Merge #6220

6220: implement binary operator overloading type inference r=flodiebold a=ruabmbua

Extend type inference of *binary operator expression*, by adding support for operator overloads.

Before this merge request, the type inference of binary expressions could only resolve operations done on built-in primitive types. This merge requests adds a code path, which is executed in case the built-in inference could not get any results. It resolves the proper operator overload trait in *core::ops* via lang items, and then resolves the associated *Output* type.

```rust
struct V2([f32; 2]);

#[lang = "add"]
pub trait Add<Rhs = Self> {
    /// The resulting type after applying the `+` operator.
    type Output;

    /// Performs the `+` operation.
    #[must_use]
    fn add(self, rhs: Rhs) -> Self::Output;
}

impl Add<V2> for V2 {
    type Output = V2;

    fn add(self, rhs: V2) -> V2 {
        let x = self.0[0] + rhs.0[0];
        let y = self.0[1] + rhs.0[1];
        V2([x, y])
    }
}

fn test() {
    let va = V2([0.0, 1.0]);
    let vb = V2([0.0, 1.0]);

    let r = va + vb; // This infers to V2 now
}
```

There is a problem with operator overloads, which do not explicitly set the *Rhs* type parameter in the respective impl block. 

**Example:**

```rust
impl Add for V2 {
    type Output = V2;

    fn add(self, rhs: V2) -> V2 {
        let x = self.0[0] + rhs.0[0];
        let y = self.0[1] + rhs.0[1];
        V2([x, y])
    }
}
```

In this case, the trait solver does not realize, that the *Rhs* type parameter is actually self in the context of the impl block. This stops type inference in its tracks, and it can not resolve the associated *Output* type.

I guess we can still merge this back, because it increases the amount of resolved types, and does not regress anything (in the tests).

Somewhat blocked by https://github.com/rust-analyzer/rust-analyzer/issues/5685
Resolves  https://github.com/rust-analyzer/rust-analyzer/issues/5544

Co-authored-by: Roland Ruckerbauer <[email protected]>

1 files changed, 14 insertions, 4 deletions

diff --git a/crates/hir_ty/src/infer/expr.rs b/crates/hir_ty/src/infer/expr.rs
index 0a141b9cb..8ac4cf89a 100644
--- a/crates/hir_ty/src/infer/expr.rs
+++ b/crates/hir_ty/src/infer/expr.rs
@@ -12,6 +12,7 @@ use hir_def::{
 };
 use hir_expand::name::{name, Name};
 use syntax::ast::RangeOp;
+use test_utils::mark;
 
 use crate::{
     autoderef, method_resolution, op,
@@ -531,13 +532,22 @@ impl<'a> InferenceContext<'a> {
                         _ => Expectation::none(),
                     };
                     let lhs_ty = self.infer_expr(*lhs, &lhs_expectation);
-                    // FIXME: find implementation of trait corresponding to operation
-                    // symbol and resolve associated `Output` type
                     let rhs_expectation = op::binary_op_rhs_expectation(*op, lhs_ty.clone());
                     let rhs_ty = self.infer_expr(*rhs, &Expectation::has_type(rhs_expectation));
 
-                    // FIXME: similar as above, return ty is often associated trait type
-                    op::binary_op_return_ty(*op, lhs_ty, rhs_ty)
+                    let ret = op::binary_op_return_ty(*op, lhs_ty.clone(), rhs_ty.clone());
+
+                    if ret == Ty::Unknown {
+                        mark::hit!(infer_expr_inner_binary_operator_overload);
+
+                        self.resolve_associated_type_with_params(
+                            lhs_ty,
+                            self.resolve_binary_op_output(op),
+                            &[rhs_ty],
+                        )
+                    } else {
+                        ret
+                    }
                 }
                 _ => Ty::Unknown,
             },