Rollup merge of #64802 - estebank:walk-parents-iterator, r=matthewjasper

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.

Turn `walk_parent_nodes` method into an iterator.

CC #39968, CC #40799.

src/librustc/hir/map/mod.rs

@@ -23,8 +23,6 @@
 use syntax::ext::base::MacroKind;
 use syntax_pos::{Span, DUMMY_SP};
 
-use std::result::Result::Err;
-
 pub mod blocks;
 mod collector;
 mod def_collector;
@@ -183,6 +181,44 @@ pub struct Map<'hir> {
     hir_to_node_id: FxHashMap<HirId, NodeId>,
 }
 
+struct ParentHirIterator<'map> {
+    current_id: HirId,
+    map: &'map Map<'map>,
+}
+
+impl<'map> ParentHirIterator<'map> {
+    fn new(current_id: HirId, map: &'map Map<'map>) -> ParentHirIterator<'map> {
+        ParentHirIterator {
+            current_id,
+            map,
+        }
+    }
+}
+
+impl<'map> Iterator for ParentHirIterator<'map> {
+    type Item = (HirId, Node<'map>);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.current_id == CRATE_HIR_ID {
+            return None;
+        }
+        loop { // There are nodes that do not have entries, so we need to skip them.
+            let parent_id = self.map.get_parent_node(self.current_id);
+
+            if parent_id == self.current_id {
+                self.current_id = CRATE_HIR_ID;
+                return None;
+            }
+
+            self.current_id = parent_id;
+            if let Some(entry) = self.map.find_entry(parent_id) {
+                return Some((parent_id, entry.node));
+            }
+            // If this `HirId` doesn't have an `Entry`, skip it and look for its `parent_id`.
+        }
+    }
+}
+
 impl<'hir> Map<'hir> {
     #[inline]
     fn lookup(&self, id: HirId) -> Option<&Entry<'hir>> {
@@ -682,45 +718,6 @@ pub fn is_hir_id_module(&self, hir_id: HirId) -> bool {
         }
     }
 
-
-    /// If there is some error when walking the parents (e.g., a node does not
-    /// have a parent in the map or a node can't be found), then we return the
-    /// last good `HirId` we found. Note that reaching the crate root (`id == 0`),
-    /// is not an error, since items in the crate module have the crate root as
-    /// parent.
-    fn walk_parent_nodes<F, F2>(&self,
-                                start_id: HirId,
-                                found: F,
-                                bail_early: F2)
-        -> Result<HirId, HirId>
-        where F: Fn(&Node<'hir>) -> bool, F2: Fn(&Node<'hir>) -> bool
-    {
-        let mut id = start_id;
-        loop {
-            let parent_id = self.get_parent_node(id);
-            if parent_id == CRATE_HIR_ID {
-                return Ok(CRATE_HIR_ID);
-            }
-            if parent_id == id {
-                return Err(id);
-            }
-
-            if let Some(entry) = self.find_entry(parent_id) {
-                if let Node::Crate = entry.node {
-                    return Err(id);
-                }
-                if found(&entry.node) {
-                    return Ok(parent_id);
-                } else if bail_early(&entry.node) {
-                    return Err(parent_id);
-                }
-                id = parent_id;
-            } else {
-                return Err(id);
-            }
-        }
-    }
-
     /// Retrieves the `HirId` for `id`'s enclosing method, unless there's a
     /// `while` or `loop` before reaching it, as block tail returns are not
     /// available in them.
@@ -744,29 +741,46 @@ fn walk_parent_nodes<F, F2>(&self,
     /// }
     /// ```
     pub fn get_return_block(&self, id: HirId) -> Option<HirId> {
-        let match_fn = |node: &Node<'_>| {
-            match *node {
+        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 { kind: 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(_) |
                 Node::TraitItem(_) |
                 Node::Expr(Expr { kind: ExprKind::Closure(..), ..}) |
-                Node::ImplItem(_) => true,
-                _ => false,
-            }
-        };
-        let match_non_returning_block = |node: &Node<'_>| {
-            match *node {
+                Node::ImplItem(_) => return Some(hir_id),
                 Node::Expr(ref expr) => {
                     match expr.kind {
-                        ExprKind::Loop(..) | ExprKind::Ret(..) => true,
-                        _ => false,
+                        // Ignore `return`s on the first iteration
+                        ExprKind::Loop(..) | ExprKind::Ret(..) => return None,
+                        _ => {}
                     }
                 }
-                _ => false,
+                Node::Local(_) => return None,
+                _ => {}
             }
-        };
-
-        self.walk_parent_nodes(id, match_fn, match_non_returning_block).ok()
+        }
+        None
     }
 
     /// Retrieves the `HirId` for `id`'s parent item, or `id` itself if no
@@ -774,16 +788,17 @@ pub fn get_return_block(&self, id: HirId) -> Option<HirId> {
     /// in the HIR which is recorded by the map and is an item, either an item
     /// in a module, trait, or impl.
     pub fn get_parent_item(&self, hir_id: HirId) -> HirId {
-        match self.walk_parent_nodes(hir_id, |node| match *node {
-            Node::Item(_) |
-            Node::ForeignItem(_) |
-            Node::TraitItem(_) |
-            Node::ImplItem(_) => true,
-            _ => false,
-        }, |_| false) {
-            Ok(id) => id,
-            Err(id) => id,
+        for (hir_id, node) in ParentHirIterator::new(hir_id, &self) {
+            match node {
+                Node::Crate |
+                Node::Item(_) |
+                Node::ForeignItem(_) |
+                Node::TraitItem(_) |
+                Node::ImplItem(_) => return hir_id,
+                _ => {}
+            }
         }
+        hir_id
     }
 
     /// Returns the `DefId` of `id`'s nearest module parent, or `id` itself if no
@@ -795,60 +810,64 @@ pub fn get_module_parent(&self, id: HirId) -> DefId {
     /// Returns the `HirId` of `id`'s nearest module parent, or `id` itself if no
     /// module parent is in this map.
     pub fn get_module_parent_node(&self, hir_id: HirId) -> HirId {
-        match self.walk_parent_nodes(hir_id, |node| match *node {
-            Node::Item(&Item { kind: ItemKind::Mod(_), .. }) => true,
-            _ => false,
-        }, |_| false) {
-            Ok(id) => id,
-            Err(id) => id,
+        for (hir_id, node) in ParentHirIterator::new(hir_id, &self) {
+            if let Node::Item(&Item { kind: ItemKind::Mod(_), .. }) = node {
+                return hir_id;
+            }
         }
+        CRATE_HIR_ID
     }
 
     /// Returns the nearest enclosing scope. A scope is roughly an item or block.
     pub fn get_enclosing_scope(&self, hir_id: HirId) -> Option<HirId> {
-        self.walk_parent_nodes(hir_id, |node| match *node {
-            Node::Item(i) => {
-                match i.kind {
-                    ItemKind::Fn(..)
-                    | ItemKind::Mod(..)
-                    | ItemKind::Enum(..)
-                    | ItemKind::Struct(..)
-                    | ItemKind::Union(..)
-                    | ItemKind::Trait(..)
-                    | ItemKind::Impl(..) => true,
-                    _ => false,
-                }
-            },
-            Node::ForeignItem(fi) => {
-                match fi.kind {
-                    ForeignItemKind::Fn(..) => true,
-                    _ => false,
-                }
-            },
-            Node::TraitItem(ti) => {
-                match ti.kind {
-                    TraitItemKind::Method(..) => true,
-                    _ => false,
-                }
-            },
-            Node::ImplItem(ii) => {
-                match ii.kind {
-                    ImplItemKind::Method(..) => true,
-                    _ => false,
-                }
-            },
-            Node::Block(_) => true,
-            _ => false,
-        }, |_| false).ok()
+        for (hir_id, node) in ParentHirIterator::new(hir_id, &self) {
+            if match node {
+                Node::Item(i) => {
+                    match i.kind {
+                        ItemKind::Fn(..)
+                        | ItemKind::Mod(..)
+                        | ItemKind::Enum(..)
+                        | ItemKind::Struct(..)
+                        | ItemKind::Union(..)
+                        | ItemKind::Trait(..)
+                        | ItemKind::Impl(..) => true,
+                        _ => false,
+                    }
+                },
+                Node::ForeignItem(fi) => {
+                    match fi.kind {
+                        ForeignItemKind::Fn(..) => true,
+                        _ => false,
+                    }
+                },
+                Node::TraitItem(ti) => {
+                    match ti.kind {
+                        TraitItemKind::Method(..) => true,
+                        _ => false,
+                    }
+                },
+                Node::ImplItem(ii) => {
+                    match ii.kind {
+                        ImplItemKind::Method(..) => true,
+                        _ => false,
+                    }
+                },
+                Node::Block(_) => true,
+                _ => false,
+            } {
+                return Some(hir_id);
+            }
+        }
+        None
     }
 
     /// Returns the defining scope for an opaque type definition.
-    pub fn get_defining_scope(&self, id: HirId) -> Option<HirId> {
+    pub fn get_defining_scope(&self, id: HirId) -> HirId {
         let mut scope = id;
         loop {
-            scope = self.get_enclosing_scope(scope)?;
+            scope = self.get_enclosing_scope(scope).unwrap_or(CRATE_HIR_ID);
             if scope == CRATE_HIR_ID {
-                return Some(CRATE_HIR_ID);
+                return CRATE_HIR_ID;
             }
             match self.get(scope) {
                 Node::Item(i) => {
@@ -861,7 +880,7 @@ pub fn get_defining_scope(&self, id: HirId) -> Option<HirId> {
                 _ => break,
             }
         }
-        Some(scope)
+        scope
     }
 
     pub fn get_parent_did(&self, id: HirId) -> DefId {

src/librustc/hir/mod.rs

@@ -1552,7 +1552,7 @@ 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/infer/opaque_types/mod.rs

@@ -1213,7 +1213,7 @@ pub fn may_define_opaque_type(
     let mut hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
 
     // Named opaque types can be defined by any siblings or children of siblings.
-    let scope = tcx.hir().get_defining_scope(opaque_hir_id).expect("could not get defining scope");
+    let scope = tcx.hir().get_defining_scope(opaque_hir_id);
     // We walk up the node tree until we hit the root or the scope of the opaque type.
     while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
         hir_id = tcx.hir().get_parent_item(hir_id);

src/librustc_typeck/check/expr.rs

@@ -620,8 +620,12 @@ fn check_expr_return(
         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/librustc_typeck/collect.rs

@@ -1717,9 +1717,7 @@ fn visit_trait_item(&mut self, it: &'tcx TraitItem) {
     }
 
     let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
-    let scope = tcx.hir()
-        .get_defining_scope(hir_id)
-        .expect("could not get defining scope");
+    let scope = tcx.hir().get_defining_scope(hir_id);
     let mut locator = ConstraintLocator {
         def_id,
         tcx,

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
    |