Apply suggestions from code review

Co-authored-by: Waffle Maybe <waffle.lapkin@gmail.com>

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -162,7 +162,7 @@ pub(super) fn get_metadata_xcoff<'a>(path: &Path, data: &'a [u8]) -> Result<&'a
             return Err(format!("Invalid metadata symbol offset: {offset}"));
         }
         // The offset specifies the location of rustc metadata in the comment section.
-        // The metadata is preceded by a 4-byte length field.
+        // The metadata is preceded by a 8-byte length field.
         let len = u64::from_le_bytes(info_data[(offset - 8)..offset].try_into().unwrap()) as usize;
         if offset + len > (info_data.len() as usize) {
             return Err(format!(

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -705,9 +705,8 @@ impl MetadataBlob {
     }
 
     fn root_pos(&self) -> NonZeroUsize {
-        let slice = &self.blob()[..];
         let offset = METADATA_HEADER.len();
-        let pos_bytes = slice[offset..][..8].try_into().unwrap();
+        let pos_bytes = self.blob()[offset..][..8].try_into().unwrap();
         let pos = u64::from_le_bytes(pos_bytes);
         NonZeroUsize::new(pos as usize).unwrap()
     }

src/tools/rust-analyzer/crates/proc-macro-api/src/version.rs

@@ -85,8 +85,8 @@ fn read_section<'a>(dylib_binary: &'a [u8], section_name: &str) -> io::Result<&'
 }
 
 /// Check the version of rustc that was used to compile a proc macro crate's
-///
 /// binary file.
+///
 /// A proc macro crate binary's ".rustc" section has following byte layout:
 /// * [b'r',b'u',b's',b't',0,0,0,5] is the first 8 bytes
 /// * ff060000 734e6150 is followed, it's the snappy format magic bytes,
@@ -96,8 +96,8 @@ fn read_section<'a>(dylib_binary: &'a [u8], section_name: &str) -> io::Result<&'
 /// The bytes you get after decompressing the snappy format portion has
 /// following layout:
 /// * [b'r',b'u',b's',b't',0,0,0,5] is the first 8 bytes(again)
-/// * [crate root bytes] next 4 bytes is to store crate root position,
+/// * [crate root bytes] next 8 bytes (4 in old versions) is to store
-///   according to rustc's source code comment
+///   crate root position, according to rustc's source code comment
 /// * [length byte] next 1 byte tells us how many bytes we should read next
 ///   for the version string's utf8 bytes
 /// * [version string bytes encoded in utf8] <- GET THIS BOI
@@ -119,7 +119,7 @@ pub fn read_version(dylib_path: &AbsPath) -> io::Result<String> {
     }
     let version = u32::from_be_bytes([dot_rustc[4], dot_rustc[5], dot_rustc[6], dot_rustc[7]]);
     // Last supported version is:
-    // https://github.com/rust-lang/rust/commit/0696e79f2740ad89309269b460579e548a5cd632
+    // https://github.com/rust-lang/rust/commit/b94cfefc860715fb2adf72a6955423d384c69318
     let (snappy_portion, bytes_before_version) = match version {
         5 | 6 => (&dot_rustc[8..], 13),
         7 | 8 => {
@@ -153,9 +153,9 @@ pub fn read_version(dylib_path: &AbsPath) -> io::Result<String> {
     // 1 byte for length of version string
     // so 13 or 17 bytes in total, and we should check the last of those bytes
     // to know the length
-    let mut bytes_before_version = vec![0u8; bytes_before_version];
+    let mut bytes = [0u8; 17];
-    uncompressed.read_exact(&mut bytes_before_version)?;
+    uncompressed.read_exact(&mut bytes[..bytes_before_version])?;
-    let length = *bytes_before_version.last().unwrap();
+    let length = bytes[bytes_before_version - 1];
 
     let mut version_string_utf8 = vec![0u8; length as usize];
     uncompressed.read_exact(&mut version_string_utf8)?;