diff --git a/prost-derive/src/field/map.rs b/prost-derive/src/field/map.rs
index 4855cc5c6..a8ccf0555 100644
--- a/prost-derive/src/field/map.rs
+++ b/prost-derive/src/field/map.rs
@@ -367,6 +367,7 @@ fn key_ty_from_str(s: &str) -> Result<scalar::Ty, Error> {
         | scalar::Ty::Sfixed32
         | scalar::Ty::Sfixed64
         | scalar::Ty::Bool
+        | scalar::Ty::CowStr
         | scalar::Ty::String => Ok(ty),
         _ => bail!("invalid map key type: {}", s),
     }
diff --git a/prost-derive/src/field/scalar.rs b/prost-derive/src/field/scalar.rs
index c2e870524..d6cae075d 100644
--- a/prost-derive/src/field/scalar.rs
+++ b/prost-derive/src/field/scalar.rs
@@ -194,6 +194,8 @@ impl Field {
             Kind::Plain(ref default) | Kind::Required(ref default) => {
                 let default = default.typed();
                 match self.ty {
+                    Ty::CowStr => quote!(#ident = ::prost::alloc::borrow::Cow::Borrowed("")),
+                    Ty::CowBytes => quote!(#ident = ::prost::alloc::borrow::Cow::Borrowed(&[])),
                     Ty::String | Ty::Bytes(..) => quote!(#ident.clear()),
                     _ => quote!(#ident = #default),
                 }
@@ -398,6 +400,8 @@ pub enum Ty {
     Sfixed64,
     Bool,
     String,
+    CowStr,
+    CowBytes,
     Bytes(BytesTy),
     Enumeration(Path),
 }
@@ -442,6 +446,8 @@ impl Ty {
             Meta::Path(ref name) if name.is_ident("sfixed64") => Ty::Sfixed64,
             Meta::Path(ref name) if name.is_ident("bool") => Ty::Bool,
             Meta::Path(ref name) if name.is_ident("string") => Ty::String,
+            Meta::Path(ref name) if name.is_ident("cow_str") => Ty::CowStr,
+            Meta::Path(ref name) if name.is_ident("cow_bytes") => Ty::CowBytes,
             Meta::Path(ref name) if name.is_ident("bytes") => Ty::Bytes(BytesTy::Vec),
             Meta::NameValue(MetaNameValue {
                 ref path,
@@ -486,6 +492,8 @@ impl Ty {
             "sfixed32" => Ty::Sfixed32,
             "sfixed64" => Ty::Sfixed64,
             "bool" => Ty::Bool,
+            "cow_str" => Ty::CowStr,
+            "cow_bytes" => Ty::CowBytes,
             "string" => Ty::String,
             "bytes" => Ty::Bytes(BytesTy::Vec),
             s if s.len() > enumeration_len && &s[..enumeration_len] == "enumeration" => {
@@ -523,6 +531,8 @@ impl Ty {
             Ty::Sfixed64 => "sfixed64",
             Ty::Bool => "bool",
             Ty::String => "string",
+            Ty::CowStr => "cow_str",
+            Ty::CowBytes => "cow_bytes",
             Ty::Bytes(..) => "bytes",
             Ty::Enumeration(..) => "enum",
         }
@@ -531,6 +541,8 @@ impl Ty {
     // TODO: rename to 'owned_type'.
     pub fn rust_type(&self) -> TokenStream {
         match self {
+            Ty::CowStr => quote!(::prost::alloc::borrow::Cow<'a, str>),
+            Ty::CowBytes => quote!(::prost::alloc::borrow::Cow<'a, [u8]>),
             Ty::String => quote!(::prost::alloc::string::String),
             Ty::Bytes(ty) => ty.rust_type(),
             _ => self.rust_ref_type(),
@@ -553,8 +565,8 @@ impl Ty {
             Ty::Sfixed32 => quote!(i32),
             Ty::Sfixed64 => quote!(i64),
             Ty::Bool => quote!(bool),
-            Ty::String => quote!(&str),
-            Ty::Bytes(..) => quote!(&[u8]),
+            Ty::CowStr | Ty::String => quote!(&str),
+            Ty::CowBytes | Ty::Bytes(..) => quote!(&[u8]),
             Ty::Enumeration(..) => quote!(i32),
         }
     }
@@ -568,7 +580,7 @@ impl Ty {
 
     /// Returns false if the scalar type is length delimited (i.e., `string` or `bytes`).
     pub fn is_numeric(&self) -> bool {
-        !matches!(self, Ty::String | Ty::Bytes(..))
+        !matches!(self, Ty::CowStr | Ty::String | Ty::CowBytes | Ty::Bytes(..))
     }
 }
 
@@ -610,6 +622,8 @@ pub enum DefaultValue {
     U64(u64),
     Bool(bool),
     String(String),
+    CowStr(std::borrow::Cow<'static, str>),
+    CowBytes(Vec<u8>),
     Bytes(Vec<u8>),
     Enumeration(TokenStream),
     Path(Path),
@@ -774,6 +788,8 @@ impl DefaultValue {
 
             Ty::Bool => DefaultValue::Bool(false),
             Ty::String => DefaultValue::String(String::new()),
+            Ty::CowStr => DefaultValue::CowStr(std::borrow::Cow::Borrowed("")),
+            Ty::CowBytes => DefaultValue::CowBytes(Vec::new()),
             Ty::Bytes(..) => DefaultValue::Bytes(Vec::new()),
             Ty::Enumeration(ref path) => DefaultValue::Enumeration(quote!(#path::default())),
         }
@@ -785,6 +801,17 @@ impl DefaultValue {
                 quote!(::prost::alloc::string::String::new())
             }
             DefaultValue::String(ref value) => quote!(#value.into()),
+            DefaultValue::CowStr(ref value) if value.is_empty() => {
+                quote!(::prost::alloc::borrow::Cow::Borrowed(""))
+            }
+            DefaultValue::CowStr(ref value) => quote!(#value.into()),
+            DefaultValue::CowBytes(ref value) if value.is_empty() => {
+                quote!(::core::default::Default::default())
+            }
+            DefaultValue::CowBytes(ref value) => {
+                let lit = LitByteStr::new(value, Span::call_site());
+                quote!(#lit.as_ref().into())
+            }
             DefaultValue::Bytes(ref value) if value.is_empty() => {
                 quote!(::core::default::Default::default())
             }
@@ -817,6 +844,11 @@ impl ToTokens for DefaultValue {
             DefaultValue::U64(value) => value.to_tokens(tokens),
             DefaultValue::Bool(value) => value.to_tokens(tokens),
             DefaultValue::String(ref value) => value.to_tokens(tokens),
+            DefaultValue::CowStr(ref value) => value.to_tokens(tokens),
+            DefaultValue::CowBytes(ref value) => {
+                let byte_str = LitByteStr::new(value, Span::call_site());
+                tokens.append_all(quote!(#byte_str as &[u8]));
+            }
             DefaultValue::Bytes(ref value) => {
                 let byte_str = LitByteStr::new(value, Span::call_site());
                 tokens.append_all(quote!(#byte_str as &[u8]));