Encode spans with relative offsets

The relative offset is always smaller than the absolute offset, and with
the LEB128 encoding, this ends up cutting the overall metadata size
considerably (~1.5 megabytes on libcore).

Author: Mark-Simulacrum

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -507,14 +507,17 @@ impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for ExpnId {
 
 impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for Span {
     fn decode(decoder: &mut DecodeContext<'a, 'tcx>) -> Span {
+        let start = decoder.position();
         let mode = SpanEncodingMode::decode(decoder);
         let data = match mode {
             SpanEncodingMode::Direct => SpanData::decode(decoder),
-            SpanEncodingMode::Shorthand(position) => decoder.with_position(position, |decoder| {
-                let mode = SpanEncodingMode::decode(decoder);
-                debug_assert!(matches!(mode, SpanEncodingMode::Direct));
-                SpanData::decode(decoder)
-            }),
+            SpanEncodingMode::Shorthand(offset) => {
+                decoder.with_position(start - offset, |decoder| {
+                    let mode = SpanEncodingMode::decode(decoder);
+                    debug_assert!(matches!(mode, SpanEncodingMode::Direct));
+                    SpanData::decode(decoder)
+                })
+            }
         };
         Span::new(data.lo, data.hi, data.ctxt, data.parent)
     }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -169,7 +169,12 @@ impl<'a, 'tcx> Encodable<EncodeContext<'a, 'tcx>> for ExpnId {
 impl<'a, 'tcx> Encodable<EncodeContext<'a, 'tcx>> for Span {
     fn encode(&self, s: &mut EncodeContext<'a, 'tcx>) {
         match s.span_shorthands.entry(*self) {
-            Entry::Occupied(o) => SpanEncodingMode::Shorthand(*o.get()).encode(s),
+            Entry::Occupied(o) => {
+                // Encode the offset rather than the absolute position. The underlying encoding uses
+                // LEB128, and offsets are always smaller than the absolute position.
+                let offset = s.opaque.position() - *o.get();
+                SpanEncodingMode::Shorthand(offset).encode(s)
+            }
             Entry::Vacant(v) => {
                 let position = s.opaque.position();
                 v.insert(position);