1 files changed, 46 insertions, 0 deletions
diff --git a/third_party/rust/fallible_collections/src/format.rs b/third_party/rust/fallible_collections/src/format.rs
new file mode 100644
index 0000000000..47a5fb9082
--- /dev/null
+++ b/third_party/rust/fallible_collections/src/format.rs
@@ -0,0 +1,46 @@
+//! A try_format! macro replacing format!
+use super::FallibleVec;
+use crate::TryReserveError;
+use alloc::fmt::{Arguments, Write};
+use alloc::string::String;
+
+/// Take a max capacity a try allocating a string with it.
+///
+/// # Warning:
+///
+/// the max capacity must be > to the formating of the
+/// arguments. If writing the argument on the string exceed the
+/// capacity, no error is return and an allocation can occurs which
+/// can lead to a panic
+pub fn try_format(max_capacity: usize, args: Arguments<'_>) -> Result<String, TryReserveError> {
+    let v = FallibleVec::try_with_capacity(max_capacity)?;
+    let mut s = String::from_utf8(v).expect("wtf an empty vec should be valid utf8");
+    s.write_fmt(args)
+        .expect("a formatting trait implementation returned an error");
+    Ok(s)
+}
+
+#[macro_export]
+/// Take a max capacity a try allocating a string with it.
+///
+/// # Warning:
+///
+/// the max capacity must be > to the formating of the
+/// arguments. If writing the argument on the string exceed the
+/// capacity, no error is return and an allocation can occurs which
+/// can lead to a panic
+macro_rules! tryformat {
+    ($max_capacity:tt, $($arg:tt)*) => (
+        $crate::format::try_format($max_capacity, format_args!($($arg)*))
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn format() {
+        assert_eq!(tryformat!(1, "1").unwrap(), format!("1"));
+        assert_eq!(tryformat!(1, "{}", 1).unwrap(), format!("{}", 1));
+        assert_eq!(tryformat!(3, "{}", 123).unwrap(), format!("{}", 123));
+    }
+}