Make llvm backend opt in as feature "llvm-backend"

Resolve #36.

Author: polybuildr

Cargo.toml

@@ -13,14 +13,15 @@ peg = { version = "0.5" }
 [features]
 cargo-clippy = []
 file-tests = []
+llvm-backend = ["llvm-sys", "libc", "itertools"]
 
 [dependencies]
 ansi_term = "0.9.0"
 rustyline = "1.0.0"
 fnv = "1.0.5"
 linear-map = "1.1.0"
 hyper = "0.10"
-llvm-sys = "40.0.0"
-itertools = "0.6.0"
-libc = "0.2"
+llvm-sys = {version = "40.0.0", optional = true}
+itertools = {version = "0.6.0", optional = true}
+libc = {version = "0.2", optional = true}
 clippy = {version = "*", optional = true}

src/main.rs

@@ -13,10 +13,11 @@ extern crate linear_map;
 
 extern crate hyper;
 
+#[cfg(feature = "llvm-backend")]
 extern crate llvm_sys;
-
+#[cfg(feature = "llvm-backend")]
 extern crate itertools;
-
+#[cfg(feature = "llvm-backend")]
 extern crate libc;
 
 // include output of rust-peg given grammar.rustpeg
@@ -28,6 +29,7 @@ mod parser {
 mod ast;
 mod runtime;
 mod ast_walk_interpreter;
+#[cfg(feature = "llvm-backend")]
 mod llvm_interpreter;
 mod value;
 mod operations;
@@ -47,17 +49,21 @@ mod file_test {
 
 use runtime::*;
 use ast_walk_interpreter::AstWalkInterpreter;
+#[cfg(feature = "llvm-backend")]
 use llvm_interpreter::LLVMInterpreter;
 
 use error::*;
 
 // FIXME: How do you represent the usage style in POSIX notation?
 fn print_usage() {
-    println!("usage: balloon [--repl-llvm | [MODE] FILE ]
-
-
---repl-llvm     launches the experimental REPL
+    if cfg!(feature = "llvm-backend") {
+        println!("usage: balloon [--repl-llvm | [MODE] FILE ]
 
+--repl-llvm     launches the experimental REPL");
+    } else {
+        println!("usage: balloon [MODE] FILE");
+    }
+    println!("
 where MODE is one of:
 --run           (default) runs the file [FILE]
 --check         type check the file [FILE]
@@ -71,6 +77,7 @@ fn main() {
         1 => repl::run_repl(AstWalkInterpreter::new()),
         2 => {
             match args[1].as_str() {
+                #[cfg(feature = "llvm-backend")]
                 "--repl-llvm" => repl::run_repl(LLVMInterpreter::new()),
                 filepath => run_file(filepath, AstWalkInterpreter::new()),
             }