Disable fuzzing both custom-page-sizes and threads (#9524)

This is an attempt to avoid #9523 in the fuzzers while that issue is
fixed independently. Fuzzing is pretty limited right now so the hope is
to let fuzzers find other issues while the refactoring here progresses.

crates/fuzzing/src/generators/module.rs

@@ -56,6 +56,12 @@ impl<'a> Arbitrary<'a> for ModuleConfig {
         // do that most of the time.
         config.disallow_traps = u.ratio(9, 10)?;
 
+        // FIXME(#9523) this `if` should ideally be deleted, requires some
+        // refactoring internally.
+        if config.custom_page_sizes_enabled && config.threads_enabled {
+            config.custom_page_sizes_enabled = false;
+        }
+
         Ok(ModuleConfig { config })
     }
 }

docs/stability-wasm-proposals.md

@@ -46,14 +46,16 @@ column is below.
 | [`function-references`]  | ✅      | ✅    | ❌       | ❌     | ✅  | ❌    |
 | [`gc`] [^6]              | ✅      | ✅    | ❌[^7]   | ❌     | ✅  | ❌    |
 | [`wide-arithmetic`]      | ❌      | ✅    | ✅       | ✅     | ✅  | ✅    |
-| [`custom-page-sizes`]    | ❌      | ✅    | ✅       | ✅     | ✅  | ❌    |
+| [`custom-page-sizes`]    | ❌      | ✅    | ⚠️[^8]    | ✅     | ✅  | ❌    |
 
 [^6]: There is also a [tracking
     issue](https://github.com/bytecodealliance/wasmtime/issues/5032) for the
     GC proposal.
 [^7]: The implementation of GC has [known performance
     issues](https://github.com/bytecodealliance/wasmtime/issues/9351) which can
     affect non-GC code when the GC proposal is enabled.
+[^8]: Using custom-page-sizes is [known to have issues when combined with shared
+    memories](https://github.com/bytecodealliance/wasmtime/issues/9523).
 
 ## Unimplemented proposals