HIR "Eager Type Inference" / Expectations dev guide page

src/SUMMARY.md

@@ -198,6 +198,7 @@
 - [Coherence checking](./coherence.md)
 - [HIR Type checking](./hir-typeck/summary.md)
     - [Coercions](./hir-typeck/coercions.md)
+    - [Eager Inference](./hir-typeck/eager-inference.md)
     - [Method lookup](./hir-typeck/method-lookup.md)
 - [Const Generics](./const-generics.md)
 - [Opaque types](./opaque-types-type-alias-impl-trait.md)

src/hir-typeck/eager-inference.md

@@ -0,0 +1,82 @@
+# Eager Type Inference
+
+There are places during compilation where we need to depend on the current state of inference. This means we need to establish the type of a {term, item, etc, figure out exact wording in review} earlier than we otherwise would.
+
+Depending on the current state of inference means _we pay attention to the set constraints we currently have_ even if we've not finished finding all constraints yet.
+
+Eager evaluation of the type of a term (type inference) is required at specific points either to make later type inference more consistent or (in the case of higher-ranked bounds / Higher Ranked Lifetime bounds) make it possible at all.
+
+Eager type inference is when we do type inference earlier than we otherwise would. To do this, we bring in Expectations as an additional piece of context.
+
+### Closures and Higher-Ranked Variables
+
+Top-level functions, such as the following, have their types fully annotated at their definition site:
+
+```rust
+fn is_even(number: i32) -> bool {
+    number % 2 == 0
+}
+```
+
+This makes inference at points where they're used relatively easy. We know it's a `fn(i32) -> bool`, so when we give it an `i32` we know the expression is a `bool`.
+
+_Closures_ need to have type inference eagerly applied to them because they are functions that are rarely fully annotated:
+
+```rust
+let closure = |a, b| if a < b {vec![1, 2, 3]} else {vec![5, 6, 7]};
+```
+
+If we didn't do eager type inference we would instead have closures whose types were filled with [inference variables](../appendix/glossary.md#inf-var).
+
+This would be able to be solved in some situations, but because we do not have Higher-Ranked Inference Variables[^higher-ranked-inference] this would make the higher-ranked bounds for lifetimes that rust can have unusable without more annotation.
+
+Another reason we infer eagerly for closures is that it's a good heuristic: We will need to know the types of the closure if it's used anywhere, so it is best to figure out what it is sooner.
+
+? TODO: higher-ranked inference, but more. Go over notes.
+
+### Coercions 
+
+[Coercions](./coercions.md) can happen in many places. We check to see if a coercion can happen, and if it can we perform the coercion. 
+
+When we successfully find a coercion, we need to eagerly perform type inference/checking on it as future inference will require or benefit from this information to be known ahead of time.
+
+? TODO: Coercions are found by eager inference, this is the other way round to what is currently written.
+
+### Trait Solving
+
+Trait solving happens in a 
+
+### Method calls, Fields, and Indexes.
+
+? This bucket of stuff should be changed.
+
+These are areas which technically take expectations, but in practice use them for diagnostics only.
+
+#### Methods
+
+Maybe Not. Maybe just point to [method lookup](./method-lookup.md).
+
+? Method calls engage in Coercion and therefore need to engage in Eager Type Inference.
+
+#### Fields?
+
+Field access is inherently typed, so when we are doing field access we want to be able to know what a type is as early as possible.
+
+? There might be something about deref here idk.
+
+#### Indexing
+
+? Indexing engages in coercion and therefore needs to engage in eager type inference.
+
+lcnr said so.
+
+
+## Expectations
+
+`Expectations` are a piece of type inference state we maintain for the cases where we need to eagerly infer the types of expressions rather than leave them to the end. They allow us to "ask questions" of the form "hey, we're expecting this term to have this type, is this true?"
+
+Papers:
+- [Practical Type Inference for Arbitrary-Rank Types, Jones ](https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/putting.pdf)
+- [Local type inference (referenced in PTIfART)]
+
+[^higher-ranked-inference]: https://github.com/rust-lang/types-team/issues/131