Add --extern-loc to augment unused crate dependency diagnostics

This allows a build system to indicate a location in its own dependency
specification files (eg Cargo's `Cargo.toml`) which can be reported
along side any unused crate dependency.

This supports several types of location:
 - 'json' - provide some json-structured data, which is included in the json diagnostics
     in a `tool_metadata` field
 - 'raw' - emit the provided string into the output. This also appears as a json string in
     `tool_metadata`.

If no `--extern-location` is explicitly provided then a default json entry of the form
`"tool_metadata":{"name":<cratename>,"path":<cratepath>}` is emitted.

compiler/rustc_errors/src/lib.rs

@@ -23,10 +23,13 @@ use rustc_data_structures::sync::{self, Lock, Lrc};
 use rustc_data_structures::AtomicRef;
 use rustc_lint_defs::FutureBreakage;
 pub use rustc_lint_defs::{pluralize, Applicability};
+use rustc_serialize::json::Json;
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use rustc_span::source_map::SourceMap;
 use rustc_span::{Loc, MultiSpan, Span};
 
 use std::borrow::Cow;
+use std::hash::{Hash, Hasher};
 use std::panic;
 use std::path::Path;
 use std::{error, fmt};
@@ -73,6 +76,35 @@ impl SuggestionStyle {
     }
 }
 
+#[derive(Clone, Debug, PartialEq, Default)]
+pub struct ToolMetadata(pub Option<Json>);
+
+impl ToolMetadata {
+    fn new(json: Json) -> Self {
+        ToolMetadata(Some(json))
+    }
+}
+
+impl Hash for ToolMetadata {
+    fn hash<H: Hasher>(&self, _state: &mut H) {}
+}
+
+// Doesn't really need to round-trip
+impl<D: Decoder> Decodable<D> for ToolMetadata {
+    fn decode(_d: &mut D) -> Result<Self, D::Error> {
+        Ok(ToolMetadata(None))
+    }
+}
+
+impl<S: Encoder> Encodable<S> for ToolMetadata {
+    fn encode(&self, e: &mut S) -> Result<(), S::Error> {
+        match &self.0 {
+            None => e.emit_unit(),
+            Some(json) => json.encode(e),
+        }
+    }
+}
+
 #[derive(Clone, Debug, PartialEq, Hash, Encodable, Decodable)]
 pub struct CodeSuggestion {
     /// Each substitute can have multiple variants due to multiple
@@ -106,6 +138,8 @@ pub struct CodeSuggestion {
     /// which are useful for users but not useful for
     /// tools like rustfix
     pub applicability: Applicability,
+    /// Tool-specific metadata
+    pub tool_metadata: ToolMetadata,
 }
 
 #[derive(Clone, Debug, PartialEq, Hash, Encodable, Decodable)]
@@ -775,7 +809,6 @@ impl HandlerInner {
         }
 
         let already_emitted = |this: &mut Self| {
-            use std::hash::Hash;
             let mut hasher = StableHasher::new();
             diagnostic.hash(&mut hasher);
             let diagnostic_hash = hasher.finish();