Rollup merge of #69792 - LenaWil:try_reserve_error/impl-error, r=sfackler

Implement Error for TryReserveError

I noticed that the Error trait wasn't implemented for TryReserveError. (#48043)

Not sure if the error messages and code style are 100% correct, it's my first time contributing to the Rust std.

src/liballoc/collections/mod.rs

@@ -42,6 +42,7 @@ pub use linked_list::LinkedList;
 pub use vec_deque::VecDeque;
 
 use crate::alloc::{Layout, LayoutErr};
+use core::fmt::Display;
 
 /// The error type for `try_reserve` methods.
 #[derive(Clone, PartialEq, Eq, Debug)]
@@ -77,6 +78,23 @@ impl From<LayoutErr> for TryReserveError {
     }
 }
 
+#[unstable(feature = "try_reserve", reason = "new API", issue = "48043")]
+impl Display for TryReserveError {
+    fn fmt(
+        &self,
+        fmt: &mut core::fmt::Formatter<'_>,
+    ) -> core::result::Result<(), core::fmt::Error> {
+        fmt.write_str("memory allocation failed")?;
+        let reason = match &self {
+            TryReserveError::CapacityOverflow => {
+                " because the computed capacity exceeded the collection's maximum"
+            }
+            TryReserveError::AllocError { .. } => " because the memory allocator returned a error",
+        };
+        fmt.write_str(reason)
+    }
+}
+
 /// An intermediate trait for specialization of `Extend`.
 #[doc(hidden)]
 trait SpecExtend<I: IntoIterator> {

src/libstd/error.rs

@@ -552,6 +552,9 @@ impl Error for char::ParseCharError {
     }
 }
 
+#[unstable(feature = "try_reserve", reason = "new API", issue = "48043")]
+impl Error for alloc::collections::TryReserveError {}
+
 // Copied from `any.rs`.
 impl dyn Error + 'static {
     /// Returns `true` if the boxed type is the same as `T`