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% `box` and `in` operators
See: https://github.com/rust-lang/rfcs/pull/1426#r48383364
Structures
=========
AST
---
UniOp for `box` and BinOp for `in` and `<-`
HIR
---
```c++
struct ExprNode_Emplace
{
/// This influences the ops trait used
enum class Type {
Placer,
Boxer,
};
Type m_type;
ExprNodeP m_place;
ExprNodeP m_value;
}
```
Type Inferrence
===============
The `_Emplace` node type has a revisit to obtain the expected result type.
1. If the place has an unknown type, the pre-coercion output type is located by taking the result type and replacing all params with fresh ivars
1. A bound is added that this new type must implement `ops::Placer<T>` (or `ops::Boxer<T>`) where `T` is the result type of `m_value`
1. Add a coercion at this node? (Or leave it up to the parent node to have inserted one)
Expansion / Lowering
====================
IDEAS:
- Convert as any other operator is - in the post-typeck expansion pass
- However, since this doesn't expand to a single function call (the current RFC is four calls) that would be interesting
- Also, the current RFC introduces bindings, which can't (easily) be done in that pass
- Keep until MIR, and lower into call sequence
- Can use temporaries
- Downside: Avoids the extra validation other operators get.
- Still the best solution.
```rust
let p = PLACE;
let mut place = Placer::make_place(p);
let raw_place = Place::pointer(&mut place);
let value = EXPR;
unsafe {
std::ptr::write(raw_place, value);
InPlace::finalize(place)
}
```
```rust
let mut place = BoxPlace::make_place();
let raw_place = Place::pointer(&mut place);
let value = EXPR;
unsafe {
::std::ptr::write(raw_place, value);
Boxed::finalize(place)
}
```
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