winch: Introduce winch-environ (#6017)

This commit introduces the `winch-environ` crate. This crate's responsibility is
to provide a shared implementatation of the `winch_codegen::FuncEnv` trait,
which is Winch's function compilation environment, used to resolve module and
runtime specific information needed by the code generation, such as resolving
all the details about a callee in a WebAssembly module, or resolving specific
information from the `VMContext`.

As of this change, the implementation only includes the necessary pieces to
resolve a function callee in a WebAssembly module. The idea is to evolve the
`winch_codegen::FuncEnv` trait as we evolve Winch's code generation.

winch/codegen/src/codegen/env.rs

@@ -0,0 +1,19 @@
+use wasmparser::FuncType;
+
+/// Function environment used the by the code generation to
+/// resolve module and runtime-specific information.
+pub trait FuncEnv {
+    /// Get the callee information from a given function index.
+    fn callee_from_index(&self, index: u32) -> Callee;
+}
+
+/// Metadata about a function callee.  Use by the code generation
+/// to emit function calls.
+pub struct Callee {
+    /// The function type.
+    pub ty: FuncType,
+    /// A flag to determine if the callee is imported.
+    pub import: bool,
+    /// The callee index in the WebAssembly function index space.
+    pub index: u32,
+}

winch/codegen/src/codegen/mod.rs

@@ -7,6 +7,8 @@ use wasmparser::{BinaryReader, FuncValidator, ValType, ValidatorResources, Visit
 
 mod context;
 pub(crate) use context::*;
+mod env;
+pub use env::*;
 
 /// The code generation abstraction.
 pub(crate) struct CodeGen<'a, M>

winch/codegen/src/lib.rs

@@ -8,6 +8,7 @@
 
 mod abi;
 mod codegen;
+pub use codegen::{Callee, FuncEnv};
 mod frame;
 pub mod isa;
 pub use isa::*;