Back io::stdin with a global singleton BufferedReader

io::stdin returns a new `BufferedReader` each time it's called, which
results in some very confusing behavior with disappearing output. It now
returns a `StdinReader`, which wraps a global singleton
`Arc<Mutex<BufferedReader<StdReader>>`. `Reader` is implemented directly
on `StdinReader`. However, `Buffer` is not, as the `fill_buf` method is
fundamentaly un-thread safe. A `lock` method is defined on `StdinReader`
which returns a smart pointer wrapping the underlying `BufferedReader`
while guaranteeing mutual exclusion.

Code that treats the return value of io::stdin as implementing `Buffer`
will break. Add a call to `lock`:

```rust
io::stdin().lines()
// =>
io::stdin().lock().lines()
```

Closes #14434

[breaking-change]

2 files changed, 3 insertions, 3 deletions

diff --git a/src/test/bench/shootout-k-nucleotide.rs b/src/test/bench/shootout-k-nucleotide.rs
index b030e7bb93e..8ed041513c4 100644
--- a/src/test/bench/shootout-k-nucleotide.rs
+++ b/src/test/bench/shootout-k-nucleotide.rs
@@ -295,7 +295,7 @@ fn main() {
         let fd = std::io::File::open(&Path::new("shootout-k-nucleotide.data"));
         get_sequence(&mut std::io::BufferedReader::new(fd), ">THREE")
     } else {
-        get_sequence(&mut std::io::stdin(), ">THREE")
+        get_sequence(&mut *std::io::stdin().lock(), ">THREE")
     };
     let input = Arc::new(input);
 
diff --git a/src/test/bench/sudoku.rs b/src/test/bench/sudoku.rs
index 6664eeecd5d..c55f85f40e8 100644
--- a/src/test/bench/sudoku.rs
+++ b/src/test/bench/sudoku.rs
@@ -65,7 +65,7 @@ impl Sudoku {
         return true;
     }
 
-    pub fn read(mut reader: BufferedReader<StdReader>) -> Sudoku {
+    pub fn read(mut reader: &mut BufferedReader<StdReader>) -> Sudoku {
         /* assert first line is exactly "9,9" */
         assert!(reader.read_line().unwrap() == "9,9".to_string());
 
@@ -284,7 +284,7 @@ fn main() {
     let mut sudoku = if use_default {
         Sudoku::from_vec(&DEFAULT_SUDOKU)
     } else {
-        Sudoku::read(io::stdin())
+        Sudoku::read(&mut *io::stdin().lock())
     };
     sudoku.solve();
     sudoku.write(&mut io::stdout());