Module snapshots

One goal of wasi-common is for multiple WASI snapshots to provide an interface to the same underlying crate::WasiCtx. This provides us a path to evolve WASI by allowing the same WASI Command to import functions from different snapshots - e.g. the user could use Rust’s std which imports snapshot 1, but also depend directly on the wasi crate which imports some future snapshot 2. Right now, this amounts to supporting snapshot 1 and “snapshot 0” aka wasi_unstable at once.

The architectural rules for snapshots are:

• Snapshots are arranged into modules under crate::snapshots::.
• Each snapshot should invoke wiggle::from_witx! with ctx: crate::WasiCtx in its module, and impl all of the required traits.
• Snapshots can be implemented in terms of other snapshots. For example, snapshot 0 is mostly implemented by calling the snapshot 1 implementation, and converting its own types back and forth with the snapshot 1 types. In a few cases, that is not feasible, so snapshot 0 carries its own implementations in terms of the WasiFile and WasiSched traits.
• Snapshots can be implemented in terms of the Wasi* traits given by WasiCtx. No further downcasting via the as_any escape hatch is permitted.

Modules

• preview_0
• preview_1