* Migrate back to `std::` stylistically
This commit moves away from idioms such as `alloc::` and `core::` as
imports of standard data structures and types. Instead it migrates all
crates to uniformly use `std::` for importing standard data structures
and types. This also removes the `std` and `core` features from all
crates to and removes any conditional checking for `feature = "std"`
All of this support was previously added in #407 in an effort to make
wasmtime/cranelift "`no_std` compatible". Unfortunately though this
change comes at a cost:
* The usage of `alloc` and `core` isn't idiomatic. Especially trying to
dual between types like `HashMap` from `std` as well as from
`hashbrown` causes imports to be surprising in some cases.
* Unfortunately there was no CI check that crates were `no_std`, so none
of them actually were. Many crates still imported from `std` or
depended on crates that used `std`.
It's important to note, however, that **this does not mean that wasmtime
will not run in embedded environments**. The style of the code today and
idioms aren't ready in Rust to support this degree of multiplexing and
makes it somewhat difficult to keep up with the style of `wasmtime`.
Instead it's intended that embedded runtime support will be added as
necessary. Currently only `std` is necessary to build `wasmtime`, and
platforms that natively need to execute `wasmtime` will need to use a
Rust target that supports `std`. Note though that not all of `std` needs
to be supported, but instead much of it could be configured off to
return errors, and `wasmtime` would be configured to gracefully handle
errors.
The goal of this PR is to move `wasmtime` back to idiomatic usage of
features/`std`/imports/etc and help development in the short-term.
Long-term when platform concerns arise (if any) they can be addressed by
moving back to `no_std` crates (but fixing the issues mentioned above)
or ensuring that the target in Rust has `std` available.
* Start filling out platform support doc
As discussed in https://github.com/bytecodealliance/cranelift/pull/1226, the context of Cranelift errors is lost after exiting the scope containing the Cranelift function. `CodegenError` then only contains something like `inst2: arg 0 (v4) has type i16x8, expected i8x16`, which is rarely enough information for investigating a codegen failure. This change uses Cranelift's `pretty_error` function to improve the error messages wrapped in `CompileError`; `CompileError` has lost the reference to `CodegenError` due to `pretty_error` taking ownership but this seems preferable since no backtrace is attached and losing the pretty-printed context would be worse (if `CodegenError` gains a `Backtrace` or implements `Clone` we can revisit this).
This commit is an attempt to reduce the number of crates necessary to
link to when using `wasmtime::Config` in "default mode" or with only one
or two tweaks. The change moves to a builder-style pattern for `Config`
to only require importing crates as necessary if you configure a
particular setting. This then also propagates that change to `Context`
as well by taking a `Config` instead of requiring that all arguments are
passed alone.
