Auto merge of #101550 - CraftSpider:link-dead-windows, r=wesleywiser

Make compressed rmeta contain compressed data length after header

Fixes #90056, which is caused by link.exe introducing padding to the `.rustc` section, since it assumes this will have no effect besides allowing it to possibly use the extra space in future links.

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -306,7 +306,13 @@ 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());
+
     let Some(mut file) = create_object_file(sess) else {
         return compressed.to_vec();
     };

compiler/rustc_metadata/src/locator.rs

@@ -789,6 +789,9 @@ fn get_metadata_section<'p>(
                 loader.get_dylib_metadata(target, filename).map_err(MetadataError::LoadFailure)?;
             // The header is uncompressed
             let header_len = METADATA_HEADER.len();
+            // header + u32 length of data
+            let data_start = header_len + 4;
+
             debug!("checking {} bytes of metadata-version stamp", header_len);
             let header = &buf[..cmp::min(header_len, buf.len())];
             if header != METADATA_HEADER {
@@ -798,8 +801,14 @@ fn get_metadata_section<'p>(
                 )));
             }
 
+            // Length of the compressed stream - this allows linkers to pad the section if they want
+            let Ok(len_bytes) = <[u8; 4]>::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 = u32::from_be_bytes(len_bytes) as usize;
+
             // Header is okay -> inflate the actual metadata
-            let compressed_bytes = &buf[header_len..];
+            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.

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -55,13 +55,13 @@ pub(crate) fn rustc_version() -> String {
 /// Metadata encoding version.
 /// N.B., increment this if you change the format of metadata such that
 /// the rustc version can't be found to compare with `rustc_version()`.
-const METADATA_VERSION: u8 = 6;
+const METADATA_VERSION: u8 = 7;
 
 /// Metadata header which includes `METADATA_VERSION`.
 ///
-/// This header is followed by the position of the `CrateRoot`,
-/// which is encoded as a 32-bit big-endian unsigned integer,
-/// and further followed by the rustc version string.
+/// This header is followed by the length of the compressed data, then
+/// the position of the `CrateRoot`, which is encoded as a 32-bit big-endian
+/// unsigned integer, and further followed by the rustc version string.
 pub const METADATA_HEADER: &[u8] = &[b'r', b'u', b's', b't', 0, 0, 0, METADATA_VERSION];
 
 /// A value of type T referred to by its absolute position