Rollup merge of #30565 - michaelwoerister:opaque_encoder, r=brson

This PR changes the `emit_opaque` and `read_opaque` methods in the RBML library to use a space-efficient binary encoder that does not emit any tags and uses the LEB128 variable-length integer format for all numbers it emits.

The space savings are nice, albeit a bit underwhelming, especially for dynamic libraries where metadata is already compressed.

| RLIBs        |  NEW   |   OLD     |
|--------------|--------|-----------|
|libstd        | 8.8 MB |  10.5 MB  |
|libcore       |15.6 MB |   19.7 MB |
|libcollections| 3.7 MB |    4.8 MB |
|librustc      |34.0 MB |   37.8 MB |
|libsyntax     |28.3 MB |   32.1 MB |

| SOs           |     NEW   |    OLD |
|---------------|-----------|--------|
| libstd        |  4.8 MB   | 5.1 MB |
| librustc      |  8.6 MB   | 9.2 MB |
| libsyntax     |  7.8 MB   | 8.4 MB |

At least this should make up for the size increase caused recently by also storing MIR in crate metadata.

Can this be a breaking change for anyone?
cc @rust-lang/compiler

1 files changed, 4 insertions, 7 deletions

diff --git a/src/libsyntax/codemap.rs b/src/libsyntax/codemap.rs
index 18659cb2e78..19236f2cd98 100644
--- a/src/libsyntax/codemap.rs
+++ b/src/libsyntax/codemap.rs
@@ -164,18 +164,15 @@ impl Eq for Span {}
 
 impl Encodable for Span {
     fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
-        // Encode spans as a single u64 in order to cut down on tagging overhead
-        // added by the RBML metadata encoding. The should be solved differently
-        // altogether some time (FIXME #21482)
-        s.emit_u64( (self.lo.0 as u64) | ((self.hi.0 as u64) << 32) )
+        try!(s.emit_u32(self.lo.0));
+        s.emit_u32(self.hi.0)
     }
 }
 
 impl Decodable for Span {
     fn decode<D: Decoder>(d: &mut D) -> Result<Span, D::Error> {
-        let lo_hi: u64 = try! { d.read_u64() };
-        let lo = BytePos(lo_hi as u32);
-        let hi = BytePos((lo_hi >> 32) as u32);
+        let lo = BytePos(try! { d.read_u32() });
+        let hi = BytePos(try! { d.read_u32() });
         Ok(mk_sp(lo, hi))
     }
 }