Documentation and error message improvements related to PassMode::Direct assert

Co-authored-by: Jubilee <46493976+workingjubilee@users.noreply.github.com>

Author: 2 people

compiler/rustc_codegen_llvm/src/abi.rs

@@ -363,13 +363,13 @@ impl<'ll, 'tcx> FnAbiLlvmExt<'ll, 'tcx> for FnAbi<'tcx, Ty<'tcx>> {
                         // shouldn't let any more targets do that.
                         // (Also see <https://github.com/rust-lang/rust/issues/115666>.)
                         //
-                        // The unstable api `PtxKernel` also uses Direct for now.
+                        // The unstable abi `PtxKernel` also uses Direct for now.
                         // It needs to switch to something else before stabilization can happen.
                         // (Tracking issue abi_ptx: https://github.com/rust-lang/rust/issues/38788)
                         assert!(
                             matches!(&*cx.tcx.sess.target.arch, "wasm32" | "wasm64")
                                 || self.conv == Conv::PtxKernel,
-                            "`PassMode::Direct` for aggregates only allowed on wasm targets\nProblematic type: {:#?}",
+                            "`PassMode::Direct` for aggregates only allowed on wasm and `extern \"ptx-kernel\"` fns\nProblematic type: {:#?}",
                             arg.layout,
                         );
                     }