Make it possible to link to custom libfuzzer in another dep

Cargo.toml

@@ -16,6 +16,8 @@ once_cell = "1"
 cc = { version = "1.0", features = ["parallel"] }
 
 [features]
+default = ["link"]
+link = []
 arbitrary-derive = ["arbitrary/derive"]
 
 [workspace]

build.rs

@@ -1,40 +1,42 @@
 fn main() {
-    println!("cargo:rerun-if-env-changed=CUSTOM_LIBFUZZER_PATH");
-    if let Ok(custom) = ::std::env::var("CUSTOM_LIBFUZZER_PATH") {
-        println!("cargo:rerun-if-changed={custom}");
+    if cfg!(feature = "link") {
+        println!("cargo:rerun-if-env-changed=CUSTOM_LIBFUZZER_PATH");
+        if let Ok(custom) = ::std::env::var("CUSTOM_LIBFUZZER_PATH") {
+            println!("cargo:rerun-if-changed={custom}");
 
-        let custom_lib_path = ::std::path::PathBuf::from(&custom);
-        let custom_lib_dir = custom_lib_path.parent().unwrap().to_string_lossy();
+            let custom_lib_path = ::std::path::PathBuf::from(&custom);
+            let custom_lib_dir = custom_lib_path.parent().unwrap().to_string_lossy();
 
-        let custom_lib_name = custom_lib_path.file_stem().unwrap().to_string_lossy();
-        let custom_lib_name = custom_lib_name
-            .strip_prefix("lib")
-            .unwrap_or(custom_lib_name.as_ref());
+            let custom_lib_name = custom_lib_path.file_stem().unwrap().to_string_lossy();
+            let custom_lib_name = custom_lib_name
+                .strip_prefix("lib")
+                .unwrap_or(custom_lib_name.as_ref());
 
-        println!("cargo:rustc-link-search=native={}", custom_lib_dir);
-        println!("cargo:rustc-link-lib=static={}", custom_lib_name);
+            println!("cargo:rustc-link-search=native={}", custom_lib_dir);
+            println!("cargo:rustc-link-lib=static={}", custom_lib_name);
 
-        match std::env::var("CUSTOM_LIBFUZZER_STD_CXX") {
-            // Default behavior for backwards compat.
-            Err(_) => println!("cargo:rustc-link-lib=stdc++"),
-            Ok(s) if s == "none" => (),
-            Ok(s) => println!("cargo:rustc-link-lib={}", s),
+            match std::env::var("CUSTOM_LIBFUZZER_STD_CXX") {
+                // Default behavior for backwards compat.
+                Err(_) => println!("cargo:rustc-link-lib=stdc++"),
+                Ok(s) if s == "none" => (),
+                Ok(s) => println!("cargo:rustc-link-lib={}", s),
+            }
+        } else {
+            let mut build = cc::Build::new();
+            let sources = ::std::fs::read_dir("libfuzzer")
+                .expect("listable source directory")
+                .map(|de| de.expect("file in directory").path())
+                .filter(|p| p.extension().map(|ext| ext == "cpp") == Some(true))
+                .collect::<Vec<_>>();
+            for source in sources.iter() {
+                println!("cargo:rerun-if-changed={}", source.display());
+                build.file(source.to_str().unwrap());
+            }
+            build.flag("-std=c++17");
+            build.flag("-fno-omit-frame-pointer");
+            build.flag("-w");
+            build.cpp(true);
+            build.compile("libfuzzer.a");
         }
-    } else {
-        let mut build = cc::Build::new();
-        let sources = ::std::fs::read_dir("libfuzzer")
-            .expect("listable source directory")
-            .map(|de| de.expect("file in directory").path())
-            .filter(|p| p.extension().map(|ext| ext == "cpp") == Some(true))
-            .collect::<Vec<_>>();
-        for source in sources.iter() {
-            println!("cargo:rerun-if-changed={}", source.display());
-            build.file(source.to_str().unwrap());
-        }
-        build.flag("-std=c++17");
-        build.flag("-fno-omit-frame-pointer");
-        build.flag("-w");
-        build.cpp(true);
-        build.compile("libfuzzer.a");
     }
 }