add tests for postgres struct field encoding

Cargo.toml

@@ -112,5 +112,9 @@ required-features = [ "mysql" ]
 name = "derives"
 required-features = [ "macros" ]
 
+[[test]]
+name = "postgres-struct"
+required-features = [ "postgres" ]
+
 [profile.release]
 lto = true

tests/postgres-struct.rs

@@ -0,0 +1,39 @@
+use sqlx::encode::Encode;
+use sqlx::postgres::{decode_struct_field, encode_struct_field};
+use sqlx::types::HasSqlType;
+use sqlx::Postgres;
+use std::convert::TryInto;
+
+#[test]
+fn test_encode_field() {
+    let value = "Foo Bar";
+    let mut raw_encoded = Vec::new();
+    <&str as Encode<Postgres>>::encode(&value, &mut raw_encoded);
+    let mut field_encoded = Vec::new();
+    encode_struct_field(&mut field_encoded, &value);
+
+    // check oid
+    let oid = <Postgres as HasSqlType<&str>>::type_info().oid();
+    let field_encoded_oid = u32::from_be_bytes(field_encoded[0..4].try_into().unwrap());
+    assert_eq!(oid, field_encoded_oid);
+
+    // check length
+    let field_encoded_length = u32::from_be_bytes(field_encoded[4..8].try_into().unwrap());
+    assert_eq!(raw_encoded.len(), field_encoded_length as usize);
+
+    // check data
+    assert_eq!(raw_encoded, &field_encoded[8..]);
+}
+
+#[test]
+fn test_decode_field() {
+    let value = "Foo Bar".to_string();
+
+    let mut buf = Vec::new();
+    encode_struct_field(&mut buf, &value);
+
+    let mut buf = buf.as_slice();
+    let value_decoded: String = decode_struct_field(&mut buf).unwrap();
+    assert_eq!(value_decoded, value);
+    assert!(buf.is_empty());
+}