Auto merge of #13209 - lowr:feat/inference-for-generator, r=Veykril

feat: type inference for generators

This PR implements basic type inference for generator and yield expressions.

Things not included in this PR:
- Generator upvars and generator witnesses are not implemented. They are only used to determine auto trait impls, so basic type inference should be fine without them, but method resolutions with auto trait bounds may not be resolved correctly.

Open questions:
- I haven't (yet) implemented `HirDisplay` for `TyKind::Generator`, so generator types are just shown as "{{generator}}" (in tests, inlay hints, hovers, etc.), which is not really nice. How should we show them?
- I added moderate amount of stuffs to minicore. I especially didn't want to add `impl<T> Deref for &T` and `impl<T> Deref for &mut T` exclusively for tests for generators; should I move them into the test fixtures or can they be placed in minicore?

cc #4309

crates/hir-ty/src/display.rs

@@ -20,6 +20,7 @@ use hir_def::{
 };
 use hir_expand::{hygiene::Hygiene, name::Name};
 use itertools::Itertools;
+use smallvec::SmallVec;
 use syntax::SmolStr;
 
 use crate::{
@@ -221,6 +222,7 @@ pub enum DisplaySourceCodeError {
     PathNotFound,
     UnknownType,
     Closure,
+    Generator,
 }
 
 pub enum HirDisplayError {
@@ -783,7 +785,34 @@ impl HirDisplay for Ty {
                 write!(f, "{{unknown}}")?;
             }
             TyKind::InferenceVar(..) => write!(f, "_")?,
-            TyKind::Generator(..) => write!(f, "{{generator}}")?,
+            TyKind::Generator(_, subst) => {
+                if f.display_target.is_source_code() {
+                    return Err(HirDisplayError::DisplaySourceCodeError(
+                        DisplaySourceCodeError::Generator,
+                    ));
+                }
+
+                let subst = subst.as_slice(Interner);
+                let a: Option<SmallVec<[&Ty; 3]>> = subst
+                    .get(subst.len() - 3..)
+                    .map(|args| args.iter().map(|arg| arg.ty(Interner)).collect())
+                    .flatten();
+
+                if let Some([resume_ty, yield_ty, ret_ty]) = a.as_deref() {
+                    write!(f, "|")?;
+                    resume_ty.hir_fmt(f)?;
+                    write!(f, "|")?;
+
+                    write!(f, " yields ")?;
+                    yield_ty.hir_fmt(f)?;
+
+                    write!(f, " -> ")?;
+                    ret_ty.hir_fmt(f)?;
+                } else {
+                    // This *should* be unreachable, but fallback just in case.
+                    write!(f, "{{generator}}")?;
+                }
+            }
             TyKind::GeneratorWitness(..) => write!(f, "{{generator witness}}")?,
         }
         Ok(())