Add WasiCtxBuilder setters for the two clock types

[rylev]

This allows users to set individual clock types on the WasiCtxBuilder.

Before

let mut clocks = clocks_ctx();
clocks.wall = Box::new(MyWallClock);
builder.set_clocks(clocks)

After

builder.set_wall_clock(MyWallClock)

I didn't add any documentation because most of the other setters don't have any, and I was unsure what form the documentation should take.

[alexcrichton]

Could this perhaps remove the set_clocks configuration method as well? I think it could otherwise be a bit surprising that set_clocks could override set_monotonic_clock for example, but if it doesn't exist then configuration should still be possible and there shouldn't be odd interactions between methods in theory.

[pchickey]

We should drop the WasiClocks intermediate struct entirely and just have two members of WasiCtx for the wall and monotonic clock - would you be willing to do that?

[pchickey]

Looks good - thank you. One last suggestion if you're up to it - in the past I used the convention WasiThing to name the host types, but as we shifted to the component model, I've preferred HostThing instead - would you mind renaming these HostMonotonicClock and HostWallClock?

Also I'm not sure if the +Send+Sync in each box is required since those traits already require send and sync, maybe those could have been removed a while ago...

[rylev]

@pchickey I've renamed the traits. Unfortunately, the compiler seems to require the Send and Sync bounds. I was surprised by this as well...

error[E0308]: mismatched types
  --> crates/wasi/src/preview2/ctx.rs:50:30
   |
50 |             monotonic_clock: monotonic_clock(),
   |                              ^^^^^^^^^^^^^^^^^ expected trait `HostMonotonicClock + std::marker::Send + Sync`, found trait `HostMonotonicClock`
   |
   = note: expected struct `Box<(dyn HostMonotonicClock + std::marker::Send + Sync + 'static)>`
              found struct `Box<(dyn HostMonotonicClock + 'static)>`

[sunfishcode]

@rylev The PR here is good, but separately, I'm curious; would you mind saying a little more about what your use case looks like? I'm interested in learning about who's using non-default clocks and what they're using them for.

[rylev]

@sunfishcode unfortunately it's not too interesting - just doing some testing around ABI conformance of wasm binaries built with old versions of wit-bindgen that have been adapted to run against the latest version of wasmtime. You can see the code here.