Rollup merge of #126967 - alexcrichton:wasm32-wasip2-tier-2, r=Mark-Simulacrum

Promote the `wasm32-wasip2` target to Tier 2

This commit promotes the `wasm32-wasip2` Rust target to tier 2 as proposed in rust-lang/compiler-team#760. There are two major changes in this PR:

1. The `dist-various-2` container, which already produces the other WASI targets, now has an extra target added for `wasm32-wasip2`.
2. A new `wasm-component-ld` binary is added to all host toolchains when LLD is enabled. This is the linker used for the `wasm32-wasip2` target.

This new linker is added for all host toolchains to ensure that all host toolchains can produce the `wasm32-wasip2` target. This is similar to how `rust-lld` was originally included for all host toolchains to be able to produce WebAssembly output when the targets were first added. The new linker is developed [here][wasm-component-ld] and is pulled in via a crates.io-based dependency to the tree here.

[wasm-component-ld]: https://github.com/bytecodealliance/wasm-component-ld

src/tools/tidy/src/deps.rs

@@ -95,7 +95,12 @@ const EXCEPTIONS: ExceptionList = &[
     ("self_cell", "Apache-2.0"),                             // rustc (fluent translations)
     ("snap", "BSD-3-Clause"),                                // rustc
     ("wasm-encoder", "Apache-2.0 WITH LLVM-exception"),      // rustc
+    ("wasm-metadata", "Apache-2.0 WITH LLVM-exception"),     // rustc
     ("wasmparser", "Apache-2.0 WITH LLVM-exception"),        // rustc
+    ("wast", "Apache-2.0 WITH LLVM-exception"),              // rustc
+    ("wat", "Apache-2.0 WITH LLVM-exception"),               // rustc
+    ("wit-component", "Apache-2.0 WITH LLVM-exception"),     // rustc
+    ("wit-parser", "Apache-2.0 WITH LLVM-exception"),        // rustc
     // tidy-alphabetical-end
 ];
 

src/tools/wasm-component-ld/Cargo.toml

@@ -0,0 +1,13 @@
+# See the `README.md` in this directory for what this tool is.
+
+[package]
+name = "wasm-component-ld-wrapper"
+version = "0.1.0"
+edition = "2021"
+
+[[bin]]
+name = "wasm-component-ld"
+path = "src/main.rs"
+
+[dependencies]
+wasm-component-ld = "0.5.4"

src/tools/wasm-component-ld/README.md

@@ -0,0 +1,62 @@
+# `wasm-component-ld`
+
+This wrapper is a wrapper around the [`wasm-component-ld`] crates.io crate. That
+crate. That crate is itself a thin wrapper around two pieces:
+
+* `wasm-ld` - the LLVM-based linker distributed as part of LLD and packaged in
+  Rust as `rust-lld`.
+* [`wit-component`] - a Rust crate for creating a [WebAssembly Component] from a
+  core wasm module.
+
+This linker is used for Rust's `wasm32-wasip2` target to natively output a
+component instead of a core WebAssembly module, unlike other WebAssembly
+targets. If you're confused about any of this here's an FAQ-style explanation of
+what's going on here:
+
+* **What's a component?** - It's a proposal to the WebAssembly standard
+  primarily developed at this time by out-of-browser use cases of WebAssembly.
+  You can find high-level documentation [here][component docs].
+
+* **What's WASIp2?** - Not to be confused with WASIp1, WASIp0,
+  `wasi_snapshot_preview1`, or `wasi_unstable`, it's a version of WASI. Released
+  in January 2024 it's the first version of WASI defined in terms of the
+  component model.
+
+* **Why does this need its own linker?** - like any target that Rust has the
+  `wasm32-wasip2` target needs a linker. What makes this different from other
+  WebAssembly targets is that WASIp2 is defined at the component level, not core
+  WebAssembly level. This means that filesystem functions take a `string`
+  instead of `i32 i32`, for example. This means that the raw output of LLVM and
+  `wasm-ld`, a core WebAssembly module, is not suitable.
+
+* **Isn't writing a linker really hard?** - Generally, yes, but this linker
+  works by first asking `wasm-ld` to do all the hard work. It invokes `wasm-ld`
+  and then uses the output core WebAssembly module to create a component.
+
+* **How do you create a component from a core module?** - this is the purpose of
+  the [`wit-component`] crate, notably the `ComponentEncoder` type. This uses
+  component type information embedded in the core module and a general set of
+  conventions/guidelines with what the core module imports/exports. A component
+  is then hooked up to codify all of these conventions in a component itself.
+
+* **Why not require users to run `wit-component` themselves?** - while possible
+  it adds friction to the usage `wasm32-wasip2` target. More importantly though
+  the "module only" output of the `wasm32-wasip2` target is not ready right now.
+  The standard library still imports from `wasi_snapshot_preview1` and it will
+  take time to migrate all usage to WASIp2.
+
+* **What exactly does this linker do?** - the `wasm-component-ld` has the same
+  CLI interface and flags as `wasm-ld`, plus some more that are
+  component-specific. These flags are used to forward most flags to `wasm-ld` to
+  produce a core wasm module. After the core wasm module is produced the
+  `wit-component` crate will read custom sections in the final binary which
+  contain component type information. After merging all this type information
+  together a component is produced which wraps the core module.
+
+If you've got any other questions about this linker or its operation don't
+hesitate to reach out to the maintainers of the `wasm32-wasip2` target.
+
+[`wasm-component-ld`]: https://crates.io/crates/wasm-component-ld
+[`wit-component`]: https://crates.io/crates/wit-component
+[WebAssembly Component]: https://github.com/webassembly/component-model
+[component docs]: https://component-model.bytecodealliance.org/

src/tools/wasm-component-ld/src/main.rs

@@ -0,0 +1,9 @@
+// See the `README.md` in this directory for what this tool is.
+
+// The source for this crate lives at
+// https://github.com/bytecodealliance/wasm-component-ld and the binary is
+// independently used in other projects such as `wasi-sdk` so the `main`
+// function is just reexported here to delegate. A Cargo dependency is used to
+// facilitate version management in the Rust repository and work well with
+// vendored/offline builds.
+use wasm_component_ld::main;