Quick draft "Result::expect" rfc

Author: thepowersgang

text/0000-result-expect.md

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+- Feature Name: `result_expect`
+- Start Date: 2015-05-13
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Add an `expect` method to the Result type, bounded to `E: Debug`
+
+# Motivation
+
+While `Result::unwrap` exists, it does not allow annotating the panic message with the operation
+attempted (e.g. what file was being opened). This is at odds to 'Option' which includes both
+`unwrap` and `expect` (with the latter taking an arbitrary failure message).
+
+# Detailed design
+
+Add a new method to the same `impl` block as `Result::unwrap` that takes a `&str` message and
+returns `T` if the `Result` was `Ok`. If the `Result` was `Err`, it panics with both the provided
+message and the error value.
+
+The format of the error message is left undefined in the documentation, but will most likely be
+the following
+```
+panic!("{}: {:?}", msg, e)
+```
+
+# Drawbacks
+
+- It involves adding a new method to a core rust type.
+- The panic message format is less obvious than it is with `Option::expect` (where the panic message is the message passed)
+
+# Alternatives
+
+- We are perfectly free to not do this.
+- A macro could be introduced to fill the same role (which would allow arbitrary formatting of the panic message).
+
+# Unresolved questions
+
+Are there any issues with the proposed format of the panic string?