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io::Write::write_fmt: panic if the formatter fails when the stream does not fail
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@ -630,7 +630,9 @@ pub fn format(args: Arguments<'_>) -> string::String {
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fn format_inner(args: Arguments<'_>) -> string::String {
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let capacity = args.estimated_capacity();
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let mut output = string::String::with_capacity(capacity);
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output.write_fmt(args).expect("a formatting trait implementation returned an error");
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output
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.write_fmt(args)
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.expect("a formatting trait implementation returned an error when the underlying stream did not");
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output
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}
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@ -1839,7 +1839,11 @@ pub trait Write {
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if output.error.is_err() {
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output.error
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} else {
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Err(error::const_io_error!(ErrorKind::Uncategorized, "formatter error"))
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// This shouldn't happen: the underlying stream did not error, but somehow
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// the formatter still errored?
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panic!(
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"a formatting trait implementation returned an error when the underlying stream did not"
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);
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}
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}
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}
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@ -1,9 +1,11 @@
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//@ run-pass
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//@ needs-unwind
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#![feature(io_error_uncategorized)]
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use std::fmt;
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use std::io::{self, Error, Write, sink};
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use std::panic::catch_unwind;
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struct ErrorDisplay;
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@ -15,7 +17,6 @@ impl fmt::Display for ErrorDisplay {
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struct ErrorWriter;
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const FORMAT_ERROR: io::ErrorKind = io::ErrorKind::Uncategorized;
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const WRITER_ERROR: io::ErrorKind = io::ErrorKind::NotConnected;
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impl Write for ErrorWriter {
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@ -27,22 +28,28 @@ impl Write for ErrorWriter {
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}
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fn main() {
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// Test that the error from the formatter is propagated.
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let res = write!(sink(), "{} {} {}", 1, ErrorDisplay, "bar");
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assert!(res.is_err(), "formatter error did not propagate");
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assert_eq!(res.unwrap_err().kind(), FORMAT_ERROR);
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// Test that an underlying error is propagated
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let res = write!(ErrorWriter, "abc");
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assert!(res.is_err(), "writer error did not propagate");
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// Writer error
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// Test that the error from the formatter is detected.
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let res = catch_unwind(|| write!(sink(), "{} {} {}", 1, ErrorDisplay, "bar"));
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let err = res.expect_err("formatter error did not lead to panic").downcast::<&str>().unwrap();
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assert!(
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err.contains("formatting trait implementation returned an error"),
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"unexpected panic: {}", err
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);
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// Writer error when there's some string before the first `{}`
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let res = write!(ErrorWriter, "abc {}", ErrorDisplay);
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assert!(res.is_err(), "writer error did not propagate");
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assert_eq!(res.unwrap_err().kind(), WRITER_ERROR);
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// Formatter error
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let res = write!(ErrorWriter, "{} abc", ErrorDisplay);
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assert!(res.is_err(), "formatter error did not propagate");
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assert_eq!(res.unwrap_err().kind(), FORMAT_ERROR);
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// Formatter error when the `{}` comes first
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let res = catch_unwind(|| write!(ErrorWriter, "{} abc", ErrorDisplay));
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let err = res.expect_err("formatter error did not lead to panic").downcast::<&str>().unwrap();
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assert!(
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err.contains("formatting trait implementation returned an error"),
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"unexpected panic: {}", err
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);
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}
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