Allow panicking in constants

Author: oli-obk

text/0000-const-panic.md

@@ -0,0 +1,95 @@
+- Feature Name: const_panic
+- Start Date: 2018-02-22
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Allow the use of `panic!`, `assert!` and `assert_eq!` within constants and
+report their evaluation as a compile-time error.
+
+# Motivation
+[motivation]: #motivation
+
+It can often be desirable to terminate a constant evaluation due to invalid
+arguments. Currently there's no way to do this other than to use `Result` to
+produce an `Err` in case of errors. Unfortunately this will end up as a runtime
+problem and not abort compilation, even though the problem has been detected at
+compile-time. There are already ways to abort compilation, e.g. by invoking
+`["some assert failed"][42]` within a constant, which will abort with a
+compile-time error pointing at the span of the index operation. But this hack is
+not very convenient to use and produces the wrong error message.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+You can now use `panic!` and `assert!` within `const fn`s. This means that when
+the const fn is invoked at runtime, you will get a regular panic, but if it is
+invoked at compile-time, the panic message will show up as an error message.
+
+As an example, imagine a function that converts strings to their corresponding
+booleans.
+
+```rust
+const fn parse_bool(s: &str) -> bool {
+    match s {
+        "true" => true,
+        "false" => false,
+        other => panic!("`{}` is not a valid bool"),
+    }
+}
+parse_bool("true");
+parse_bool("false");
+parse_bool("foo");
+```
+
+will produce an error with your custom error message:
+
+```
+error[E0080]: `foo` is not a valid bool
+ --> src/main.rs: 5:25
+  |
+5 |        other => panic!("`{}` is not a valid bool"),
+  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: during the evaluation of
+   |
+10 | parse_bool("foo");
+   | ^^^^^^^^^^^^^^^^^
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+MIR interpretation gets a special case for the panic machinery (which isn't
+const fn). If the `panic` lang item is entered, instead of producing an error
+about it not being const fn, we produce a specialized error with the panic's
+message. This panic reporting machinery is already present in the mir
+interpreter, but needs the lang item detection in order to work.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+We have to implement some magic around processing `fmt::Arguments` objects and
+producing the panic message from that.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+* We could add a special constant error reporting mechanism. This has the
+  disadvantage of widening the gap between const eval and runtime execution.
+* We could make `String` and formatting const enough to allow the panic
+  formatting machinery to be interpreted and made const fn
+* Don't produce a good error message, just say "const eval encountered an error"
+  and point the user to the panic location. This already works out of the box
+  right now. We can improve the error message in the future with the `String` +
+  formatting alternative. This is the most minimalistic alternative to this RFC
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+* Should there be some additional message in the error about this being a panic
+  turned error? Or do we just produce the exact message the panic would produce?
+
+* This change becomes really useful if `Result::unwrap` and `Option::unwrap`
+  become const fn, doing both in one go might be a good idea