Add fn fn_decl to Hir, for looking up the FnDecl of a body owner.

src/librustc/hir/map/mod.rs

@@ -170,6 +170,40 @@ impl<'hir> MapEntry<'hir> {
         })
     }
 
+    fn fn_decl(&self) -> Option<&FnDecl> {
+        match self {
+            EntryItem(_, _, ref item) => {
+                match item.node {
+                    ItemFn(ref fn_decl, _, _, _, _, _) => Some(&fn_decl),
+                    _ => None,
+                }
+            }
+
+            EntryTraitItem(_, _, ref item) => {
+                match item.node {
+                    TraitItemKind::Method(ref method_sig, _) => Some(&method_sig.decl),
+                    _ => None
+                }
+            }
+
+            EntryImplItem(_, _, ref item) => {
+                match item.node {
+                    ImplItemKind::Method(ref method_sig, _) => Some(&method_sig.decl),
+                    _ => None,
+                }
+            }
+
+            EntryExpr(_, _, ref expr) => {
+                match expr.node {
+                    ExprClosure(_, ref fn_decl, ..) => Some(&fn_decl),
+                    _ => None,
+                }
+            }
+
+            _ => None
+        }
+    }
+
     fn associated_body(self) -> Option<BodyId> {
         match self {
             EntryItem(_, _, item) => {
@@ -502,6 +536,14 @@ impl<'hir> Map<'hir> {
         self.forest.krate.body(id)
     }
 
+    pub fn fn_decl(&self, node_id: ast::NodeId) -> Option<FnDecl> {
+        if let Some(entry) = self.find_entry(node_id) {
+            entry.fn_decl().map(|fd| fd.clone())
+        } else {
+            bug!("no entry for node_id `{}`", node_id)
+        }
+    }
+
     /// Returns the `NodeId` that corresponds to the definition of
     /// which this is the body of, i.e. a `fn`, `const` or `static`
     /// item (possibly associated), a closure, or a `hir::AnonConst`.