From 09960e03194626738b3533b6c2d06a0c505117d3 Mon Sep 17 00:00:00 2001
From: Ben Kimock <kimockb@gmail.com>
Date: Fri, 22 Sep 2023 12:34:30 -0400
Subject: [PATCH] Open the FileEncoder file for reading and writing

---
 compiler/rustc_serialize/src/opaque.rs | 7 ++++++-
 tests/ui/stats/meta-stats.rs           | 7 +++++++
 2 files changed, 13 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/stats/meta-stats.rs

diff --git a/compiler/rustc_serialize/src/opaque.rs b/compiler/rustc_serialize/src/opaque.rs
index 44855ae629c..55255439051 100644
--- a/compiler/rustc_serialize/src/opaque.rs
+++ b/compiler/rustc_serialize/src/opaque.rs
@@ -38,11 +38,16 @@ pub struct FileEncoder {
 
 impl FileEncoder {
     pub fn new<P: AsRef<Path>>(path: P) -> io::Result<Self> {
+        // File::create opens the file for writing only. When -Zmeta-stats is enabled, the metadata
+        // encoder rewinds the file to inspect what was written. So we need to always open the file
+        // for reading and writing.
+        let file = File::options().read(true).write(true).create(true).truncate(true).open(path)?;
+
         Ok(FileEncoder {
             buf: vec![0u8; BUF_SIZE].into_boxed_slice().try_into().unwrap(),
             buffered: 0,
             flushed: 0,
-            file: File::create(path)?,
+            file,
             res: Ok(()),
         })
     }
diff --git a/tests/ui/stats/meta-stats.rs b/tests/ui/stats/meta-stats.rs
new file mode 100644
index 00000000000..2d38e088286
--- /dev/null
+++ b/tests/ui/stats/meta-stats.rs
@@ -0,0 +1,7 @@
+// build-pass
+// dont-check-compiler-stderr
+// compile-flags: -Zmeta-stats
+
+#![crate_type = "lib"]
+
+pub fn a() {}