RFC: Better temporary lifetimes (so e.g. .as_slice() works)

Author: lilyball

active/0000-better-temporary-lifetimes.md

@@ -0,0 +1,62 @@
+- Start Date: 2014-05-04
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+# Summary
+
+Temporaries live for the enclosing block when found in a let-binding. This only
+holds when the reference to the temporary is taken directly. This logic should
+be extended to extend the cleanup scope of any temporary whose lifetime ends up
+in the let-binding.
+
+For example, the following doesn't work now, but should:
+
+```rust
+use std::os;
+
+fn main() {
+	let x = os::args().slice_from(1);
+	println!("{}", x);
+}
+```
+
+# Motivation
+
+Temporary lifetimes are a bit confusing right now. Sometimes you can keep
+references to them, and sometimes you get the dreaded "borrowed value does not
+live long enough" error. Sometimes one operation works but an equivalent
+operation errors, e.g. autoref of `~[T]` to `&[T]` works but calling
+`.as_slice()` doesn't. In general it feels as though the compiler is simply
+being overly restrictive when it decides the temporary doesn't live long
+enough.
+
+# Drawbacks
+
+I can't think of any drawbacks.
+
+# Detailed design
+
+When a reference to a temporary is passed to a function (either as a regular
+argument or as the `self` argument of a method), and the function returns a
+value with the same lifetime as the temporary reference, the lifetime of the
+temporary should be extended the same way it would if the function was not
+invoked.
+
+For example, `~[T].as_slice()` takes `&'a self` and returns `&'a [T]`. Calling
+`as_slice()` on a temporary of type `~[T]` will implicitly take a reference
+`&'a ~[T]` and return a value `&'a [T]` This return value should be considered
+to extend the lifetime of the `~[T]` temporary just as taking an explicit
+reference (and skipping the method call) would.
+
+# Alternatives
+
+Don't do this. We live with the surprising borrowck errors and the ugly workarounds that look like
+
+```rust
+let x = os::args();
+let x = x.slice_from(1);
+```
+
+# Unresolved questions
+
+None that I know of.