diff --git a/lib/std/compress/flate/inflate.zig b/lib/std/compress/flate/inflate.zig
index ec8c3a408283..5e286d2af396 100644
--- a/lib/std/compress/flate/inflate.zig
+++ b/lib/std/compress/flate/inflate.zig
@@ -288,6 +288,14 @@ pub fn Inflate(comptime container: Container, comptime ReaderType: type) type {
             }
         }
 
+        /// Returns the number of bytes that have been read from the internal
+        /// reader but not yet consumed by the decompressor.
+        pub fn unreadBytes(self: Self) usize {
+            // There can be no error here: the denominator is not zero, and
+            // overflow is not possible since the type is unsigned.
+            return std.math.divCeil(usize, self.bits.nbits, 8) catch unreachable;
+        }
+
         // Iterator interface
 
         /// Can be used in iterator like loop without memcpy to another buffer:
diff --git a/src/Package/Fetch/git.zig b/src/Package/Fetch/git.zig
index abbb03194825..db50ddfab7c1 100644
--- a/src/Package/Fetch/git.zig
+++ b/src/Package/Fetch/git.zig
@@ -1091,6 +1091,86 @@ pub fn indexPack(allocator: Allocator, pack: std.fs.File, index_writer: anytype)
     try index_writer.writeAll(&index_checksum);
 }
 
+/// A reader that stores read data in a growable internal buffer. The read
+/// position can be rewound to allow previously read data to be read again.
+fn AccumulatingReader(comptime ReaderType: type) type {
+    return struct {
+        child_reader: ReaderType,
+        buffer: std.ArrayListUnmanaged(u8) = .{},
+        /// The position in `buffer` from which reads should start, returning
+        /// buffered data. If this is `buffer.items.len`, data will be read from
+        /// `child_reader` instead.
+        read_start: usize = 0,
+        allocator: Allocator,
+
+        const Self = @This();
+
+        fn deinit(self: *Self) void {
+            self.buffer.deinit(self.allocator);
+            self.* = undefined;
+        }
+
+        const ReadError = ReaderType.Error || Allocator.Error;
+        const Reader = std.io.Reader(*Self, ReadError, read);
+
+        fn read(self: *Self, buf: []u8) ReadError!usize {
+            if (self.read_start < self.buffer.items.len) {
+                // Previously buffered data is available and should be used
+                // before reading more data from the underlying reader.
+                const available = self.buffer.items.len - self.read_start;
+                const count = @min(available, buf.len);
+                @memcpy(buf[0..count], self.buffer.items[self.read_start..][0..count]);
+                self.read_start += count;
+                return count;
+            }
+
+            try self.buffer.ensureUnusedCapacity(self.allocator, buf.len);
+            const read_buffer = self.buffer.unusedCapacitySlice();
+            const count = try self.child_reader.read(read_buffer[0..buf.len]);
+            @memcpy(buf[0..count], read_buffer[0..count]);
+            self.buffer.items.len += count;
+            self.read_start += count;
+            return count;
+        }
+
+        fn reader(self: *Self) Reader {
+            return .{ .context = self };
+        }
+
+        /// Returns a slice of the buffered data that has already been read,
+        /// except the last `count_before_end` bytes.
+        fn readDataExcept(self: Self, count_before_end: usize) []const u8 {
+            assert(count_before_end <= self.read_start);
+            return self.buffer.items[0 .. self.read_start - count_before_end];
+        }
+
+        /// Discards the first `count` bytes of buffered data.
+        fn discard(self: *Self, count: usize) void {
+            assert(count <= self.buffer.items.len);
+            const retain = self.buffer.items.len - count;
+            mem.copyForwards(
+                u8,
+                self.buffer.items[0..retain],
+                self.buffer.items[count..][0..retain],
+            );
+            self.buffer.items.len = retain;
+            self.read_start -= @min(self.read_start, count);
+        }
+
+        /// Rewinds the read position to the beginning of buffered data.
+        fn rewind(self: *Self) void {
+            self.read_start = 0;
+        }
+    };
+}
+
+fn accumulatingReader(
+    allocator: Allocator,
+    reader: anytype,
+) AccumulatingReader(@TypeOf(reader)) {
+    return .{ .child_reader = reader, .allocator = allocator };
+}
+
 /// Performs the first pass over the packfile data for index construction.
 /// This will index all non-delta objects, queue delta objects for further
 /// processing, and return the pack checksum (which is part of the index
@@ -1102,59 +1182,106 @@ fn indexPackFirstPass(
     pending_deltas: *std.ArrayListUnmanaged(IndexEntry),
 ) ![Sha1.digest_length]u8 {
     var pack_buffered_reader = std.io.bufferedReader(pack.reader());
-    var pack_counting_reader = std.io.countingReader(pack_buffered_reader.reader());
-    var pack_hashed_reader = std.compress.hashedReader(pack_counting_reader.reader(), Sha1.init(.{}));
-    const pack_reader = pack_hashed_reader.reader();
+    var pack_accumulating_reader = accumulatingReader(allocator, pack_buffered_reader.reader());
+    defer pack_accumulating_reader.deinit();
+    var pack_position: usize = 0;
+    var pack_hash = Sha1.init(.{});
+    const pack_reader = pack_accumulating_reader.reader();
 
     const pack_header = try PackHeader.read(pack_reader);
+    const pack_header_bytes = pack_accumulating_reader.readDataExcept(0);
+    pack_position += pack_header_bytes.len;
+    pack_hash.update(pack_header_bytes);
+    pack_accumulating_reader.discard(pack_header_bytes.len);
 
     var current_entry: u32 = 0;
     while (current_entry < pack_header.total_objects) : (current_entry += 1) {
-        const entry_offset = pack_counting_reader.bytes_read;
-        var entry_crc32_reader = std.compress.hashedReader(pack_reader, std.hash.Crc32.init());
-        const entry_header = try EntryHeader.read(entry_crc32_reader.reader());
+        const entry_offset = pack_position;
+        var entry_crc32 = std.hash.Crc32.init();
+
+        const entry_header = try EntryHeader.read(pack_reader);
+        const entry_header_bytes = pack_accumulating_reader.readDataExcept(0);
+        pack_position += entry_header_bytes.len;
+        pack_hash.update(entry_header_bytes);
+        entry_crc32.update(entry_header_bytes);
+        pack_accumulating_reader.discard(entry_header_bytes.len);
+
         switch (entry_header) {
-            inline .commit, .tree, .blob, .tag => |object, tag| {
-                var entry_decompress_stream = std.compress.zlib.decompressor(entry_crc32_reader.reader());
-                var entry_counting_reader = std.io.countingReader(entry_decompress_stream.reader());
+            .commit, .tree, .blob, .tag => |object| {
+                var entry_decompress_stream = std.compress.zlib.decompressor(pack_reader);
+                var entry_data_size: usize = 0;
                 var entry_hashed_writer = hashedWriter(std.io.null_writer, Sha1.init(.{}));
                 const entry_writer = entry_hashed_writer.writer();
+
                 // The object header is not included in the pack data but is
                 // part of the object's ID
-                try entry_writer.print("{s} {}\x00", .{ @tagName(tag), object.uncompressed_length });
-                var fifo = std.fifo.LinearFifo(u8, .{ .Static = 4096 }).init();
-                try fifo.pump(entry_counting_reader.reader(), entry_writer);
-                if (entry_counting_reader.bytes_read != object.uncompressed_length) {
+                try entry_writer.print("{s} {}\x00", .{ @tagName(entry_header), object.uncompressed_length });
+
+                while (try entry_decompress_stream.next()) |decompressed_data| {
+                    entry_data_size += decompressed_data.len;
+                    try entry_writer.writeAll(decompressed_data);
+
+                    const compressed_bytes = pack_accumulating_reader.readDataExcept(entry_decompress_stream.unreadBytes());
+                    pack_position += compressed_bytes.len;
+                    pack_hash.update(compressed_bytes);
+                    entry_crc32.update(compressed_bytes);
+                    pack_accumulating_reader.discard(compressed_bytes.len);
+                }
+                const footer_bytes = pack_accumulating_reader.readDataExcept(entry_decompress_stream.unreadBytes());
+                pack_position += footer_bytes.len;
+                pack_hash.update(footer_bytes);
+                entry_crc32.update(footer_bytes);
+                pack_accumulating_reader.discard(footer_bytes.len);
+                pack_accumulating_reader.rewind();
+
+                if (entry_data_size != object.uncompressed_length) {
                     return error.InvalidObject;
                 }
+
                 const oid = entry_hashed_writer.hasher.finalResult();
                 try index_entries.put(allocator, oid, .{
                     .offset = entry_offset,
-                    .crc32 = entry_crc32_reader.hasher.final(),
+                    .crc32 = entry_crc32.final(),
                 });
             },
             inline .ofs_delta, .ref_delta => |delta| {
-                var entry_decompress_stream = std.compress.zlib.decompressor(entry_crc32_reader.reader());
-                var entry_counting_reader = std.io.countingReader(entry_decompress_stream.reader());
-                var fifo = std.fifo.LinearFifo(u8, .{ .Static = 4096 }).init();
-                try fifo.pump(entry_counting_reader.reader(), std.io.null_writer);
-                if (entry_counting_reader.bytes_read != delta.uncompressed_length) {
+                var entry_decompress_stream = std.compress.zlib.decompressor(pack_reader);
+                var entry_data_size: usize = 0;
+
+                while (try entry_decompress_stream.next()) |decompressed_data| {
+                    entry_data_size += decompressed_data.len;
+
+                    const compressed_bytes = pack_accumulating_reader.readDataExcept(entry_decompress_stream.unreadBytes());
+                    pack_position += compressed_bytes.len;
+                    pack_hash.update(compressed_bytes);
+                    entry_crc32.update(compressed_bytes);
+                    pack_accumulating_reader.discard(compressed_bytes.len);
+                }
+                const footer_bytes = pack_accumulating_reader.readDataExcept(entry_decompress_stream.unreadBytes());
+                pack_position += footer_bytes.len;
+                pack_hash.update(footer_bytes);
+                entry_crc32.update(footer_bytes);
+                pack_accumulating_reader.discard(footer_bytes.len);
+                pack_accumulating_reader.rewind();
+
+                if (entry_data_size != delta.uncompressed_length) {
                     return error.InvalidObject;
                 }
+
                 try pending_deltas.append(allocator, .{
                     .offset = entry_offset,
-                    .crc32 = entry_crc32_reader.hasher.final(),
+                    .crc32 = entry_crc32.final(),
                 });
             },
         }
     }
 
-    const pack_checksum = pack_hashed_reader.hasher.finalResult();
-    const recorded_checksum = try pack_buffered_reader.reader().readBytesNoEof(Sha1.digest_length);
+    const pack_checksum = pack_hash.finalResult();
+    const recorded_checksum = try pack_reader.readBytesNoEof(Sha1.digest_length);
     if (!mem.eql(u8, &pack_checksum, &recorded_checksum)) {
         return error.CorruptedPack;
     }
-    _ = pack_buffered_reader.reader().readByte() catch |e| switch (e) {
+    _ = pack_reader.readByte() catch |e| switch (e) {
         error.EndOfStream => return pack_checksum,
         else => |other| return other,
     };
@@ -1385,7 +1512,11 @@ test "packfile indexing and checkout" {
     defer worktree.cleanup();
 
     const commit_id = try parseOid("dd582c0720819ab7130b103635bd7271b9fd4feb");
-    try repository.checkout(worktree.dir, commit_id);
+
+    var diagnostics: Diagnostics = .{ .allocator = testing.allocator };
+    defer diagnostics.deinit();
+    try repository.checkout(worktree.dir, commit_id, &diagnostics);
+    try testing.expect(diagnostics.errors.items.len == 0);
 
     const expected_files: []const []const u8 = &.{
         "dir/file",