[incremental] Specialize encoding and decoding of Fingerprints

This saves the storage space used by about 32 bits per `Fingerprint`.
On average, this reduces the size of the `/target/{mode}/incremental`
folder by roughly 5%.

Fixes #45875

Author: wesleywiser

src/librustc/ich/fingerprint.rs

@@ -8,9 +8,12 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use std::mem;
 use rustc_data_structures::stable_hasher;
+use serialize;
+use serialize::opaque::{EncodeResult, Encoder, Decoder};
 
-#[derive(Eq, PartialEq, Ord, PartialOrd, Hash, Debug, Clone, Copy, RustcEncodable, RustcDecodable)]
+#[derive(Eq, PartialEq, Ord, PartialOrd, Hash, Debug, Clone, Copy)]
 pub struct Fingerprint(u64, u64);
 
 impl Fingerprint {
@@ -46,6 +49,21 @@ impl Fingerprint {
         format!("{:x}{:x}", self.0, self.1)
     }
 
+    pub fn encode_opaque(&self, encoder: &mut Encoder) -> EncodeResult {
+        let bytes: [u8; 16] = unsafe { mem::transmute([self.0.to_le(), self.1.to_le()]) };
+
+        encoder.emit_raw_bytes(&bytes)
+    }
+
+    pub fn decode_opaque<'a>(decoder: &mut Decoder<'a>) -> Result<Fingerprint, String> {
+        let mut bytes = [0; 16];
+
+        decoder.read_raw_bytes(&mut bytes)?;
+
+        let [l, r]: [u64; 2] = unsafe { mem::transmute(bytes) };
+
+        Ok(Fingerprint(u64::from_le(l), u64::from_le(r)))
+    }
 }
 
 impl ::std::fmt::Display for Fingerprint {
@@ -69,3 +87,19 @@ impl<CTX> stable_hasher::HashStable<CTX> for Fingerprint {
         ::std::hash::Hash::hash(self, hasher);
     }
 }
+
+impl serialize::UseSpecializedEncodable for Fingerprint { }
+
+impl serialize::UseSpecializedDecodable for Fingerprint { }
+
+impl<'a> serialize::SpecializedEncoder<Fingerprint> for serialize::opaque::Encoder<'a> {
+    fn specialized_encode(&mut self, f: &Fingerprint) -> Result<(), Self::Error> {
+        f.encode_opaque(self)
+    }
+}
+
+impl<'a> serialize::SpecializedDecoder<Fingerprint> for serialize::opaque::Decoder<'a> {
+    fn specialized_decode(&mut self) -> Result<Fingerprint, Self::Error> {
+        Fingerprint::decode_opaque(self)
+    }
+}

src/librustc/ty/maps/on_disk_cache.rs

@@ -14,7 +14,7 @@ use hir;
 use hir::def_id::{CrateNum, DefIndex, DefId, LocalDefId,
                   RESERVED_FOR_INCR_COMP_CACHE, LOCAL_CRATE};
 use hir::map::definitions::DefPathHash;
-use ich::CachingCodemapView;
+use ich::{CachingCodemapView, Fingerprint};
 use mir;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::indexed_vec::{IndexVec, Idx};
@@ -660,6 +660,12 @@ impl<'a, 'tcx, 'x> SpecializedDecoder<NodeId> for CacheDecoder<'a, 'tcx, 'x> {
     }
 }
 
+impl<'a, 'tcx, 'x> SpecializedDecoder<Fingerprint> for CacheDecoder<'a, 'tcx, 'x> {
+    fn specialized_decode(&mut self) -> Result<Fingerprint, Self::Error> {
+        Fingerprint::decode_opaque(&mut self.opaque)
+    }
+}
+
 impl<'a, 'tcx, 'x, T: Decodable> SpecializedDecoder<mir::ClearCrossCrate<T>>
 for CacheDecoder<'a, 'tcx, 'x> {
     #[inline]
@@ -879,6 +885,14 @@ impl<'enc, 'a, 'tcx, E> SpecializedEncoder<NodeId> for CacheEncoder<'enc, 'a, 't
     }
 }
 
+impl<'enc, 'a, 'tcx> SpecializedEncoder<Fingerprint>
+for CacheEncoder<'enc, 'a, 'tcx, opaque::Encoder<'enc>>
+{
+    fn specialized_encode(&mut self, f: &Fingerprint) -> Result<(), Self::Error> {
+        f.encode_opaque(&mut self.encoder)
+    }
+}
+
 impl<'enc, 'a, 'tcx, E, T> SpecializedEncoder<mir::ClearCrossCrate<T>>
 for CacheEncoder<'enc, 'a, 'tcx, E>
     where E: 'enc + ty_codec::TyEncoder,

src/librustc_metadata/decoder.rs

@@ -336,6 +336,12 @@ impl<'a, 'tcx> SpecializedDecoder<Span> for DecodeContext<'a, 'tcx> {
     }
 }
 
+impl<'a, 'tcx> SpecializedDecoder<Fingerprint> for DecodeContext<'a, 'tcx> {
+    fn specialized_decode(&mut self) -> Result<Fingerprint, Self::Error> {
+        Fingerprint::decode_opaque(&mut self.opaque)
+    }
+}
+
 impl<'a, 'tcx, T: Decodable> SpecializedDecoder<mir::ClearCrossCrate<T>>
 for DecodeContext<'a, 'tcx> {
     #[inline]

src/librustc_metadata/encoder.rs

@@ -18,6 +18,7 @@ use rustc::middle::cstore::{LinkMeta, LinkagePreference, NativeLibrary,
 use rustc::hir::def::CtorKind;
 use rustc::hir::def_id::{CrateNum, CRATE_DEF_INDEX, DefIndex, DefId, LOCAL_CRATE};
 use rustc::hir::map::definitions::DefPathTable;
+use rustc::ich::Fingerprint;
 use rustc::middle::dependency_format::Linkage;
 use rustc::middle::lang_items;
 use rustc::mir;
@@ -154,6 +155,12 @@ impl<'a, 'tcx> SpecializedEncoder<ty::GenericPredicates<'tcx>> for EncodeContext
     }
 }
 
+impl<'a, 'tcx> SpecializedEncoder<Fingerprint> for EncodeContext<'a, 'tcx> {
+    fn specialized_encode(&mut self, f: &Fingerprint) -> Result<(), Self::Error> {
+        f.encode_opaque(&mut self.opaque)
+    }
+}
+
 impl<'a, 'tcx, T: Encodable> SpecializedEncoder<mir::ClearCrossCrate<T>>
 for EncodeContext<'a, 'tcx> {
     fn specialized_encode(&mut self,

src/libserialize/opaque.rs

@@ -175,11 +175,12 @@ impl<'a> Decoder<'a> {
     }
 
     pub fn read_raw_bytes(&mut self, s: &mut [u8]) -> Result<(), String> {
-        let len = s.len();
+        let start = self.position;
+        let end = start + s.len();
 
-        self.position += len;
+        s.copy_from_slice(&self.data[start..end]);
 
-        s.copy_from_slice(&self.data[0..len]);
+        self.position = end;
 
         Ok(())
     }