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/diagnostic.rs

@@ -4,7 +4,9 @@ use crate::Level;
 use crate::Substitution;
 use crate::SubstitutionPart;
 use crate::SuggestionStyle;
+use crate::ToolMetadata;
 use rustc_lint_defs::Applicability;
+use rustc_serialize::json::Json;
 use rustc_span::{MultiSpan, Span, DUMMY_SP};
 use std::fmt;
 
@@ -303,6 +305,7 @@ impl Diagnostic {
             msg: msg.to_owned(),
             style: SuggestionStyle::ShowCode,
             applicability,
+            tool_metadata: Default::default(),
         });
         self
     }
@@ -328,6 +331,7 @@ impl Diagnostic {
             msg: msg.to_owned(),
             style: SuggestionStyle::ShowCode,
             applicability,
+            tool_metadata: Default::default(),
         });
         self
     }
@@ -354,6 +358,7 @@ impl Diagnostic {
             msg: msg.to_owned(),
             style: SuggestionStyle::CompletelyHidden,
             applicability,
+            tool_metadata: Default::default(),
         });
         self
     }
@@ -408,6 +413,7 @@ impl Diagnostic {
             msg: msg.to_owned(),
             style,
             applicability,
+            tool_metadata: Default::default(),
         });
         self
     }
@@ -446,6 +452,7 @@ impl Diagnostic {
             msg: msg.to_owned(),
             style: SuggestionStyle::ShowCode,
             applicability,
+            tool_metadata: Default::default(),
         });
         self
     }
@@ -515,6 +522,23 @@ impl Diagnostic {
         self
     }
 
+    /// Adds a suggestion intended only for a tool. The intent is that the metadata encodes
+    /// the suggestion in a tool-specific way, as it may not even directly involve Rust code.
+    pub fn tool_only_suggestion_with_metadata(
+        &mut self,
+        msg: &str,
+        applicability: Applicability,
+        tool_metadata: Json,
+    ) {
+        self.suggestions.push(CodeSuggestion {
+            substitutions: vec![],
+            msg: msg.to_owned(),
+            style: SuggestionStyle::CompletelyHidden,
+            applicability,
+            tool_metadata: ToolMetadata::new(tool_metadata),
+        })
+    }
+
     pub fn set_span<S: Into<MultiSpan>>(&mut self, sp: S) -> &mut Self {
         self.span = sp.into();
         if let Some(span) = self.span.primary_span() {