6 files changed, 48 insertions, 8 deletions
diff --git a/dhall/src/syntax/ast/expr.rs b/dhall/src/syntax/ast/expr.rs
index 48c48d8..fe49448 100644
--- a/dhall/src/syntax/ast/expr.rs
+++ b/dhall/src/syntax/ast/expr.rs
@@ -163,6 +163,8 @@ pub enum ExprKind<SubExpr, Embed> {
     Projection(SubExpr, DupTreeSet<Label>),
     ///  `e.(t)`
     ProjectionByExpr(SubExpr, SubExpr),
+    ///  `x::y`
+    Completion(SubExpr, SubExpr),
     /// `./some/path`
     Import(Import<SubExpr>),
     /// Embeds the result of resolving an import
diff --git a/dhall/src/syntax/ast/visitor.rs b/dhall/src/syntax/ast/visitor.rs
index b557995..424048b 100644
--- a/dhall/src/syntax/ast/visitor.rs
+++ b/dhall/src/syntax/ast/visitor.rs
@@ -164,6 +164,9 @@ where
         ProjectionByExpr(e, x) => {
             ProjectionByExpr(v.visit_subexpr(e)?, v.visit_subexpr(x)?)
         }
+        Completion(e, x) => {
+            Completion(v.visit_subexpr(e)?, v.visit_subexpr(x)?)
+        }
         Assert(e) => Assert(v.visit_subexpr(e)?),
         Import(i) => Import(i.traverse_ref(|e| v.visit_subexpr(e))?),
         Embed(a) => Embed(v.visit_embed(a)?),
@@ -281,6 +284,10 @@ where
             v.visit_subexpr(e)?;
             v.visit_subexpr(x)?;
         }
+        Completion(x, y) => {
+            v.visit_subexpr(x)?;
+            v.visit_subexpr(y)?;
+        }
         Assert(e) => v.visit_subexpr(e)?,
         Import(i) => i.traverse_mut(|e| v.visit_subexpr(e))?,
         Embed(a) => v.visit_embed(a)?,
diff --git a/dhall/src/syntax/binary/decode.rs b/dhall/src/syntax/binary/decode.rs
index 254ab07..c18deb5 100644
--- a/dhall/src/syntax/binary/decode.rs
+++ b/dhall/src/syntax/binary/decode.rs
@@ -98,6 +98,11 @@ fn cbor_value_to_dhall(data: &cbor::Value) -> Result<DecodedExpr, DecodeError> {
                 let l = Label::from(l.as_str());
                 Pi(l, x, y)
             }
+            [U64(3), U64(13), x, y] => {
+                let x = cbor_value_to_dhall(&x)?;
+                let y = cbor_value_to_dhall(&y)?;
+                Completion(x, y)
+            }
             [U64(3), U64(n), x, y] => {
                 let x = cbor_value_to_dhall(&x)?;
                 let y = cbor_value_to_dhall(&y)?;
diff --git a/dhall/src/syntax/binary/encode.rs b/dhall/src/syntax/binary/encode.rs
index 25a545c..867fc9c 100644
--- a/dhall/src/syntax/binary/encode.rs
+++ b/dhall/src/syntax/binary/encode.rs
@@ -164,6 +164,9 @@ where
         ProjectionByExpr(x, y) => {
             ser_seq!(ser; tag(10), expr(x), vec![expr(y)])
         }
+        Completion(x, y) => {
+            ser_seq!(ser; tag(3), tag(13), expr(x), expr(y))
+        }
         Import(import) => serialize_import(ser, import),
         Embed(_) => unimplemented!(
             "An expression with resolved imports cannot be binary-encoded"
diff --git a/dhall/src/syntax/text/parser.rs b/dhall/src/syntax/text/parser.rs
index 832472b..fe34aee 100644
--- a/dhall/src/syntax/text/parser.rs
+++ b/dhall/src/syntax/text/parser.rs
@@ -766,6 +766,25 @@ impl DhallParser {
     }
 
     #[alias(expression, shortcut = true)]
+    fn completion_expression(input: ParseInput) -> ParseResult<Expr> {
+        Ok(match_nodes!(input.children();
+            [expression(e)] => e,
+            [expression(first), expression(rest)..] => {
+                rest.fold(
+                    first,
+                    |acc, e| {
+                        spanned_union(
+                            acc.span(),
+                            e.span(),
+                            Completion(acc, e),
+                        )
+                    }
+                )
+            },
+        ))
+    }
+
+    #[alias(expression, shortcut = true)]
     fn selector_expression(input: ParseInput) -> ParseResult<Expr> {
         Ok(match_nodes!(input.children();
             [expression(e)] => e,
diff --git a/dhall/src/syntax/text/printer.rs b/dhall/src/syntax/text/printer.rs
index dff3479..759e7e6 100644
--- a/dhall/src/syntax/text/printer.rs
+++ b/dhall/src/syntax/text/printer.rs
@@ -64,6 +64,7 @@ impl<E: Display + Clone> UnspannedExpr<E> {
             Field(a, b) => Field(a.phase(Primitive), b),
             Projection(e, ls) => Projection(e.phase(Primitive), ls),
             ProjectionByExpr(a, b) => ProjectionByExpr(a.phase(Primitive), b),
+            Completion(a, b) => Completion(a.phase(Primitive), b.phase(Primitive)),
             e => e,
         }
     }
@@ -89,7 +90,7 @@ impl<E: Display + Clone> UnspannedExpr<E> {
             // Precedence is magically handled by the ordering of BinOps.
             ExprKind::BinOp(op, _, _) => phase > PrintPhase::BinOp(*op),
             ExprKind::App(_, _) => phase > PrintPhase::App,
-            Field(_, _) | Projection(_, _) | ProjectionByExpr(_, _) => {
+            Field(_, _) | Projection(_, _) | ProjectionByExpr(_, _) | Completion(_, _) => {
                 phase > PrintPhase::Import
             }
             _ => false,
@@ -189,13 +190,6 @@ impl<SE: Display + Clone, E: Display> Display for ExprKind<SE, E> {
             Field(a, b) => {
                 write!(f, "{}.{}", a, b)?;
             }
-            Projection(e, ls) => {
-                write!(f, "{}.", e)?;
-                fmt_list("{ ", ", ", " }", ls, f, Display::fmt)?;
-            }
-            ProjectionByExpr(a, b) => {
-                write!(f, "{}.({})", a, b)?;
-            }
             Var(a) => a.fmt(f)?,
             Const(k) => k.fmt(f)?,
             Builtin(v) => v.fmt(f)?,
@@ -224,6 +218,16 @@ impl<SE: Display + Clone, E: Display> Display for ExprKind<SE, E> {
                 }
                 Ok(())
             })?,
+            Projection(e, ls) => {
+                write!(f, "{}.", e)?;
+                fmt_list("{ ", ", ", " }", ls, f, Display::fmt)?;
+            }
+            ProjectionByExpr(a, b) => {
+                write!(f, "{}.({})", a, b)?;
+            }
+            Completion(a, b) => {
+                write!(f, "{}::{}", a, b)?;
+            }
             Import(a) => a.fmt(f)?,
             Embed(a) => a.fmt(f)?,
         }