Remove --extern-location and all associated code

`--extern-location` was an experiment to investigate the best way to
generate useful diagnostics for unused dependency warnings by enabling a
build system to identify the corresponding build config.

While I did successfully use this, I've since been convinced the
alternative `--json unused-externs` mechanism is the way to go, and
there's no point in having two mechanisms with basically the same
functionality.

This effectively reverts https://github.com/rust-lang/rust/pull/72603

compiler/rustc_errors/src/json.rs

@@ -14,7 +14,6 @@ use rustc_span::source_map::{FilePathMapping, SourceMap};
 use crate::emitter::{Emitter, HumanReadableErrorType};
 use crate::registry::Registry;
 use crate::DiagnosticId;
-use crate::ToolMetadata;
 use crate::{
     CodeSuggestion, FluentBundle, LazyFallbackBundle, MultiSpan, SpanLabel, SubDiagnostic,
 };
@@ -30,7 +29,6 @@ use std::sync::{Arc, Mutex};
 use std::vec;
 
 use rustc_serialize::json::{as_json, as_pretty_json};
-use rustc_serialize::{Encodable, Encoder};
 
 #[cfg(test)]
 mod tests;
@@ -205,8 +203,7 @@ impl Emitter for JsonEmitter {
 
 // The following data types are provided just for serialisation.
 
-// NOTE: this has a manual implementation of Encodable which needs to be updated in
-// parallel.
+#[derive(Encodable)]
 struct Diagnostic {
     /// The primary error message.
     message: String,
@@ -218,65 +215,6 @@ struct Diagnostic {
     children: Vec<Diagnostic>,
     /// The message as rustc would render it.
     rendered: Option<String>,
-    /// Extra tool metadata
-    tool_metadata: ToolMetadata,
-}
-
-macro_rules! encode_fields {
-    (
-        $enc:expr,                  // encoder
-        $idx:expr,                  // starting field index
-        $struct:expr,               // struct we're serializing
-        $struct_name:ident,         // struct name
-        [ $($name:ident),+$(,)? ],  // fields to encode
-        [ $($ignore:ident),+$(,)? ] // fields we're skipping
-    ) => {
-        {
-            // Pattern match to make sure all fields are accounted for
-            let $struct_name { $($name,)+ $($ignore: _,)+ } = $struct;
-            let mut idx = $idx;
-            $(
-                $enc.emit_struct_field(
-                    stringify!($name),
-                    idx == 0,
-                    |enc| $name.encode(enc),
-                )?;
-                idx += 1;
-            )+
-            idx
-        }
-    };
-}
-
-// Special-case encoder to skip tool_metadata if not set
-impl<E: Encoder> Encodable<E> for Diagnostic {
-    fn encode(&self, s: &mut E) -> Result<(), E::Error> {
-        s.emit_struct(false, |s| {
-            let mut idx = 0;
-
-            idx = encode_fields!(
-                s,
-                idx,
-                self,
-                Self,
-                [message, code, level, spans, children, rendered],
-                [tool_metadata]
-            );
-            if self.tool_metadata.is_set() {
-                idx = encode_fields!(
-                    s,
-                    idx,
-                    self,
-                    Self,
-                    [tool_metadata],
-                    [message, code, level, spans, children, rendered]
-                );
-            }
-
-            let _ = idx;
-            Ok(())
-        })
-    }
 }
 
 #[derive(Encodable)]
@@ -380,7 +318,6 @@ impl Diagnostic {
                 spans: DiagnosticSpan::from_suggestion(sugg, &args, je),
                 children: vec![],
                 rendered: None,
-                tool_metadata: sugg.tool_metadata.clone(),
             }
         });
 
@@ -428,7 +365,6 @@ impl Diagnostic {
                 .chain(sugg)
                 .collect(),
             rendered: Some(output),
-            tool_metadata: ToolMetadata::default(),
         }
     }
 
@@ -449,7 +385,6 @@ impl Diagnostic {
                 .unwrap_or_else(|| DiagnosticSpan::from_multispan(&diag.span, args, je)),
             children: vec![],
             rendered: None,
-            tool_metadata: ToolMetadata::default(),
         }
     }
 }