Account for tail expressions when pointing at return type

When there's a type mismatch we make an effort to check if it was
caused by a function's return type. This logic now makes sure to
only point at the return type if the error happens in a tail
expression.

Author: estebank

src/librustc/hir/map/mod.rs

@@ -743,7 +743,28 @@ impl<'hir> Map<'hir> {
     /// }
     /// ```
     pub fn get_return_block(&self, id: HirId) -> Option<HirId> {
-        for (hir_id, node) in ParentHirIterator::new(id, &self) {
+        let mut iter = ParentHirIterator::new(id, &self).peekable();
+        let mut ignore_tail = false;
+        if let Some(entry) = self.find_entry(id) {
+            if let Node::Expr(Expr { node: ExprKind::Ret(_), .. }) = entry.node {
+                // When dealing with `return` statements, we don't care about climbing only tail
+                // expressions.
+                ignore_tail = true;
+            }
+        }
+        while let Some((hir_id, node)) = iter.next() {
+            if let (Some((_, next_node)), false) = (iter.peek(), ignore_tail) {
+                match next_node {
+                    Node::Block(Block { expr: None, .. }) => return None,
+                    Node::Block(Block { expr: Some(expr), .. }) => {
+                        if hir_id != expr.hir_id {
+                            // The current node is not the tail expression of its parent.
+                            return None;
+                        }
+                    }
+                    _ => {}
+                }
+            }
             match node {
                 Node::Item(_) |
                 Node::ForeignItem(_) |
@@ -752,10 +773,12 @@ impl<'hir> Map<'hir> {
                 Node::ImplItem(_) => return Some(hir_id),
                 Node::Expr(ref expr) => {
                     match expr.node {
-                        ExprKind::Loop(..) | ExprKind::Ret(..) => return None,
+                        // Ignore `return`s on the first iteration
+                        ExprKind::Ret(..) | ExprKind::Loop(..) => return None,
                         _ => {}
                     }
                 }
+                Node::Local(_) => return None,
                 _ => {}
             }
         }

src/librustc/hir/mod.rs

@@ -1535,7 +1535,7 @@ pub enum ExprKind {
     /// Thus, `x.foo::<Bar, Baz>(a, b, c, d)` is represented as
     /// `ExprKind::MethodCall(PathSegment { foo, [Bar, Baz] }, [x, a, b, c, d])`.
     MethodCall(P<PathSegment>, Span, HirVec<Expr>),
-    /// A tuple (e.g., `(a, b, c ,d)`).
+    /// A tuple (e.g., `(a, b, c, d)`).
     Tup(HirVec<Expr>),
     /// A binary operation (e.g., `a + b`, `a * b`).
     Binary(BinOp, P<Expr>, P<Expr>),

src/librustc_typeck/check/expr.rs

@@ -620,8 +620,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expr: &'tcx hir::Expr
     ) -> Ty<'tcx> {
         if self.ret_coercion.is_none() {
-            struct_span_err!(self.tcx.sess, expr.span, E0572,
-                                "return statement outside of function body").emit();
+            struct_span_err!(
+                self.tcx.sess,
+                expr.span,
+                E0572,
+                "return statement outside of function body",
+            ).emit();
         } else if let Some(ref e) = expr_opt {
             if self.ret_coercion_span.borrow().is_none() {
                 *self.ret_coercion_span.borrow_mut() = Some(e.span);

src/test/ui/struct-literal-variant-in-if.stderr

@@ -49,9 +49,6 @@ LL |     if x == E::V { field } {}
 error[E0308]: mismatched types
   --> $DIR/struct-literal-variant-in-if.rs:10:20
    |
-LL | fn test_E(x: E) {
-   |                 - help: try adding a return type: `-> bool`
-LL |     let field = true;
 LL |     if x == E::V { field } {}
    |                    ^^^^^ expected (), found bool
    |