diff --git a/autogen/Cargo.toml b/autogen/Cargo.toml
index 43c85fd..10a0174 100644
--- a/autogen/Cargo.toml
+++ b/autogen/Cargo.toml
@@ -6,7 +6,6 @@ authors = [
 	"Lei Zhang <antiagainst@gmail.com>",
 ]
 edition = "2018"
-rust-version = "1.58"
 
 publish = false
 
diff --git a/autogen/src/dr.rs b/autogen/src/dr.rs
index bc5b412..90ecbd2 100644
--- a/autogen/src/dr.rs
+++ b/autogen/src/dr.rs
@@ -391,7 +391,7 @@ pub fn gen_dr_operand_kinds(grammar: &[structs::OperandKind]) -> TokenStream {
                 let mut seen_discriminator = BTreeMap::new();
 
                 for e in enumerators {
-                    if seen_discriminator.get(&e.value).is_none() {
+                    if let std::collections::btree_map::Entry::Vacant(seen_entry) = seen_discriminator.entry(e.value) {
                         let name = match category {
                             structs::Category::BitEnum => {
                                 use heck::ShoutySnakeCase;
@@ -412,7 +412,7 @@ pub fn gen_dr_operand_kinds(grammar: &[structs::OperandKind]) -> TokenStream {
                             _ => panic!("Unexpected operand type"),
                         };
 
-                        seen_discriminator.insert(e.value, name.clone());
+                        seen_entry.insert(name.clone());
 
                         capability_clauses
                             .entry(&e.capabilities)