Rewrite for loop desugaring to use language items

text/0000-for-loop-lang-items.md

@@ -0,0 +1,87 @@
+- Feature Name: for_loop_lang_items
+- Start Date: 2016-01-10
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Rewrite the `for` loop desugaring to use language items instead of hardcoded paths.
+
+# Motivation
+[motivation]: #motivation
+
+As noted in issue [#30803], pull request [#20790] changed the `for` loop desugaring to use the
+`IntoIterator` trait, removing the `iterator` language item. As a result, `for` loops are the only
+syntax that involves desugaring based on hardcoded paths to items instead of language items. In
+particular, the desugaring references `core::iter::IntoIterator`, `core::iter::Iterator`, and
+`core::option::Option`.
+
+[#20790]: https://github.com/rust-lang/rust/pull/20790
+[#30803]: https://github.com/rust-lang/rust/issues/30803
+
+This makes them inconsistent, and results in poor error messages for code using `#![no_core]`:
+
+```rust
+#![feature(lang_items, no_core)]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+
+fn main() {
+    for _ in () {}
+}
+```
+
+Compilation output:
+
+```
+foo.rs:8:5: 8:9 error: failed to resolve. Maybe a missing `extern crate iter`? [E0433]
+foo.rs:8     for _ in () {}
+                          ^~~~
+foo.rs:8:5: 8:9 help: run `rustc --explain E0433` to see a detailed explanation
+foo.rs:8:5: 8:19 error: unresolved name `iter::IntoIterator::into_iter` [E0425]
+foo.rs:8     for _ in () {}
+                          ^~~~~~~~~~~~~~
+foo.rs:8:5: 8:19 help: run `rustc --explain E0425` to see a detailed explanation
+foo.rs:8:5: 8:9 error: failed to resolve. Maybe a missing `extern crate iter`? [E0433]
+foo.rs:8     for _ in () {}
+                          ^~~~
+foo.rs:8:5: 8:9 help: run `rustc --explain E0433` to see a detailed explanation
+foo.rs:8:5: 8:19 error: unresolved name `iter::Iterator::next` [E0425]
+foo.rs:8     for _ in () {}
+                          ^~~~~~~~~~~~~~
+foo.rs:8:5: 8:19 help: run `rustc --explain E0425` to see a detailed explanation
+foo.rs:8:5: 8:19 error: unresolved enum variant, struct or const `Some` [E0419]
+foo.rs:8     for _ in () {}
+                          ^~~~~~~~~~~~~~
+foo.rs:8:5: 8:19 help: run `rustc --explain E0419` to see a detailed explanation
+foo.rs:8:5: 8:19 error: unresolved enum variant, struct or const `None` [E0419]
+foo.rs:8     for _ in () {}
+                          ^~~~~~~~~~~~~~
+foo.rs:8:5: 8:19 help: run `rustc --explain E0419` to see a detailed explanation
+```
+
+# Detailed design
+[design]: #detailed-design
+
+1. Add an `into_iterator` language item and provide it with `core::iter::IntoIterator`.
+2. Restore the `iterator` language item and provide it with `core::iter::Iterator`.
+3. Add an `option` language item and provide it with `core::option::Option`.
+4. Rewrite the `for` loop desugaring to use the new language items.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Possible backward compatibility concerns with existing `#![no_core]` code.
+
+# Alternatives
+[alternatives]: #alternatives
+
+Retain the use of hardcoded paths.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.