fuzz: Add a script for downloading the correct LLVM version

Avoid downloading LLVM as part of the build script, which speeds the
rebuild process down significantly. This also simplifies full paths from
std.

Author: tgross35

etc/download-llvm.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+# Download the version of LLVM matching this crate's port version to test
+# against.
+
+set -xux
+
+# Find the version and extract everything after the `-` to get the LLVM hash
+version="$(
+    cargo metadata --no-deps --format-version=1 |
+    jq -r '.packages | .[] | select(.name == "rustc_apfloat") | .version'
+)"
+llvm_hash="${version##*-}"
+
+target_dir="${CARGO_TARGET_DIR:-target}"
+out_dir="$target_dir/llvm-downloads/"
+mkdir -p "$out_dir"
+
+tgz_url="https://codeload.github.com/llvm/llvm-project/tar.gz/$llvm_hash"
+curl -sS "$tgz_url" | tar -C "$out_dir" -xz

fuzz/build.rs

@@ -1,8 +1,12 @@
+use std::env;
+use std::fs;
+use std::io;
+use std::path::PathBuf;
 use std::process::{Command, ExitCode};
 
 mod ops;
 
-fn main() -> std::io::Result<ExitCode> {
+fn main() -> io::Result<ExitCode> {
     // HACK(eddyb) disable the default of re-running the build script on *any*
     // change to *the entire source tree* (i.e. the default is roughly `./`).
     println!("cargo:rerun-if-changed=build.rs");
@@ -11,8 +15,8 @@ fn main() -> std::io::Result<ExitCode> {
     let (_, llvm_commit_hash) = env!("CARGO_PKG_VERSION").split_once("+llvm-").unwrap();
     assert_eq!(llvm_commit_hash.len(), 12);
 
-    let out_dir = std::path::PathBuf::from(std::env::var_os("OUT_DIR").unwrap());
-    std::fs::write(out_dir.join("generated_fuzz_ops.rs"), ops::generate_rust())?;
+    let out_dir = PathBuf::from(env::var_os("OUT_DIR").unwrap());
+    fs::write(out_dir.join("generated_fuzz_ops.rs"), ops::generate_rust())?;
 
     // FIXME(eddyb) add a way to disable the C++ build below, or automatically
     // disable it if on an unsupported target (e.g. Windows).
@@ -22,27 +26,41 @@ fn main() -> std::io::Result<ExitCode> {
     }
 
     let mut cxx_exported_symbols = vec![];
-    std::fs::write(
+    fs::write(
         out_dir.join("cxx_apf_fuzz.cpp"),
         ops::generate_cxx(&mut cxx_exported_symbols),
     )?;
 
+    let manifest_dir =
+        PathBuf::from(env::var_os("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR unset"));
+    let target_dir = env::var_os("CARGO_TARGET_DIR")
+        .map(PathBuf::from)
+        .unwrap_or_else(|| manifest_dir.parent().unwrap().join("target"));
+    let llvm_dir = target_dir.join(format!("llvm-downloads/llvm-project-{llvm_commit_hash}"));
+    if !llvm_dir.try_exists().is_ok_and(|val| val) {
+        panic!(
+            "llvm dir `{llvm_dir:?}` does not exist or cannot be reached. \
+            Perhaps you need to run etc/download-llvm.sh?"
+        )
+    }
+
     // HACK(eddyb) work around https://github.com/rust-lang/cargo/issues/3676,
     // by removing the env vars that Cargo appears to hardcode.
     const CARGO_HARDCODED_ENV_VARS: &[(&str, &str)] = &[
         ("SSL_CERT_DIR", "/etc/pki/tls/certs"),
         ("SSL_CERT_FILE", "/etc/pki/tls/certs/ca-bundle.crt"),
     ];
     for &(var_name, cargo_hardcoded_value) in CARGO_HARDCODED_ENV_VARS {
-        if let Ok(value) = std::env::var(var_name) {
+        if let Ok(value) = env::var(var_name) {
             if value == cargo_hardcoded_value {
-                std::env::remove_var(var_name);
+                env::remove_var(var_name);
             }
         }
     }
 
     let sh_script_exit_status = Command::new("bash")
         .args(["-c", SH_SCRIPT])
+        .env("llvm", &llvm_dir.join("llvm"))
         .envs([
             ("llvm_project_git_hash", llvm_commit_hash),
             ("cxx_apf_fuzz_exports", &cxx_exported_symbols.join(",")),
@@ -61,11 +79,6 @@ fn main() -> std::io::Result<ExitCode> {
 
 // HACK(eddyb) should avoid shelling out, but for now this will suffice.
 const SH_SCRIPT: &str = r#"
-set -e
-
-llvm_project_tgz_url="https://codeload.github.com/llvm/llvm-project/tar.gz/$llvm_project_git_hash"
-curl -sS "$llvm_project_tgz_url" | tar -C "$OUT_DIR" -xz
-llvm="$OUT_DIR"/llvm-project-"$llvm_project_git_hash"/llvm
 set -eux
 
 mkdir -p "$OUT_DIR"/fake-config/llvm/Config