_rules! gc_assert {
+    ($cond:expr $(,)?) => {
+        if cfg!(gc_zeal) {
+            assert!($cond);
+        }
+    };
+    ($cond:expr, $($arg:tt)+) => {
+        if cfg!(gc_zeal) {
+            assert!($cond, $($arg)+);
+        }
+    };
+}
+
 /// Discriminant to check whether GC reference is an `i31ref` or not.
 pub const I31_DISCRIMINANT: u32 = 1;
 
@@ -485,15 +504,26 @@ impl VMGcKind {
     #[inline]
     pub fn from_high_bits_of_u32(val: u32) -> VMGcKind {
         let masked = val & Self::MASK;
-        match masked {
+        let result = match masked {
             x if x == Self::ExternRef.as_u32() => Self::ExternRef,
             x if x == Self::AnyRef.as_u32() => Self::AnyRef,
             x if x == Self::EqRef.as_u32() => Self::EqRef,
             x if x == Self::ArrayRef.as_u32() => Self::ArrayRef,
             x if x == Self::StructRef.as_u32() => Self::StructRef,
             x if x == Self::ExnRef.as_u32() => Self::ExnRef,
             _ => panic!("invalid `VMGcKind`: {masked:#032b}"),
+        };
+
+        #[cfg(debug_assertions)]
+        {
+            let poison_kind = u32::from_le_bytes([POISON, POISON, POISON, POISON]) & VMGcKind::MASK;
+            debug_assert_ne!(
+                masked, poison_kind,
+                "No valid `VMGcKind` should overlap with the poison pattern"
+            );
         }
+
+        result
     }
 
     /// Does this kind match the other kind?

crates/wasmtime/build.rs

@@ -35,6 +35,10 @@ fn main() {
     custom_cfg("has_custom_sync", has_custom_sync);
     custom_cfg("has_host_compiler_backend", has_host_compiler_backend);
 
+    // When enabled, `cfg(gc_zeal)` activates aggressive GC debugging
+    // assertions.
+    println!("cargo:rustc-check-cfg=cfg(gc_zeal)");
+
     // If this OS isn't supported and no debug-builtins or if Cranelift doesn't support
     // the host or there's no need to build these helpers.
     #[cfg(feature = "runtime")]