Provide `cabi_realloc` on `wasm32-wasip2` by default

This commit provides a component model intrinsic in the standard library
by default on the `wasm32-wasip2` target. This intrinsic is not
required by the component model itself but is quite common to use, for
example it's needed if a wasm module receives a string or a list.

The intention of this commit is to provide an overridable definition in
the standard library through a weak definition of this function. That
means that downstream crates can provide their own customized and more
specific versions if they'd like, but the standard library's version
should suffice for general-purpose use.

library/std/src/sys/pal/wasip2/cabi_realloc.rs

@@ -0,0 +1,65 @@
+//! This module contains a canonical definition of the `cabi_realloc` function
+//! for the component model.
+//!
+//! The component model's canonical ABI for representing datatypes in memory
+//! makes use of this function when transferring lists and strings, for example.
+//! This function behaves like C's `realloc` but also takes alignment into
+//! account.
+//!
+//! Components are notably not required to export this function, but nearly
+//! all components end up doing so currently. This definition in the standard
+//! library removes the need for all compilations to define this themselves.
+//!
+//! More information about the canonical ABI can be found at
+//! <https://github.com/WebAssembly/component-model/blob/main/design/mvp/CanonicalABI.md>
+//!
+//! Note that the name of this function is not standardized in the canonical ABI
+//! at this time. Instead it's a convention of the "componentization process"
+//! where a core wasm module is converted to a component to use this name.
+//! Additionally this is not the only possible definition of this function, so
+//! this is defined as a "weak" symbol. This means that other definitions are
+//! allowed to overwrite it if they are present in a compilation.
+
+use crate::alloc::{self, Layout};
+use crate::ptr;
+
+#[used]
+static FORCE_CODEGEN_OF_CABI_REALLOC: unsafe extern "C" fn(
+    *mut u8,
+    usize,
+    usize,
+    usize,
+) -> *mut u8 = cabi_realloc;
+
+#[linkage = "weak"]
+#[no_mangle]
+pub unsafe extern "C" fn cabi_realloc(
+    old_ptr: *mut u8,
+    old_len: usize,
+    align: usize,
+    new_len: usize,
+) -> *mut u8 {
+    let layout;
+    let ptr = if old_len == 0 {
+        if new_len == 0 {
+            return ptr::without_provenance_mut(align);
+        }
+        layout = Layout::from_size_align_unchecked(new_len, align);
+        alloc::alloc(layout)
+    } else {
+        debug_assert_ne!(new_len, 0, "non-zero old_len requires non-zero new_len!");
+        layout = Layout::from_size_align_unchecked(old_len, align);
+        alloc::realloc(old_ptr, layout, new_len)
+    };
+    if ptr.is_null() {
+        // Print a nice message in debug mode, but in release mode don't
+        // pull in so many dependencies related to printing so just emit an
+        // `unreachable` instruction.
+        if cfg!(debug_assertions) {
+            alloc::handle_alloc_error(layout);
+        } else {
+            super::abort_internal();
+        }
+    }
+    return ptr;
+}

library/std/src/sys/pal/wasip2/mod.rs

@@ -66,3 +66,5 @@ pub use helpers::decode_error_kind;
 use helpers::err2io;
 pub use helpers::hashmap_random_keys;
 pub use helpers::is_interrupted;
+
+mod cabi_realloc;