Don't compress dylib metadata

Author: bjorn3

Cargo.lock

@@ -3367,7 +3367,6 @@ dependencies = [
  "rustc_type_ir",
  "serde_json",
  "smallvec",
- "snap",
  "tempfile",
  "thorin-dwp",
  "tracing",

compiler/rustc_codegen_ssa/Cargo.toml

@@ -17,7 +17,6 @@ tempfile = "3.2"
 thorin-dwp = "0.6"
 pathdiff = "0.2.0"
 serde_json = "1.0.59"
-snap = "1"
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 regex = "1.4"
 

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -10,8 +10,6 @@ use object::{
     ObjectSymbol, SectionFlags, SectionKind, SymbolFlags, SymbolKind, SymbolScope,
 };
 
-use snap::write::FrameEncoder;
-
 use rustc_data_structures::memmap::Mmap;
 use rustc_data_structures::owned_slice::{try_slice_owned, OwnedSlice};
 use rustc_metadata::fs::METADATA_FILENAME;
@@ -482,12 +480,8 @@ pub fn create_compressed_metadata_file(
     symbol_name: &str,
 ) -> Vec<u8> {
     let mut compressed = rustc_metadata::METADATA_HEADER.to_vec();
-    // Our length will be backfilled once we're done writing
-    compressed.write_all(&[0; 4]).unwrap();
-    FrameEncoder::new(&mut compressed).write_all(metadata.raw_data()).unwrap();
-    let meta_len = rustc_metadata::METADATA_HEADER.len();
-    let data_len = (compressed.len() - meta_len - 4) as u32;
-    compressed[meta_len..meta_len + 4].copy_from_slice(&data_len.to_be_bytes());
+    compressed.write_all(&(metadata.raw_data().len() as u32).to_be_bytes()).unwrap();
+    compressed.extend(metadata.raw_data());
 
     let Some(mut file) = create_object_file(sess) else {
         return compressed.to_vec();

compiler/rustc_metadata/src/locator.rs

@@ -806,19 +806,26 @@ fn get_metadata_section<'p>(
             let compressed_len = u32::from_be_bytes(len_bytes) as usize;
 
             // Header is okay -> inflate the actual metadata
-            let compressed_bytes = &buf[data_start..(data_start + compressed_len)];
-            debug!("inflating {} bytes of compressed metadata", compressed_bytes.len());
-            // Assume the decompressed data will be at least the size of the compressed data, so we
-            // don't have to grow the buffer as much.
-            let mut inflated = Vec::with_capacity(compressed_bytes.len());
-            FrameDecoder::new(compressed_bytes).read_to_end(&mut inflated).map_err(|_| {
-                MetadataError::LoadFailure(format!(
-                    "failed to decompress metadata: {}",
-                    filename.display()
-                ))
-            })?;
+            let compressed_bytes = buf.slice(|buf| &buf[data_start..(data_start + compressed_len)]);
+            if &compressed_bytes[..cmp::min(METADATA_HEADER.len(), compressed_bytes.len())]
+                == METADATA_HEADER
+            {
+                // The metadata was not actually compressed.
+                compressed_bytes
+            } else {
+                debug!("inflating {} bytes of compressed metadata", compressed_bytes.len());
+                // Assume the decompressed data will be at least the size of the compressed data, so we
+                // don't have to grow the buffer as much.
+                let mut inflated = Vec::with_capacity(compressed_bytes.len());
+                FrameDecoder::new(&*compressed_bytes).read_to_end(&mut inflated).map_err(|_| {
+                    MetadataError::LoadFailure(format!(
+                        "failed to decompress metadata: {}",
+                        filename.display()
+                    ))
+                })?;
 
-            slice_owned(inflated, Deref::deref)
+                slice_owned(inflated, Deref::deref)
+            }
         }
         CrateFlavor::Rmeta => {
             // mmap the file, because only a small fraction of it is read.