Remove support for decompressing dylib metadata

We haven't been compressing dylib metadata for a while now. Removing
decompression support will regress error messages about an incompatible
rustc version being used, but dylibs are pretty rare anyway.

Author: bjorn3

Cargo.lock

@@ -4004,7 +4004,6 @@ dependencies = [
  "rustc_span",
  "rustc_target",
  "rustc_type_ir",
- "snap",
  "tempfile",
  "tracing",
 ]
@@ -4889,12 +4888,6 @@ version = "1.13.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3c5e1a9a646d36c3599cd173a41282daf47c44583ad367b8e6837255952e5c67"
 
-[[package]]
-name = "snap"
-version = "1.1.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1b6b67fb9a61334225b5b790716f609cd58395f895b3fe8b328786812a40bc3b"
-
 [[package]]
 name = "socket2"
 version = "0.5.7"

compiler/rustc_metadata/Cargo.toml

@@ -27,7 +27,6 @@ rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
 rustc_target = { path = "../rustc_target" }
 rustc_type_ir = { path = "../rustc_type_ir" }
-snap = "1"
 tempfile = "3.2"
 tracing = "0.1"
 # tidy-alphabetical-end

compiler/rustc_metadata/src/locator.rs

@@ -213,7 +213,7 @@
 //! metadata::locator or metadata::creader for all the juicy details!
 
 use std::borrow::Cow;
-use std::io::{Read, Result as IoResult, Write};
+use std::io::{Result as IoResult, Write};
 use std::ops::Deref;
 use std::path::{Path, PathBuf};
 use std::{cmp, fmt};
@@ -232,7 +232,6 @@ use rustc_session::utils::CanonicalizedPath;
 use rustc_span::Span;
 use rustc_span::symbol::Symbol;
 use rustc_target::spec::{Target, TargetTriple};
-use snap::read::FrameDecoder;
 use tracing::{debug, info};
 
 use crate::creader::{Library, MetadataLoader};
@@ -792,7 +791,6 @@ fn get_metadata_section<'p>(
         CrateFlavor::Dylib => {
             let buf =
                 loader.get_dylib_metadata(target, filename).map_err(MetadataError::LoadFailure)?;
-            // The header is uncompressed
             let header_len = METADATA_HEADER.len();
             // header + u64 length of data
             let data_start = header_len + 8;
@@ -806,37 +804,18 @@ fn get_metadata_section<'p>(
                 )));
             }
 
-            // Length of the compressed stream - this allows linkers to pad the section if they want
+            // Length of the metadata - this allows linkers to pad the section if they want
             let Ok(len_bytes) =
                 <[u8; 8]>::try_from(&buf[header_len..cmp::min(data_start, buf.len())])
             else {
                 return Err(MetadataError::LoadFailure(
                     "invalid metadata length found".to_string(),
                 ));
             };
-            let compressed_len = u64::from_le_bytes(len_bytes) as usize;
+            let metadata_len = u64::from_le_bytes(len_bytes) as usize;
 
             // Header is okay -> inflate the actual metadata
-            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)
-            }
+            buf.slice(|buf| &buf[data_start..(data_start + metadata_len)])
         }
         CrateFlavor::Rmeta => {
             // mmap the file, because only a small fraction of it is read.

src/tools/tidy/src/deps.rs

@@ -100,7 +100,6 @@ const EXCEPTIONS: ExceptionList = &[
     ("rustc_apfloat", "Apache-2.0 WITH LLVM-exception"),     // rustc (license is the same as LLVM uses)
     ("ryu", "Apache-2.0 OR BSL-1.0"), // BSL is not acceptble, but we use it under Apache-2.0                       // cargo/... (because of serde)
     ("self_cell", "Apache-2.0"),                             // rustc (fluent translations)
-    ("snap", "BSD-3-Clause"),                                // rustc
     ("wasi-preview1-component-adapter-provider", "Apache-2.0 WITH LLVM-exception"), // rustc
     // tidy-alphabetical-end
 ];
@@ -390,7 +389,6 @@ const PERMITTED_RUSTC_DEPENDENCIES: &[&str] = &[
     "sharded-slab",
     "shlex",
     "smallvec",
-    "snap",
     "stable_deref_trait",
     "stacker",
     "static_assertions",