rustc_metadata: dedupe strings to prevent multiple copies in rmeta/query cache blow file size

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -637,6 +637,35 @@ impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for Span {
     }
 }
 
+impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for Symbol {
+    fn decode(d: &mut DecodeContext<'a, 'tcx>) -> Self {
+        let tag = d.read_u8();
+
+        match tag {
+            SYMBOL_STR => {
+                let s = d.read_str();
+                Symbol::intern(s)
+            }
+            SYMBOL_OFFSET => {
+                // read str offset
+                let pos = d.read_usize();
+                let old_pos = d.opaque.position();
+
+                // move to str ofset and read
+                d.opaque.set_position(pos);
+                let s = d.read_str();
+                let sym = Symbol::intern(s);
+
+                // restore position
+                d.opaque.set_position(old_pos);
+
+                sym
+            }
+            _ => unreachable!(),
+        }
+    }
+}
+
 impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for &'tcx [ty::abstract_const::Node<'tcx>] {
     fn decode(d: &mut DecodeContext<'a, 'tcx>) -> Self {
         ty::codec::RefDecodable::decode(d)

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -39,6 +39,7 @@ use rustc_span::{
 };
 use rustc_target::abi::VariantIdx;
 use std::borrow::Borrow;
+use std::collections::hash_map::Entry;
 use std::hash::Hash;
 use std::io::{Read, Seek, Write};
 use std::iter;
@@ -75,6 +76,7 @@ pub(super) struct EncodeContext<'a, 'tcx> {
     required_source_files: Option<GrowableBitSet<usize>>,
     is_proc_macro: bool,
     hygiene_ctxt: &'a HygieneEncodeContext,
+    symbol_table: FxHashMap<Symbol, usize>,
 }
 
 /// If the current crate is a proc-macro, returns early with `Lazy:empty()`.
@@ -307,6 +309,24 @@ impl<'a, 'tcx> Encodable<EncodeContext<'a, 'tcx>> for Span {
     }
 }
 
+impl<'a, 'tcx> Encodable<EncodeContext<'a, 'tcx>> for Symbol {
+    fn encode(&self, s: &mut EncodeContext<'a, 'tcx>) {
+        match s.symbol_table.entry(*self) {
+            Entry::Vacant(o) => {
+                s.opaque.emit_u8(SYMBOL_STR);
+                let pos = s.opaque.position();
+                o.insert(pos);
+                s.emit_str(self.as_str());
+            }
+            Entry::Occupied(o) => {
+                let x = o.get().clone();
+                s.emit_u8(SYMBOL_OFFSET);
+                s.emit_usize(x);
+            }
+        }
+    }
+}
+
 impl<'a, 'tcx> TyEncoder for EncodeContext<'a, 'tcx> {
     const CLEAR_CROSS_CRATE: bool = true;
 
@@ -2259,6 +2279,7 @@ fn encode_metadata_impl(tcx: TyCtxt<'_>, path: &Path) {
         required_source_files,
         is_proc_macro: tcx.sess.crate_types().contains(&CrateType::ProcMacro),
         hygiene_ctxt: &hygiene_ctxt,
+        symbol_table: Default::default(),
     };
 
     // Encode the rustc version string in a predictable location.

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -445,6 +445,10 @@ const TAG_VALID_SPAN_LOCAL: u8 = 0;
 const TAG_VALID_SPAN_FOREIGN: u8 = 1;
 const TAG_PARTIAL_SPAN: u8 = 2;
 
+// Tags for encoding Symbol's
+const SYMBOL_STR: u8 = 0;
+const SYMBOL_OFFSET: u8 = 1;
+
 pub fn provide(providers: &mut Providers) {
     encoder::provide(providers);
     decoder::provide(providers);

compiler/rustc_query_impl/src/on_disk_cache.rs

@@ -22,8 +22,9 @@ use rustc_span::hygiene::{
     ExpnId, HygieneDecodeContext, HygieneEncodeContext, SyntaxContext, SyntaxContextData,
 };
 use rustc_span::source_map::{SourceMap, StableSourceFileId};
-use rustc_span::CachingSourceMapView;
 use rustc_span::{BytePos, ExpnData, ExpnHash, Pos, SourceFile, Span};
+use rustc_span::{CachingSourceMapView, Symbol};
+use std::collections::hash_map::Entry;
 use std::io;
 use std::mem;
 
@@ -38,6 +39,10 @@ const TAG_RELATIVE_SPAN: u8 = 2;
 const TAG_SYNTAX_CONTEXT: u8 = 0;
 const TAG_EXPN_DATA: u8 = 1;
 
+// Tags for encoding Symbol's
+const SYMBOL_STR: u8 = 0;
+const SYMBOL_OFFSET: u8 = 1;
+
 /// Provides an interface to incremental compilation data cached from the
 /// previous compilation session. This data will eventually include the results
 /// of a few selected queries (like `typeck` and `mir_optimized`) and
@@ -254,6 +259,7 @@ impl<'sess> rustc_middle::ty::OnDiskCache<'sess> for OnDiskCache<'sess> {
                 source_map: CachingSourceMapView::new(tcx.sess.source_map()),
                 file_to_file_index,
                 hygiene_context: &hygiene_encode_context,
+                symbol_table: Default::default(),
             };
 
             // Encode query results.
@@ -714,6 +720,36 @@ impl<'a, 'tcx> Decodable<CacheDecoder<'a, 'tcx>> for Span {
     }
 }
 
+// copy&paste impl from rustc_metadata
+impl<'a, 'tcx> Decodable<CacheDecoder<'a, 'tcx>> for Symbol {
+    fn decode(d: &mut CacheDecoder<'a, 'tcx>) -> Self {
+        let tag = d.read_u8();
+
+        match tag {
+            SYMBOL_STR => {
+                let s = d.read_str();
+                Symbol::intern(s)
+            }
+            SYMBOL_OFFSET => {
+                // read str offset
+                let pos = d.read_usize();
+                let old_pos = d.opaque.position();
+
+                // move to str ofset and read
+                d.opaque.set_position(pos);
+                let s = d.read_str();
+                let sym = Symbol::intern(s);
+
+                // restore position
+                d.opaque.set_position(old_pos);
+
+                sym
+            }
+            _ => unreachable!(),
+        }
+    }
+}
+
 impl<'a, 'tcx> Decodable<CacheDecoder<'a, 'tcx>> for CrateNum {
     fn decode(d: &mut CacheDecoder<'a, 'tcx>) -> Self {
         let stable_id = StableCrateId::decode(d);
@@ -815,6 +851,7 @@ pub struct CacheEncoder<'a, 'tcx> {
     source_map: CachingSourceMapView<'tcx>,
     file_to_file_index: FxHashMap<*const SourceFile, SourceFileIndex>,
     hygiene_context: &'a HygieneEncodeContext,
+    symbol_table: FxHashMap<Symbol, usize>,
 }
 
 impl<'a, 'tcx> CacheEncoder<'a, 'tcx> {
@@ -899,6 +936,25 @@ impl<'a, 'tcx> Encodable<CacheEncoder<'a, 'tcx>> for Span {
     }
 }
 
+// copy&paste impl from rustc_metadata
+impl<'a, 'tcx> Encodable<CacheEncoder<'a, 'tcx>> for Symbol {
+    fn encode(&self, s: &mut CacheEncoder<'a, 'tcx>) {
+        match s.symbol_table.entry(*self) {
+            Entry::Vacant(o) => {
+                s.encoder.emit_u8(SYMBOL_STR);
+                let pos = s.encoder.position();
+                o.insert(pos);
+                s.emit_str(self.as_str());
+            }
+            Entry::Occupied(o) => {
+                let x = o.get().clone();
+                s.emit_u8(SYMBOL_OFFSET);
+                s.emit_usize(x);
+            }
+        }
+    }
+}
+
 impl<'a, 'tcx> TyEncoder for CacheEncoder<'a, 'tcx> {
     type I = TyCtxt<'tcx>;
     const CLEAR_CROSS_CRATE: bool = false;

compiler/rustc_span/src/symbol.rs

@@ -1852,14 +1852,14 @@ impl fmt::Display for Symbol {
 }
 
 impl<S: Encoder> Encodable<S> for Symbol {
-    fn encode(&self, s: &mut S) {
+    default fn encode(&self, s: &mut S) {
         s.emit_str(self.as_str());
     }
 }
 
 impl<D: Decoder> Decodable<D> for Symbol {
     #[inline]
-    fn decode(d: &mut D) -> Symbol {
+    default fn decode(d: &mut D) -> Symbol {
         Symbol::intern(&d.read_str())
     }
 }