Merge pull request #59 from dtolnay/extern

Allow macro calls anywhere in declaration crate

Author: dtolnay

build.rs

@@ -0,0 +1,30 @@
+use std::env;
+use std::process::Command;
+use std::str;
+
+fn main() {
+    let compiler = match rustc_version() {
+        Some(compiler) => compiler,
+        None => return,
+    };
+
+    if compiler.minor < 33 {
+        println!("cargo:rustc-cfg=proc_macro_hack_no_crate_as_underscore");
+    }
+}
+
+struct Compiler {
+    minor: u32,
+}
+
+fn rustc_version() -> Option<Compiler> {
+    let rustc = env::var_os("RUSTC")?;
+    let output = Command::new(rustc).arg("--version").output().ok()?;
+    let version = str::from_utf8(&output.stdout).ok()?;
+    let mut pieces = version.split('.');
+    if pieces.next() != Some("rustc 1") {
+        return None;
+    }
+    let minor = pieces.next()?.parse().ok()?;
+    Some(Compiler { minor })
+}

src/lib.rs

@@ -332,6 +332,16 @@ fn expand_export(export: Export, args: ExportArgs) -> TokenStream {
         });
     }
 
+    let extern_crate = if cfg!(proc_macro_hack_no_crate_as_underscore) {
+        quote!()
+    } else {
+        quote! {
+            #[allow(unused_imports)]
+            #[macro_use]
+            extern crate #from as _;
+        }
+    };
+
     if export.macros.len() != 1 {
         export_dispatch = quote!({#export_dispatch});
         export_call_site = quote!({#export_call_site});
@@ -357,6 +367,8 @@ fn expand_export(export: Export, args: ExportArgs) -> TokenStream {
     };
 
     let expanded = quote! {
+        #extern_crate
+
         #[doc(hidden)]
         #vis use #from::#actual_names;