We can only have one mutable borrow at a time after bytecodealliance#…

…8277

Co-authored-by: Nick Fitzgerald <fitzgen@gmail.com>

crates/wasi-common/src/snapshots/preview_1.rs

@@ -340,12 +340,15 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
  return Ok(0);
  }
  } else {
- // Convert all of the unsafe guest slices to safe ones--this uses
- // Wiggle's internal borrow checker to ensure no overlaps. We assume
- // here that, because the memory is not shared, there are no other
- // threads to access it while it is written to.
+ // Convert the first unsafe guest slice into a safe one--Wiggle
+ // can only track mutable borrows for an entire region, and converting
+ // all guest pointers to slices would cause a runtime borrow-checking
+ // error. As read is allowed to return less than the requested amount,
+ // it's valid (though not as efficient) for us to only perform the
+ // read of the first buffer.
  let mut guest_slices: Vec<wiggle::GuestSliceMut<u8>> = iovs
  .into_iter()
+ .take(1)
  .map(|iov| Ok(iov.as_slice_mut()?.unwrap()))
  .collect::<Result<_, Error>>()?;