Use a unique name for libwit_bindgen_cabi_realloc.a per-crate-versi…

…on (#928)

* Use a unique name for `libwit_bindgen_cabi_realloc.a` per-crate-version

This commit updates how the weak `cabi_realloc` symbol is linked.
Previously rustc was told to use `-lwit_bindgen_cabi_realloc` with a
`-L` pointing to the source directory of the crate which has a
precompiled copy. This caused issues, however, when two versions were
present in a crate graph but only one was used. With two versions
present in a crate graph rustc would use two `-L` flags with directories
that both contain the same-named library. Which one was picked depended
on the order that rustc passed flags. If only one crate was used,
however, then rustc would not pass the rlib for the other crate. This
could end up in a situation where with two wit-bindgen versions A and B:

* During linking, A's `libwit_bindgen_cabi_realloc.a` file was used.
* Rustc only passed B's rlib since A wasn't actually used anywhere.
* The linker then tried to load A's version of the actual cabi_realloc
  symbol, but only B's was present.

The fix in this commit is to uniquely name the library name based on the
crate version. This matches how the symbol name is different per crate
version, for example.

* Remove copy/paste

crates/guest-rust/rt/build.rs

@@ -1,6 +1,10 @@
+use std::env;
+use std::path::PathBuf;
+
 fn main() {
-    let target_arch = std::env::var("CARGO_CFG_TARGET_ARCH").unwrap_or(String::new());
-    let target_family = std::env::var("CARGO_CFG_TARGET_FAMILY").unwrap_or(String::new());
+    let target_arch = env::var("CARGO_CFG_TARGET_ARCH").unwrap_or(String::new());
+    let target_family = env::var("CARGO_CFG_TARGET_FAMILY").unwrap_or(String::new());
+    let out_dir = PathBuf::from(env::var_os("OUT_DIR").unwrap());
 
     if target_family != "wasm" {
         return;
@@ -10,8 +14,18 @@ fn main() {
         panic!("only wasm32 supports cabi-realloc right now");
     }
 
-    println!("cargo:rustc-link-lib=wit_bindgen_cabi_realloc");
-    let cwd = std::env::current_dir().unwrap();
-    let cwd = cwd.display();
-    println!("cargo:rustc-link-search=native={cwd}/src");
+    let mut src = env::current_dir().unwrap();
+    src.push("src");
+    src.push("libwit_bindgen_cabi_realloc.a");
+
+    let dst_name = format!(
+        "wit_bindgen_cabi_realloc{}",
+        env!("CARGO_PKG_VERSION").replace(".", "_")
+    );
+    let dst = out_dir.join(format!("lib{dst_name}.a"));
+
+    std::fs::copy(&src, &dst).unwrap();
+
+    println!("cargo:rustc-link-lib={dst_name}");
+    println!("cargo:rustc-link-search=native={}", out_dir.display());
 }