refactor(postgres): make better use of traits to improve protocol handling

sqlx-postgres/src/message/row_description.rs

@@ -1,7 +1,8 @@
 use sqlx_core::bytes::{Buf, Bytes};
 
 use crate::error::Error;
-use crate::io::{BufExt, Decode};
+use crate::io::BufExt;
+use crate::message::{BackendMessage, BackendMessageFormat};
 use crate::types::Oid;
 
 #[derive(Debug)]
@@ -40,13 +41,30 @@ pub struct Field {
     pub format: i16,
 }
 
-impl Decode<'_> for RowDescription {
-    fn decode_with(mut buf: Bytes, _: ()) -> Result<Self, Error> {
+impl BackendMessage for RowDescription {
+    const FORMAT: BackendMessageFormat = BackendMessageFormat::RowDescription;
+
+    fn decode_body(mut buf: Bytes) -> Result<Self, Error> {
+        if buf.len() < 2 {
+            return Err(err_protocol!(
+                "expected at least 2 bytes, got {}",
+                buf.len()
+            ));
+        }
+
         let cnt = buf.get_u16();
         let mut fields = Vec::with_capacity(cnt as usize);
 
         for _ in 0..cnt {
             let name = buf.get_str_nul()?.to_owned();
+
+            if buf.len() < 18 {
+                return Err(err_protocol!(
+                    "expected at least 18 bytes after field name {name:?}, got {}",
+                    buf.len()
+                ));
+            }
+
             let relation_id = buf.get_i32();
             let relation_attribute_no = buf.get_i16();
             let data_type_id = Oid(buf.get_u32());