From 458fdb37f62fd3d6f9ae2ddf9f1bbdfcd0d126cd Mon Sep 17 00:00:00 2001
From: Esteban Kuber <esteban@kuber.com.ar>
Date: Tue, 10 May 2022 19:41:22 +0000
Subject: [PATCH] Print type of every call in a method call chain

```
error[E0271]: type mismatch resolving `<std::collections::hash_map::Iter<'_, _, _> as Iterator>::Item == &_`
  --> $DIR/issue-33941.rs:6:36
   |
LL |     for _ in HashMap::new().iter().cloned() {}
   |              ------------   ----   ^^^^^^ expected reference, found tuple
   |              |              |
   |              |              this call is of type `std::collections::hash_map::Iter<'_, _, _>`
   |              this call is of type `&HashMap<_, _>`
   |
   = note: expected reference `&_`
                  found tuple `(&_, &_)`
note: required by a bound in `cloned`
  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
   |
LL |         Self: Sized + Iterator<Item = &'a T>,
   |                                ^^^^^^^^^^^^ required by this bound in `cloned`
```

Partially address #33941.
---
 .../src/traits/error_reporting/mod.rs         | 51 +++++++++++++++++++
 src/test/ui/issues/issue-31173.stderr         |  3 ++
 src/test/ui/issues/issue-33941.stderr         |  5 +-
 3 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 7a3579eb1cc85..0fa90dc7b9756 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -1621,6 +1621,36 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 "type mismatch resolving `{}`",
                 predicate
             );
+            let mut v = HirFinder { needle: obligation.cause.span, found: None };
+            let body = self.tcx.hir().get(obligation.cause.body_id);
+            if let hir::Node::Expr(expr) = body {
+                v.visit_expr(expr);
+            }
+            if let Some(expr) = v.found {
+                // FIXME(estebank): look for method calls that can be applied on the previous
+                // element of the chain, that would make the next call to typeck.
+                let mut receiver = expr;
+                if let (Some(typeck_results), hir::ExprKind::MethodCall(..)) =
+                    (self.in_progress_typeck_results, &receiver.kind)
+                {
+                    let typeck_results = typeck_results.borrow();
+                    while let hir::ExprKind::MethodCall(method_segment, args, _) = receiver.kind {
+                        if let Some(ty) = typeck_results.expr_ty_adjusted_opt(receiver) {
+                            diag.span_label(
+                                method_segment.ident.span,
+                                &format!("this call is of type `{ty}`"),
+                            );
+                        }
+                        receiver = &args[0];
+                    }
+                    while let hir::ExprKind::Call(path, _args) = receiver.kind {
+                        if let Some(ty) = typeck_results.expr_ty_adjusted_opt(receiver) {
+                            diag.span_label(path.span, &format!("this call is of type `{ty}`"));
+                        }
+                        receiver = &path;
+                    }
+                }
+            }
             let secondary_span = match predicate.kind().skip_binder() {
                 ty::PredicateKind::Projection(proj) => self
                     .tcx
@@ -2689,3 +2719,24 @@ impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
         }
     }
 }
+
+struct HirFinder<'tcx> {
+    needle: Span,
+    found: Option<&'tcx hir::Expr<'tcx>>,
+}
+
+impl<'v> Visitor<'v> for HirFinder<'v> {
+    fn visit_expr(&mut self, expr: &'v hir::Expr<'v>) {
+        if expr.span == self.needle {
+            self.found = Some(expr);
+            return;
+        }
+        if let hir::ExprKind::MethodCall(segment, _, _) = expr.kind {
+            if segment.ident.span == self.needle {
+                self.found = Some(expr);
+                return;
+            }
+        }
+        hir::intravisit::walk_expr(self, expr);
+    }
+}
diff --git a/src/test/ui/issues/issue-31173.stderr b/src/test/ui/issues/issue-31173.stderr
index 982b6118ce659..820696b03f1bf 100644
--- a/src/test/ui/issues/issue-31173.stderr
+++ b/src/test/ui/issues/issue-31173.stderr
@@ -1,6 +1,9 @@
 error[E0271]: type mismatch resolving `<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 9:6]> as Iterator>::Item == &_`
   --> $DIR/issue-31173.rs:10:10
    |
+LL |     let temp: Vec<u8> = it.take_while(|&x| {
+   |                            ---------- this call is of type `TakeWhile<&mut std::vec::IntoIter<u8>, _>`
+...
 LL |         .cloned()
    |          ^^^^^^ expected reference, found `u8`
    |
diff --git a/src/test/ui/issues/issue-33941.stderr b/src/test/ui/issues/issue-33941.stderr
index e1ce6eed98efb..f32ec0831c5d8 100644
--- a/src/test/ui/issues/issue-33941.stderr
+++ b/src/test/ui/issues/issue-33941.stderr
@@ -2,7 +2,10 @@ error[E0271]: type mismatch resolving `<std::collections::hash_map::Iter<'_, _,
   --> $DIR/issue-33941.rs:6:36
    |
 LL |     for _ in HashMap::new().iter().cloned() {}
-   |                                    ^^^^^^ expected reference, found tuple
+   |              ------------   ----   ^^^^^^ expected reference, found tuple
+   |              |              |
+   |              |              this call is of type `std::collections::hash_map::Iter<'_, _, _>`
+   |              this call is of type `&HashMap<_, _>`
    |
    = note: expected reference `&_`
                   found tuple `(&_, &_)`