From f5d2151d35942b957230c3081a928af3619d9400 Mon Sep 17 00:00:00 2001
From: Nadrieril
Date: Sun, 31 Mar 2019 18:47:34 +0200
Subject: Make SubExpr a newtype

---
 dhall/src/binary.rs               |   3 +-
 dhall/src/main.rs                 |   3 +-
 dhall/src/normalize.rs            |  39 ++++++++-------
 dhall/src/typecheck.rs            | 103 ++++++++++++++++++--------------------
 dhall_core/src/core.rs            |  45 ++++++++++++-----
 dhall_core/src/printer.rs         |   6 +++
 dhall_core/src/text.rs            |  11 ++--
 dhall_generator/src/dhall_expr.rs |   7 ++-
 8 files changed, 117 insertions(+), 100 deletions(-)

diff --git a/dhall/src/binary.rs b/dhall/src/binary.rs
index 235a55b..988891b 100644
--- a/dhall/src/binary.rs
+++ b/dhall/src/binary.rs
@@ -1,9 +1,8 @@
 use dhall_core::*;
 use itertools::*;
 use serde_cbor::value::value as cbor;
-use std::rc::Rc;
 
-type ParsedExpr = Rc<Expr<X, Import>>;
+type ParsedExpr = SubExpr<X, Import>;
 
 #[derive(Debug)]
 pub enum DecodeError {
diff --git a/dhall/src/main.rs b/dhall/src/main.rs
index 1a2e309..eb2206b 100644
--- a/dhall/src/main.rs
+++ b/dhall/src/main.rs
@@ -1,6 +1,5 @@
 use std::error::Error;
 use std::io::{self, Read};
-use std::rc::Rc;
 use term_painter::ToStyle;
 
 use dhall::*;
@@ -66,7 +65,7 @@ fn main() {
         }
     };
 
-    let expr: Rc<Expr<_, _>> = rc(imports::panic_imports(&expr));
+    let expr: SubExpr<_, _> = rc(imports::panic_imports(&expr));
 
     let type_expr = match typecheck::type_of(expr.clone()) {
         Err(e) => {
diff --git a/dhall/src/normalize.rs b/dhall/src/normalize.rs
index a3a2318..5405e88 100644
--- a/dhall/src/normalize.rs
+++ b/dhall/src/normalize.rs
@@ -2,7 +2,6 @@
 use dhall_core::*;
 use dhall_generator::dhall_expr;
 use std::fmt;
-use std::rc::Rc;
 
 fn apply_builtin<S, A>(
     b: Builtin,
@@ -44,13 +43,13 @@ where
     }
     // Normalize the important argument
     if let Some(i) = arg_to_eval {
-        args[i] = Rc::clone(&normalize_whnf(&args[i]));
+        args[i] = SubExpr::clone(&normalize_whnf(&args[i]));
     }
     let evaled_arg = arg_to_eval.map(|i| args[i].as_ref());
 
     let ret = match (b, evaled_arg, args.as_slice()) {
-        (OptionalSome, _, [x, ..]) => rc(NEOptionalLit(Rc::clone(x))),
-        (OptionalNone, _, [t, ..]) => rc(EmptyOptionalLit(Rc::clone(t))),
+        (OptionalSome, _, [x, ..]) => rc(NEOptionalLit(SubExpr::clone(x))),
+        (OptionalNone, _, [t, ..]) => rc(EmptyOptionalLit(SubExpr::clone(t))),
         (NaturalIsZero, Some(NaturalLit(n)), _) => rc(BoolLit(*n == 0)),
         (NaturalEven, Some(NaturalLit(n)), _) => rc(BoolLit(*n % 2 == 0)),
         (NaturalOdd, Some(NaturalLit(n)), _) => rc(BoolLit(*n % 2 != 0)),
@@ -76,7 +75,7 @@ where
             rc(NEListLit(ys))
         }
         (ListIndexed, Some(EmptyListLit(t)), _) => {
-            let t = Rc::clone(t);
+            let t = SubExpr::clone(t);
             dhall_expr!([] : List ({ index : Natural, value : t }))
         }
         (ListIndexed, Some(NEListLit(xs)), _) => {
@@ -101,7 +100,7 @@ where
                         break rc(App(x.clone(), rest.to_vec()));
                     }
                 };
-                let a0 = Rc::clone(a0);
+                let a0 = SubExpr::clone(a0);
                 let a1 = shift(1, &V("a".into(), 0), &a0);
                 // TODO: use Embed to avoid reevaluating g
                 break dhall_expr!(
@@ -122,7 +121,7 @@ where
                         break rc(App(x.clone(), rest.to_vec()));
                     }
                 };
-                let a0 = Rc::clone(a0);
+                let a0 = SubExpr::clone(a0);
                 // TODO: use Embed to avoid reevaluating g
                 break dhall_expr!(
                     g
@@ -133,13 +132,13 @@ where
             }
         }
         (ListFold, Some(EmptyListLit(_)), [_, _, _, _, nil, ..]) => {
-            Rc::clone(nil)
+            SubExpr::clone(nil)
         }
         (ListFold, Some(NEListLit(xs)), [_, _, _, cons, nil, ..]) => {
-            xs.iter().rev().fold(Rc::clone(nil), |acc, x| {
+            xs.iter().rev().fold(SubExpr::clone(nil), |acc, x| {
                 let x = x.clone();
                 let acc = acc.clone();
-                let cons = Rc::clone(cons);
+                let cons = SubExpr::clone(cons);
                 dhall_expr!(cons x acc)
             })
         }
@@ -149,14 +148,14 @@ where
         // }
         (OptionalFold, Some(NEOptionalLit(x)), [_, _, _, just, _, ..]) => {
             let x = x.clone();
-            let just = Rc::clone(just);
+            let just = SubExpr::clone(just);
             dhall_expr!(just x)
         }
         (
             OptionalFold,
             Some(EmptyOptionalLit(_)),
             [_, _, _, _, nothing, ..],
-        ) => Rc::clone(nothing),
+        ) => SubExpr::clone(nothing),
         // // fold/build fusion
         // (OptionalFold, [_, App(box Builtin(OptionalBuild), [_, x, rest..]), rest..]) => {
         //     normalize_whnf(&App(bx(x.clone()), rest.to_vec()))
@@ -175,13 +174,15 @@ where
                 break dhall_expr!(g Natural (λ(x : Natural) -> x + 1) 0);
             }
         }
-        (NaturalFold, Some(NaturalLit(0)), [_, _, _, zero]) => Rc::clone(zero),
+        (NaturalFold, Some(NaturalLit(0)), [_, _, _, zero]) => {
+            SubExpr::clone(zero)
+        }
         (NaturalFold, Some(NaturalLit(n)), [_, t, succ, zero]) => {
             let fold = rc(Builtin(NaturalFold));
             let n = rc(NaturalLit(n - 1));
-            let t = Rc::clone(t);
-            let succ = Rc::clone(succ);
-            let zero = Rc::clone(zero);
+            let t = SubExpr::clone(t);
+            let succ = SubExpr::clone(succ);
+            let zero = SubExpr::clone(zero);
             dhall_expr!(succ (fold n t succ zero))
         }
         // (NaturalFold, Some(App(f2, args2)), _) => {
@@ -273,7 +274,7 @@ where
                 }
                 (TextAppend, TextLit(x), TextLit(y)) => TextLit(x + y),
                 (ListAppend, EmptyListLit(t), EmptyListLit(_)) => {
-                    EmptyListLit(Rc::clone(t))
+                    EmptyListLit(SubExpr::clone(t))
                 }
                 (ListAppend, EmptyListLit(_), _) => return y,
                 (ListAppend, _, EmptyListLit(_)) => return x,
@@ -326,7 +327,7 @@ where
                 _ => rc(Projection(e, ls.clone())),
             }
         }
-        _ => Rc::clone(e),
+        _ => SubExpr::clone(e),
     }
 }
 
@@ -344,5 +345,5 @@ where
     S: fmt::Debug,
     A: fmt::Debug,
 {
-    map_subexpr_rc(&normalize_whnf(&e), |x| normalize(Rc::clone(x)))
+    map_subexpr_rc(&normalize_whnf(&e), |x| normalize(SubExpr::clone(x)))
 }
diff --git a/dhall/src/typecheck.rs b/dhall/src/typecheck.rs
index f21721d..e769c7f 100644
--- a/dhall/src/typecheck.rs
+++ b/dhall/src/typecheck.rs
@@ -1,7 +1,6 @@
 #![allow(non_snake_case)]
 use std::collections::HashSet;
 use std::fmt;
-use std::rc::Rc;
 
 use crate::normalize::normalize;
 use dhall_core;
@@ -113,7 +112,7 @@ where
     go::<S, T>(&mut ctx, eL0, eR0)
 }
 
-fn type_of_builtin<S>(b: Builtin) -> Rc<Expr<S, X>> {
+fn type_of_builtin<S>(b: Builtin) -> SubExpr<S, X> {
     use dhall_core::Builtin::*;
     match b {
         Bool | Natural | Integer | Double | Text => dhall_expr!(Type),
@@ -184,16 +183,15 @@ fn type_of_builtin<S>(b: Builtin) -> Rc<Expr<S, X>> {
 /// is not necessary for just type-checking.  If you actually care about the
 /// returned type then you may want to `normalize` it afterwards.
 pub fn type_with<S>(
-    ctx: &Context<Label, Rc<Expr<S, X>>>,
-    e: Rc<Expr<S, X>>,
-) -> Result<Rc<Expr<S, X>>, TypeError<S>>
+    ctx: &Context<Label, SubExpr<S, X>>,
+    e: SubExpr<S, X>,
+) -> Result<SubExpr<S, X>, TypeError<S>>
 where
     S: ::std::fmt::Debug,
 {
     use dhall_core::BinOp::*;
     use dhall_core::Builtin::*;
     use dhall_core::Const::*;
-    use dhall_core::Expr;
     use dhall_core::Expr::*;
     let mkerr = |msg: TypeMessage<_>| TypeError::new(ctx, e.clone(), msg);
     let ensure_const = |x: &SubExpr<_, _>, msg: TypeMessage<_>| match x.as_ref()
@@ -266,7 +264,7 @@ where
                     return Err(mkerr(NotAFunction(f.clone(), tf)));
                 }
             };
-            let tA = normalize(Rc::clone(tA));
+            let tA = normalize(SubExpr::clone(tA));
             let tA2 = normalized_type_with(ctx, a.clone())?;
             if prop_equal(tA.as_ref(), tA2.as_ref()) {
                 let vx0 = &V(x.clone(), 0);
@@ -280,9 +278,9 @@ where
         }
         Let(f, mt, r, b) => {
             let r = if let Some(t) = mt {
-                rc(Annot(Rc::clone(r), Rc::clone(t)))
+                rc(Annot(SubExpr::clone(r), SubExpr::clone(t)))
             } else {
-                Rc::clone(r)
+                SubExpr::clone(r)
             };
 
             let tR = type_with(ctx, r)?;
@@ -351,7 +349,7 @@ where
         EmptyListLit(t) => {
             let s = normalized_type_with(ctx, t.clone())?;
             ensure_is_type(s, InvalidListType(t.clone()))?;
-            let t = normalize(Rc::clone(t));
+            let t = normalize(SubExpr::clone(t));
             return Ok(dhall_expr!(List t));
         }
         NEListLit(xs) => {
@@ -377,7 +375,7 @@ where
             return Ok(dhall_expr!(Optional t));
         }
         NEOptionalLit(x) => {
-            let t: Rc<Expr<_, _>> = type_with(ctx, x.clone())?;
+            let t: SubExpr<_, _> = type_with(ctx, x.clone())?;
             let s = normalized_type_with(ctx, t.clone())?;
             ensure_is_type(s, InvalidOptionalType(t.clone()))?;
             let t = normalize(t);
@@ -451,9 +449,9 @@ where
 }
 
 pub fn normalized_type_with<S>(
-    ctx: &Context<Label, Rc<Expr<S, X>>>,
-    e: Rc<Expr<S, X>>,
-) -> Result<Rc<Expr<S, X>>, TypeError<S>>
+    ctx: &Context<Label, SubExpr<S, X>>,
+    e: SubExpr<S, X>,
+) -> Result<SubExpr<S, X>, TypeError<S>>
 where
     S: ::std::fmt::Debug,
 {
@@ -464,8 +462,8 @@ where
 /// expression must be closed (i.e. no free variables), otherwise type-checking
 /// will fail.
 pub fn type_of<S: ::std::fmt::Debug>(
-    e: Rc<Expr<S, X>>,
-) -> Result<Rc<Expr<S, X>>, TypeError<S>> {
+    e: SubExpr<S, X>,
+) -> Result<SubExpr<S, X>, TypeError<S>> {
     let ctx = Context::new();
     type_with(&ctx, e) //.map(|e| e.into_owned())
 }
@@ -474,63 +472,58 @@ pub fn type_of<S: ::std::fmt::Debug>(
 #[derive(Debug)]
 pub enum TypeMessage<S> {
     UnboundVariable,
-    InvalidInputType(Rc<Expr<S, X>>),
-    InvalidOutputType(Rc<Expr<S, X>>),
-    NotAFunction(Rc<Expr<S, X>>, Rc<Expr<S, X>>),
-    TypeMismatch(
-        Rc<Expr<S, X>>,
-        Rc<Expr<S, X>>,
-        Rc<Expr<S, X>>,
-        Rc<Expr<S, X>>,
-    ),
-    AnnotMismatch(Rc<Expr<S, X>>, Rc<Expr<S, X>>, Rc<Expr<S, X>>),
+    InvalidInputType(SubExpr<S, X>),
+    InvalidOutputType(SubExpr<S, X>),
+    NotAFunction(SubExpr<S, X>, SubExpr<S, X>),
+    TypeMismatch(SubExpr<S, X>, SubExpr<S, X>, SubExpr<S, X>, SubExpr<S, X>),
+    AnnotMismatch(SubExpr<S, X>, SubExpr<S, X>, SubExpr<S, X>),
     Untyped,
-    InvalidListElement(usize, Rc<Expr<S, X>>, Rc<Expr<S, X>>, Rc<Expr<S, X>>),
-    InvalidListType(Rc<Expr<S, X>>),
-    InvalidOptionalElement(Rc<Expr<S, X>>, Rc<Expr<S, X>>, Rc<Expr<S, X>>),
+    InvalidListElement(usize, SubExpr<S, X>, SubExpr<S, X>, SubExpr<S, X>),
+    InvalidListType(SubExpr<S, X>),
+    InvalidOptionalElement(SubExpr<S, X>, SubExpr<S, X>, SubExpr<S, X>),
     InvalidOptionalLiteral(usize),
-    InvalidOptionalType(Rc<Expr<S, X>>),
-    InvalidPredicate(Rc<Expr<S, X>>, Rc<Expr<S, X>>),
+    InvalidOptionalType(SubExpr<S, X>),
+    InvalidPredicate(SubExpr<S, X>, SubExpr<S, X>),
     IfBranchMismatch(
-        Rc<Expr<S, X>>,
-        Rc<Expr<S, X>>,
-        Rc<Expr<S, X>>,
-        Rc<Expr<S, X>>,
+        SubExpr<S, X>,
+        SubExpr<S, X>,
+        SubExpr<S, X>,
+        SubExpr<S, X>,
     ),
-    IfBranchMustBeTerm(bool, Rc<Expr<S, X>>, Rc<Expr<S, X>>, Rc<Expr<S, X>>),
-    InvalidField(Label, Rc<Expr<S, X>>),
-    InvalidFieldType(Label, Rc<Expr<S, X>>),
-    InvalidAlternative(Label, Rc<Expr<S, X>>),
-    InvalidAlternativeType(Label, Rc<Expr<S, X>>),
+    IfBranchMustBeTerm(bool, SubExpr<S, X>, SubExpr<S, X>, SubExpr<S, X>),
+    InvalidField(Label, SubExpr<S, X>),
+    InvalidFieldType(Label, SubExpr<S, X>),
+    InvalidAlternative(Label, SubExpr<S, X>),
+    InvalidAlternativeType(Label, SubExpr<S, X>),
     DuplicateAlternative(Label),
-    MustCombineARecord(Rc<Expr<S, X>>, Rc<Expr<S, X>>),
+    MustCombineARecord(SubExpr<S, X>, SubExpr<S, X>),
     FieldCollision(Label),
-    MustMergeARecord(Rc<Expr<S, X>>, Rc<Expr<S, X>>),
-    MustMergeUnion(Rc<Expr<S, X>>, Rc<Expr<S, X>>),
+    MustMergeARecord(SubExpr<S, X>, SubExpr<S, X>),
+    MustMergeUnion(SubExpr<S, X>, SubExpr<S, X>),
     UnusedHandler(HashSet<Label>),
     MissingHandler(HashSet<Label>),
-    HandlerInputTypeMismatch(Label, Rc<Expr<S, X>>, Rc<Expr<S, X>>),
-    HandlerOutputTypeMismatch(Label, Rc<Expr<S, X>>, Rc<Expr<S, X>>),
-    HandlerNotAFunction(Label, Rc<Expr<S, X>>),
-    NotARecord(Label, Rc<Expr<S, X>>, Rc<Expr<S, X>>),
-    MissingField(Label, Rc<Expr<S, X>>),
-    BinOpTypeMismatch(BinOp, Rc<Expr<S, X>>, Rc<Expr<S, X>>),
-    NoDependentLet(Rc<Expr<S, X>>, Rc<Expr<S, X>>),
-    NoDependentTypes(Rc<Expr<S, X>>, Rc<Expr<S, X>>),
+    HandlerInputTypeMismatch(Label, SubExpr<S, X>, SubExpr<S, X>),
+    HandlerOutputTypeMismatch(Label, SubExpr<S, X>, SubExpr<S, X>),
+    HandlerNotAFunction(Label, SubExpr<S, X>),
+    NotARecord(Label, SubExpr<S, X>, SubExpr<S, X>),
+    MissingField(Label, SubExpr<S, X>),
+    BinOpTypeMismatch(BinOp, SubExpr<S, X>, SubExpr<S, X>),
+    NoDependentLet(SubExpr<S, X>, SubExpr<S, X>),
+    NoDependentTypes(SubExpr<S, X>, SubExpr<S, X>),
 }
 
 /// A structured type error that includes context
 #[derive(Debug)]
 pub struct TypeError<S> {
-    pub context: Context<Label, Rc<Expr<S, X>>>,
-    pub current: Rc<Expr<S, X>>,
+    pub context: Context<Label, SubExpr<S, X>>,
+    pub current: SubExpr<S, X>,
     pub type_message: TypeMessage<S>,
 }
 
 impl<S> TypeError<S> {
     pub fn new(
-        context: &Context<Label, Rc<Expr<S, X>>>,
-        current: Rc<Expr<S, X>>,
+        context: &Context<Label, SubExpr<S, X>>,
+        current: SubExpr<S, X>,
         type_message: TypeMessage<S>,
     ) -> Self {
         TypeError {
diff --git a/dhall_core/src/core.rs b/dhall_core/src/core.rs
index 0ea8c83..b49515a 100644
--- a/dhall_core/src/core.rs
+++ b/dhall_core/src/core.rs
@@ -170,7 +170,8 @@ pub type ParsedExpr = SubExpr<X, Import>;
 pub type ResolvedExpr = SubExpr<X, X>;
 pub type DhallExpr = ResolvedExpr;
 
-pub type SubExpr<Note, Embed> = Rc<Expr<Note, Embed>>;
+#[derive(Debug, PartialEq, Eq)]
+pub struct SubExpr<Note, Embed>(pub Rc<Expr<Note, Embed>>);
 
 /// Syntax tree for expressions
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -315,13 +316,33 @@ impl<S: Clone, A: Clone> Expr<S, Expr<S, A>> {
     }
 }
 
+impl<N, E> Clone for SubExpr<N, E> {
+    fn clone(&self) -> Self {
+        SubExpr(Rc::clone(&self.0))
+    }
+}
+
+impl<N, E> std::ops::Deref for SubExpr<N, E> {
+    type Target = Rc<Expr<N, E>>;
+    #[inline(always)]
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+// impl<N, E> SubExpr<N, E> {
+//     pub fn as_ref(&self) -> &Expr<N, E> {
+//         self.0.as_ref()
+//     }
+// }
+
 // Remains of a previous life, where everything was in Boxes
-pub fn bx<T>(x: T) -> Rc<T> {
-    Rc::new(x)
+pub fn bx<N, E>(x: Expr<N, E>) -> SubExpr<N, E> {
+    SubExpr(Rc::new(x))
 }
 
-pub fn rc<T>(x: T) -> Rc<T> {
-    Rc::new(x)
+pub fn rc<N, E>(x: Expr<N, E>) -> SubExpr<N, E> {
+    SubExpr(Rc::new(x))
 }
 
 fn add_ui(u: usize, i: isize) -> usize {
@@ -401,7 +422,7 @@ where
     F2: FnOnce(&Label, &SubExpr<S, A>) -> SubExpr<S, A>,
 {
     match e.as_ref() {
-        Expr::Embed(_) => Rc::clone(e),
+        Expr::Embed(_) => SubExpr::clone(e),
         Expr::Note(_, e) => {
             map_subexpr_rc_binder(e, map_expr, map_under_binder)
         }
@@ -500,8 +521,8 @@ where
 pub fn shift<S, A>(
     delta: isize,
     var: &V,
-    in_expr: &Rc<Expr<S, A>>,
-) -> Rc<Expr<S, A>> {
+    in_expr: &SubExpr<S, A>,
+) -> SubExpr<S, A> {
     use crate::Expr::*;
     let V(x, n) = var;
     let under_binder = |y: &Label, e: &SubExpr<_, _>| {
@@ -529,9 +550,9 @@ pub fn shift<S, A>(
 ///
 pub fn subst<S, A>(
     var: &V,
-    value: &Rc<Expr<S, A>>,
-    in_expr: &Rc<Expr<S, A>>,
-) -> Rc<Expr<S, A>> {
+    value: &SubExpr<S, A>,
+    in_expr: &SubExpr<S, A>,
+) -> SubExpr<S, A> {
     use crate::Expr::*;
     let under_binder = |y: &Label, e: &SubExpr<_, _>| {
         let V(x, n) = var;
@@ -540,7 +561,7 @@ pub fn subst<S, A>(
         subst(newvar, &shift(1, &V(y.clone(), 0), value), e)
     };
     match in_expr.as_ref() {
-        Var(v) if var == v => Rc::clone(value),
+        Var(v) if var == v => SubExpr::clone(value),
         _ => map_subexpr_rc_binder(
             in_expr,
             |e| subst(var, value, e),
diff --git a/dhall_core/src/printer.rs b/dhall_core/src/printer.rs
index 1d1b063..b153d1b 100644
--- a/dhall_core/src/printer.rs
+++ b/dhall_core/src/printer.rs
@@ -8,6 +8,12 @@ impl<S, A: Display> Display for Expr<S, A> {
     }
 }
 
+impl<S, A: Display> Display for SubExpr<S, A> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        self.as_ref().fmt(f)
+    }
+}
+
 //  There is a one-to-one correspondence between the formatter and the grammar. Each phase is
 //  named after a corresponding grammar group, and the structure of the formatter reflects
 //  the relationship between the corresponding grammar rules. This leads to the nice property
diff --git a/dhall_core/src/text.rs b/dhall_core/src/text.rs
index d377877..eef0797 100644
--- a/dhall_core/src/text.rs
+++ b/dhall_core/src/text.rs
@@ -1,18 +1,17 @@
 use crate::*;
 use std::iter::FromIterator;
 use std::ops::Add;
-use std::rc::Rc;
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct InterpolatedText<Note, Embed> {
     head: String,
-    tail: Vec<(Rc<Expr<Note, Embed>>, String)>,
+    tail: Vec<(SubExpr<Note, Embed>, String)>,
 }
 
-impl<N, E> From<(String, Vec<(Rc<Expr<N, E>>, String)>)>
+impl<N, E> From<(String, Vec<(SubExpr<N, E>, String)>)>
     for InterpolatedText<N, E>
 {
-    fn from(x: (String, Vec<(Rc<Expr<N, E>>, String)>)) -> Self {
+    fn from(x: (String, Vec<(SubExpr<N, E>, String)>)) -> Self {
         InterpolatedText {
             head: x.0,
             tail: x.1,
@@ -38,7 +37,7 @@ pub enum InterpolatedTextContents<Note, Embed> {
 impl<N, E> InterpolatedText<N, E> {
     pub fn map<N2, E2, F>(&self, mut f: F) -> InterpolatedText<N2, E2>
     where
-        F: FnMut(&Rc<Expr<N, E>>) -> Rc<Expr<N2, E2>>,
+        F: FnMut(&SubExpr<N, E>) -> SubExpr<N2, E2>,
     {
         InterpolatedText {
             head: self.head.clone(),
@@ -52,7 +51,7 @@ impl<N, E> InterpolatedText<N, E> {
         use std::iter::once;
         once(InterpolatedTextContents::Text(self.head.clone())).chain(
             self.tail.iter().flat_map(|(e, s)| {
-                once(InterpolatedTextContents::Expr(Rc::clone(e)))
+                once(InterpolatedTextContents::Expr(SubExpr::clone(e)))
                     .chain(once(InterpolatedTextContents::Text(s.clone())))
             }),
         )
diff --git a/dhall_generator/src/dhall_expr.rs b/dhall_generator/src/dhall_expr.rs
index 41e558b..e2187d6 100644
--- a/dhall_generator/src/dhall_expr.rs
+++ b/dhall_generator/src/dhall_expr.rs
@@ -4,7 +4,6 @@ use dhall_core::*;
 use proc_macro2::TokenStream;
 use quote::quote;
 use std::collections::BTreeMap;
-use std::rc::Rc;
 
 pub fn dhall_expr(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
     let input_str = input.to_string();
@@ -17,7 +16,7 @@ pub fn dhall_expr(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
 }
 
 // Returns an expression of type Expr<_, _>. Expects interpolated variables
-// to be of type Rc<Expr<_, _>>.
+// to be of type SubExpr<_, _>.
 fn dhall_to_tokenstream(
     expr: &DhallExpr,
     ctx: &Context<Label, ()>,
@@ -97,7 +96,7 @@ fn dhall_to_tokenstream(
     }
 }
 
-// Returns an expression of type Rc<Expr<_, _>>
+// Returns an expression of type SubExpr<_, _>
 fn dhall_to_tokenstream_bx(
     expr: &DhallExpr,
     ctx: &Context<Label, ()>,
@@ -146,7 +145,7 @@ fn label_to_tokenstream(l: &Label) -> TokenStream {
 }
 
 fn map_to_tokenstream(
-    m: &BTreeMap<Label, Rc<Expr<X, X>>>,
+    m: &BTreeMap<Label, SubExpr<X, X>>,
     ctx: &Context<Label, ()>,
 ) -> TokenStream {
     let (keys, values): (Vec<TokenStream>, Vec<TokenStream>) = m
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