Point at LHS on binop type err if relevant

Author: estebank

Diff for: compiler/rustc_hir_typeck/src/demand.rs

@@ -189,7 +189,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         &self,
         err: &mut Diagnostic,
         expr: &hir::Expr<'_>,
-        error: Option<TypeError<'_>>,
+        error: Option<TypeError<'tcx>>,
     ) {
         let parent = self.tcx.hir().get_parent_node(expr.hir_id);
         match (self.tcx.hir().find(parent), error) {
@@ -286,6 +286,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     err.downgrade_to_delayed_bug();
                 }
             }
+            (
+                Some(hir::Node::Expr(hir::Expr {
+                    kind: hir::ExprKind::Binary(_, lhs, rhs), ..
+                })),
+                Some(TypeError::Sorts(ExpectedFound { expected, .. })),
+            ) if rhs.hir_id == expr.hir_id
+                && self.typeck_results.borrow().expr_ty_adjusted_opt(lhs) == Some(expected) =>
+            {
+                err.span_label(lhs.span, &format!("expected because this is `{expected}`"));
+            }
             _ => {}
         }
     }

Diff for: src/test/ui/const-generics/generic_const_exprs/issue-79518-default_trait_method_normalization.stderr

@@ -2,7 +2,9 @@ error[E0308]: mismatched types
   --> $DIR/issue-79518-default_trait_method_normalization.rs:16:32
    |
 LL |         Self::AssocInstance == [(); std::mem::size_of::<Self::Assoc>()];
-   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected associated type, found array `[(); _]`
+   |         -------------------    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected associated type, found array `[(); _]`
+   |         |
+   |         expected because this is `<Self as Foo>::Assoc`
    |
    = note: expected associated type `<Self as Foo>::Assoc`
                         found array `[(); _]`

Diff for: src/test/ui/inference/deref-suggestion.stderr

@@ -87,7 +87,10 @@ error[E0308]: mismatched types
   --> $DIR/deref-suggestion.rs:37:5
    |
 LL |     assert_eq!(3i32, &3i32);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^ expected `i32`, found `&i32`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
+   |     |
+   |     expected `i32`, found `&i32`
+   |     expected because this is `i32`
    |
    = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
 

Diff for: src/test/ui/issues/issue-47486.stderr

@@ -2,7 +2,9 @@ error[E0308]: mismatched types
   --> $DIR/issue-47486.rs:2:10
    |
 LL |     () < std::mem::size_of::<_>();
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found `usize`
+   |     --   ^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found `usize`
+   |     |
+   |     expected because this is `()`
 
 error[E0282]: type annotations needed
   --> $DIR/issue-47486.rs:3:11