Only require the target spec if the c-feature is enabled

Cargo.toml

@@ -5,7 +5,7 @@ name = "compiler_builtins"
 version = "0.1.0"
 
 [build-dependencies]
-rustc-cfg = "0.3.0"
+rustc-cfg = { version = "0.3.0", optional = true }
 
 [build-dependencies.gcc]
 optional = true
@@ -19,7 +19,7 @@ compiler-rt = { path = "compiler-rt" }
 
 [features]
 # Build the missing intrinsics from compiler-rt C source code
-c = ["gcc"]
+c = ["gcc", "rustc-cfg"]
 # Mark this crate as the #![compiler_builtins] crate
 compiler-builtins = []
 default = ["compiler-builtins"]

build.rs

@@ -1,14 +1,20 @@
 #[cfg(feature = "c")]
 extern crate gcc;
+#[cfg(feature = "c")]
 extern crate rustc_cfg;
 
 #[cfg(feature = "c")]
 use std::collections::BTreeMap;
+#[cfg(feature = "c")]
 use std::io::Write;
 #[cfg(feature = "c")]
 use std::path::Path;
-use std::{env, io, process};
+use std::env;
+
+#[cfg(feature = "c")]
+use std::{io, process};
 
+#[cfg(feature = "c")]
 use rustc_cfg::Cfg;
 
 #[cfg(feature = "c")]
@@ -61,20 +67,19 @@ fn main() {
         return;
     }
 
-    let Cfg { ref target_arch, ref target_os, ref target_env, ref target_vendor, .. } =
-        Cfg::new(&target).unwrap_or_else(|e| {
-            writeln!(io::stderr(), "{}", e).ok();
-            process::exit(1)
-        });
     // NOTE we are going to assume that llvm-target, what determines our codegen option, matches the
     // target triple. This is usually correct for our built-in targets but can break in presence of
     // custom targets, which can have arbitrary names.
     let llvm_target = target.split('-').collect::<Vec<_>>();
 
     // Build missing intrinsics from compiler-rt C source code
-    match () {
     #[cfg(feature = "c")]
-        () => {
+    {
+        let Cfg { ref target_arch, ref target_os, ref target_env, ref target_vendor, .. } =
+            Cfg::new(&target).unwrap_or_else(|e| {
+                writeln!(io::stderr(), "{}", e).ok();
+                process::exit(1)
+            });
         let target_vendor = target_vendor.as_ref().unwrap();
         let cfg = &mut gcc::Config::new();
 
@@ -413,9 +418,6 @@ fn main() {
 
         cfg.compile("libcompiler-rt.a");
     }
-        #[cfg(not(feature = "c"))]
-        () => {}
-    }
 
     // To filter away some flaky test (see src/float/add.rs for details)
     if llvm_target[0].starts_with("arm") &&

src/bin/intrinsics.rs

@@ -3,7 +3,7 @@
 // compiling a C implementation and forget to implement that intrinsic in Rust, this file will fail
 // to link due to the missing intrinsic (symbol).
 
-#![allow(unused_features)]
+#![allow(unused_features, private_no_mangle_fns)]
 #![cfg_attr(thumb, no_main)]
 #![deny(dead_code)]
 #![feature(asm)]