Rollup merge of #139498 - alexcrichton:wasm-zst-safe, r=wesleywiser

Ignore zero-sized types in wasm future-compat warning

This commit fixes a false positive of the warning triggered for #138762 and the fix is to codify that zero-sized types are "safe" in both the old and new ABIs.

2 files changed, 11 insertions, 0 deletions

diff --git a/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs b/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs
index 0f5bdc8d768..94ee34c8b7b 100644
--- a/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs
+++ b/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs
@@ -111,6 +111,11 @@ fn wasm_abi_safe<'tcx>(tcx: TyCtxt<'tcx>, arg: &ArgAbi<'tcx, Ty<'tcx>>) -> bool
         }
     }
 
+    // Zero-sized types are dropped in both ABIs, so they're safe
+    if arg.layout.is_zst() {
+        return true;
+    }
+
     false
 }
 
diff --git a/tests/ui/lint/wasm_c_abi_transition.rs b/tests/ui/lint/wasm_c_abi_transition.rs
index 1fe81679e65..6a933a0de03 100644
--- a/tests/ui/lint/wasm_c_abi_transition.rs
+++ b/tests/ui/lint/wasm_c_abi_transition.rs
@@ -39,3 +39,9 @@ pub fn call_other_fun(x: MyType) {
     unsafe { other_fun(x) } //~ERROR: wasm ABI transition
     //~^WARN: previously accepted
 }
+
+// Zero-sized types are safe in both ABIs
+#[repr(C)]
+pub struct MyZstType;
+#[allow(improper_ctypes_definitions)]
+pub extern "C" fn zst_safe(_x: (), _y: MyZstType) {}