add notes from 2016-09-15

lang-team/2016-09-15.md

@@ -0,0 +1,147 @@
+# impl Trait
+
+impl Iterator for Foo {
+    type Item = impl Iterator<Item=String>;
+    fn next(&mut self) -> Option<Self::Item> {...}
+}
+
+type FooIter = impl Iterator<Item = u32>;
+type FooIter2 = impl Iterator<Item = u32>;
+
+// Upstream crate: 
+
+fn foo() -> impl Iterator<Item = u32> {
+    ...
+}
+
+// Downstream crate:
+
+pub type MyFooIter = impl Iterator<Item = u32>;
+
+pub type MyFooIter: impl Iterator<Item = u32> = <BlahIter as Iterator>::next::Output;
+
+
+pub fn my_foo() -> MyFooIter {
+    foo()
+}
+
+pub struct HasFoo {
+    pub iter: MyFooIter
+}
+
+HasFoo { iter: foo() } // <- the pack!!!!
+
+
+trait SomeTrait {
+    type Assoc;
+    fn foo(&self) -> Assoc;
+    fn bar(&self) -> Assoc;
+}
+
+impl SomeTrait for Blah {
+    type Assoc = impl AnotherTrait;
+    fn foo(&self) -> 
+
+
+
+impl HasFoo {
+    pub type FooIter: Iterator<Item = u32> = <... witness ...>;
+    pub type FooIter: impl Iterator<Item = u32> = _;
+    pub type FooIter = impl Iterator<Item = u32>;  // witness inferred from function bodies
+    pub type FooIter: impl Iterator<Item = u32> = <... witness ...>;
+}
+
+HasFoo::FooIter
+
+trait Foo {
+    fn foo(&self) -> impl Iterator<Item = u32>; <- pack
+    fn bar(&self) -> impl Iterator<Item = u32>; <- pack, again
+}
+
+impl Foo {
+    fn foo(&self) -> Concrete { ... }
+    fn bar(&self) -> Concrete { ... }
+}
+
+# Action items
+
+- [x] advertise FCP for stabilisation issues
+- [ ] merge/close FCP'd RFCs
+- [ ] thread about private-in-public
+
+Default arguments etc
+
+fn foo(x: u32, y: u32 = x)
+
+
+
+
+- [x] B-RFC-accepted and B-unstable - talk to brson, triage - nrc
+- [ ] stabilise https://github.com/rust-lang/rust/issues/27245
+- [ ] FCP remaining nominated things in the way you expect
+
+```
+struct Bar<T: Iterator> {
+    x: Option<Box<Bar<T::Item>>>
+}
+```
+
+```
+unsafe impl<T: Iterator> Send for Bar<T>
+    where T::Item: Send
+{ }
+```
+
+```
+Bar<Baz> where Baz: Iterator<Item=Bar<Baz>>
+```
+
+
+struct Foo<X: Bar> { ... }
+
+impl<..> Foo<..> {
+}
+
+impl<'a, ..> Trait<'a> for Foo<..> { ... }
+
+trait Example {
+    fn foo<T>(&self, t: T)
+}
+
+impl Example for [u8] { ... }
+impl Example for SomeOtherTrait { ... }
+
+```
+trait Trait { fn method(&self, self: &Self) // <-- should not be invokable when Self == Trait }
+fn foo<T:?Sized + Trait>(t: &T) { t.method(t) } // 
+foo::<Trait>(...) // what prevents this?
+```
+
+- Key idea:
+    - Could we add `where Self: Sized` implicitly so as to make all traits (or most traits) object-safe, but disallow methods from being called?
+- Answer: 
+    - No, because backwards compat. There are traits like this one that are not object-safe but still usable with unsized self:
+
+```
+trait Trait { fn method(&self, self: &Self) // <-- should not be invokable when Self == Trait }
+fn foo<T:?Sized + Trait>(t: &T) { t.method(t) } // 
+```
+
+	- So if we added `Self: Sized` that would be bad
+	- If we tried to make some other criteria that is not `Self: Sized`, it would accept the fn `foo` above, but then what stops us from calling `foo::<Trait>(t: &Trait)` // since `Trait` is now object-safe?
+
+- [x] Close https://github.com/rust-lang/rfcs/pull/1180
+- [x] Merge https://github.com/rust-lang/rfcs/pull/1561
+- [x] Close https://github.com/rust-lang/rfcs/pull/1573
+- [x] Close https://github.com/rust-lang/rfcs/pull/1583
+- [x] Merge https://github.com/rust-lang/rfcs/pull/1623
+- [x] Merge https://github.com/rust-lang/rfcs/pull/1681 but checkin with acrichto about unresolved questions
+- [x] Close https://github.com/rust-lang/rfcs/pull/1694
+- [x] FCP to stabilize https://github.com/rust-lang/rust/issues/31436
+- [x] FCP to remove https://github.com/rust-lang/rust/issues/29721
+- [x] Calling for an RFC to do better -- https://github.com/rust-lang/rust/issues/28237
+- [x] FCP to stabilize statements https://github.com/rust-lang/rust/issues/15701
+      - seek feedback on expressions
+- [x] FCP https://github.com/rust-lang/rfcs/pull/1696
+- [x] FCP to deprecate the Reflect trait
+- [x] FCP to deprecate https://github.com/rust-lang/rust/issues/27749