Open the FileEncoder file for reading and writing

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(()),
         })
     }

src/tools/tidy/src/ui_tests.rs

@@ -11,7 +11,7 @@ use std::path::{Path, PathBuf};
 const ENTRY_LIMIT: usize = 900;
 // FIXME: The following limits should be reduced eventually.
 const ISSUES_ENTRY_LIMIT: usize = 1854;
-const ROOT_ENTRY_LIMIT: usize = 865;
+const ROOT_ENTRY_LIMIT: usize = 867;
 
 const EXPECTED_TEST_FILE_EXTENSIONS: &[&str] = &[
     "rs",     // test source files

tests/ui/meta-stats.rs

@@ -0,0 +1,7 @@
+// build-pass
+// dont-check-compiler-stderr
+// compile-flags: -Zmeta-stats
+
+#![crate_type = "lib"]
+
+pub fn a() {}