Log when buffering a diagnostic.

This is useful in debugging when and where errors are emitted in
logs.

src/Cargo.lock

@@ -2183,6 +2183,7 @@ name = "rustc_errors"
 version = "0.0.0"
 dependencies = [
  "atty 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)",
+ "log 0.4.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc_cratesio_shim 0.0.0",
  "rustc_data_structures 0.0.0",
  "serialize 0.0.0",

src/librustc_errors/Cargo.toml

@@ -9,6 +9,7 @@ path = "lib.rs"
 crate-type = ["dylib"]
 
 [dependencies]
+log = "0.4"
 serialize = { path = "../libserialize" }
 syntax_pos = { path = "../libsyntax_pos" }
 rustc_data_structures = { path = "../librustc_data_structures" }

src/librustc_errors/diagnostic_builder.rs

@@ -108,6 +108,9 @@ impl<'a> DiagnosticBuilder<'a> {
             diagnostic = ::std::ptr::read(&self.diagnostic);
             ::std::mem::forget(self);
         };
+        // Logging here is useful to help track down where in logs an error was
+        // actually emitted.
+        debug!("buffer: diagnostic={:?}", diagnostic);
         buffered_diagnostics.push(diagnostic);
     }
 

src/librustc_errors/lib.rs

@@ -23,6 +23,8 @@ extern crate atty;
 extern crate termcolor;
 #[cfg(unix)]
 extern crate libc;
+#[macro_use]
+extern crate log;
 extern crate rustc_data_structures;
 extern crate serialize as rustc_serialize;
 extern crate syntax_pos;