Enforce in type system that a Bundle contains at least one Action

Cargo.toml

@@ -20,6 +20,7 @@ rustdoc-args = [ "--html-in-header", "katex-header.html" ]
 
 [dependencies]
 halo2 = { git = "https://github.com/zcash/halo2.git", branch = "main" }
+nonempty = "0.6"
 
 [dev-dependencies]
 criterion = "0.3"

src/bundle.rs

@@ -1,5 +1,7 @@
 //! Structs related to bundles of Orchard actions.
 
+use nonempty::NonEmpty;
+
 use crate::{
     circuit::Proof,
     note::{EncryptedNote, NoteCommitment, Nullifier},
@@ -36,7 +38,7 @@ pub struct Action {
 #[derive(Debug)]
 pub struct Bundle {
     anchor: Anchor,
-    actions: Vec<Action>,
+    actions: NonEmpty<Action>,
     value_balance: ValueSum,
 }