discussion of nocapture

nikomatsakis · nikomatsakis · commit d0b9eb313ffa · 2016-02-24T11:19:40.000-05:00
diff --git a/optimizations/nocapture_by_safe_fn.md b/optimizations/nocapture_by_safe_fn.md
@@ -0,0 +1,42 @@
+### Example
+
+```rust
+fn escape_simple(x: &i32) -> usize {
+    22
+}
+
+fn escape_as_usize(x: &i32) -> usize {
+    x as usize
+}
+
+fn escape_as_ptr(x: &i32) -> *const i32 {
+    x
+}
+
+pub struct MyType(*const i32);
+
+fn escape_as_newtype(x: &i32) -> MyType {
+    MyType(x)
+}
+```
+
+### Explanation
+
+To be very LLVM specific, we'd like to add "nocapture" attributes.
+More generally, we'd like to be able to assume that if you have an
+argument of type `fn foo<'a>(x: &'a T) -> U`, and `'a` (nor any
+lifetime related to `'a`) does not appear in `U`, then we can assume
+that no alias of `x` escapes `foo`. However, it's not clear that we
+can always assume that.
+
+- `escape_simple` -- no capture would be appropriate
+- `escape_as_newtype` -- same types as previous example, but in this case
+  one might imagine that the user expects to coerce the result back to
+  a pointer and use it. Is that legal?
+- `escape_as_ptr` -- same as above, but they were more explicit in their types.
+- `escape_as_newtype` -- same as above, but there is a newtype wrapping this
+  `*const` -- maybe it forms a kind of abstraction boundary?
+  
+### Source
+
+IRC conversation: https://botbot.me/mozilla/rustc/2016-02-24/?msg=60823650&page=1
