filter: Don't print to stdout the warnings due to an invalid spec.

This makes programs that rely on the output of other programs not work properly
in these cases. stderr is more appropriate for such things.

Indeed, if you `export RUST_LOG="xxx=xxx"` before building this crate for the
first time, the build fails :)

See rust-lang/rust-bindgen#1502.

Author: emilio

src/filter/mod.rs

@@ -306,8 +306,8 @@ fn parse_spec(spec: &str) -> (Vec<Directive>, Option<inner::Filter>) {
     let mods = parts.next();
     let filter = parts.next();
     if parts.next().is_some() {
-        println!("warning: invalid logging spec '{}', \
-                 ignoring it (too many '/'s)", spec);
+        eprintln!("warning: invalid logging spec '{}', \
+                  ignoring it (too many '/'s)", spec);
         return (dirs, None);
     }
     mods.map(|m| { for s in m.split(',') {
@@ -327,15 +327,15 @@ fn parse_spec(spec: &str) -> (Vec<Directive>, Option<inner::Filter>) {
                 match part1.parse() {
                     Ok(num) => (num, Some(part0)),
                     _ => {
-                        println!("warning: invalid logging spec '{}', \
-                                 ignoring it", part1);
+                        eprintln!("warning: invalid logging spec '{}', \
+                                  ignoring it", part1);
                         continue
                     }
                 }
             },
             _ => {
-                println!("warning: invalid logging spec '{}', \
-                         ignoring it", s);
+                eprintln!("warning: invalid logging spec '{}', \
+                          ignoring it", s);
                 continue
             }
         };
@@ -349,7 +349,7 @@ fn parse_spec(spec: &str) -> (Vec<Directive>, Option<inner::Filter>) {
         match inner::Filter::new(filter) {
             Ok(re) => Some(re),
             Err(e) => {
-                println!("warning: invalid regex filter - {}", e);
+                eprintln!("warning: invalid regex filter - {}", e);
                 None
             }
         }