make rustfmt optional

- when rustfmt is not detected (by attempting to run ``rustfmt --version)
  no formatting will be attempted
- this should resolve issue mthom/scryer-prolog#1504

build/main.rs

@@ -1,15 +1,15 @@
-mod static_string_indexing;
 mod instructions_template;
+mod static_string_indexing;
 
-use static_string_indexing::index_static_strings;
 use instructions_template::generate_instructions_rs;
+use static_string_indexing::index_static_strings;
 
 use std::env;
 use std::fs;
 use std::fs::File;
 use std::io::Write;
 use std::path::Path;
-use std::process::{self, Command};
+use std::process::{self, Command, Stdio};
 
 fn find_prolog_files(libraries: &mut File, prefix: &str, current_dir: &Path) {
     let entries = match current_dir.read_dir() {
@@ -42,6 +42,16 @@ fn find_prolog_files(libraries: &mut File, prefix: &str, current_dir: &Path) {
 }
 
 fn main() {
+    let has_rustfmt = Command::new("rustfmt")
+        .arg("--version")
+        .stdin(Stdio::inherit())
+        .status()
+        .is_ok();
+
+    if !has_rustfmt {
+        println!("Failed to run rustfmt, will skip formatting generated files.")
+    }
+
     let out_dir = env::var("OUT_DIR").unwrap();
     let dest_path = Path::new(&out_dir).join("libraries.rs");
 
@@ -68,7 +78,9 @@ fn main() {
         .write_all(quoted_output.to_string().as_bytes())
         .unwrap();
 
+    if has_rustfmt {
         format_generated_file(instructions_path.as_path());
+    }
 
     let static_atoms_path = Path::new(&out_dir).join("static_atoms.rs");
     let mut static_atoms_file = File::create(&static_atoms_path).unwrap();
@@ -79,7 +91,9 @@ fn main() {
         .write_all(quoted_output.to_string().as_bytes())
         .unwrap();
 
+    if has_rustfmt {
         format_generated_file(static_atoms_path.as_path());
+    }
 
     println!("cargo:rerun-if-changed=src/");
 }