From b18fee64ddfd717e778d0bf44bfd2aaa3d0dee87 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/meta-stats.rs                 |  6 ++++++
 tests/ui/meta-stats.stderr             | 30 ++++++++++++++++++++++++++
 3 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/meta-stats.rs
 create mode 100644 tests/ui/meta-stats.stderr

diff --git a/compiler/rustc_serialize/src/opaque.rs b/compiler/rustc_serialize/src/opaque.rs
index 44855ae629c13..86bb542ef170a 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
+        // 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/meta-stats.rs b/tests/ui/meta-stats.rs
new file mode 100644
index 0000000000000..7fd3b7b3c63bc
--- /dev/null
+++ b/tests/ui/meta-stats.rs
@@ -0,0 +1,6 @@
+// build-pass
+// compile-flags: -Zmeta-stats
+
+#![crate_type = "lib"]
+
+pub fn a() {}
diff --git a/tests/ui/meta-stats.stderr b/tests/ui/meta-stats.stderr
new file mode 100644
index 0000000000000..643920645dfe0
--- /dev/null
+++ b/tests/ui/meta-stats.stderr
@@ -0,0 +1,30 @@
+meta-stats METADATA STATS
+meta-stats Section                      Size
+meta-stats ----------------------------------------------------------------
+meta-stats lib-features                    0 ( 0.0%)
+meta-stats stability-implications          0 ( 0.0%)
+meta-stats lang-items                      0 ( 0.0%)
+meta-stats stripped-cfg-items              0 ( 0.0%)
+meta-stats diagnostic-items                0 ( 0.0%)
+meta-stats native-libs                     0 ( 0.0%)
+meta-stats foreign-modules                 0 ( 0.0%)
+meta-stats traits                          0 ( 0.0%)
+meta-stats impls                           0 ( 0.0%)
+meta-stats incoherent-impls                0 ( 0.0%)
+meta-stats mir                             0 ( 0.0%)
+meta-stats interpret-alloc-index           0 ( 0.0%)
+meta-stats proc-macro-data                 0 ( 0.0%)
+meta-stats debugger-visualizers            0 ( 0.0%)
+meta-stats exported-symbols                6 ( 0.3%)
+meta-stats def-path-table                 22 ( 1.0%)
+meta-stats preamble                       30 ( 1.3%)
+meta-stats source-map                    105 ( 4.7%)
+meta-stats hygiene                       111 ( 5.0%)
+meta-stats def-ids                       153 ( 6.9%)
+meta-stats tables                        192 ( 8.6%)
+meta-stats final                         317 (14.2%)
+meta-stats def-path-hash-map             386 (17.3%)
+meta-stats dep                           909 (40.7%)
+meta-stats ----------------------------------------------------------------
+meta-stats Total                       2_231 (of which 33.2% are zero bytes)
+meta-stats