Mark method receivers in builtin derives as being from the derive.

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -1052,7 +1052,7 @@ fn create_method(
 
         let args = {
             let self_arg = explicit_self.map(|explicit_self| {
-                let ident = Ident::with_dummy_span(kw::SelfLower).with_span_pos(span);
+                let ident = Ident::new(kw::SelfLower, span);
                 ast::Param::from_self(ast::AttrVec::default(), explicit_self, ident)
             });
             let nonself_args =

compiler/rustc_expand/src/build.rs

@@ -315,7 +315,7 @@ pub fn expr_ident(&self, span: Span, id: Ident) -> Box<ast::Expr> {
         self.expr_path(self.path_ident(span, id))
     }
     pub fn expr_self(&self, span: Span) -> Box<ast::Expr> {
-        self.expr_ident(span, Ident::with_dummy_span(kw::SelfLower))
+        self.expr_ident(span, Ident::new(kw::SelfLower, span))
     }
 
     pub fn expr_macro_call(&self, span: Span, call: Box<ast::MacCall>) -> Box<ast::Expr> {

tests/ui/associated-types/issue-38821.stderr

@@ -272,6 +272,7 @@ LL | impl<T: NotNull> IntoNullable for T {
    |         -------  ^^^^^^^^^^^^     ^
    |         |
    |         unsatisfied trait bound introduced here
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
   --> $DIR/issue-38821.rs:23:23
@@ -301,6 +302,7 @@ LL | impl<T: NotNull> IntoNullable for T {
    |         -------  ^^^^^^^^^^^^     ^
    |         |
    |         unsatisfied trait bound introduced here
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
   --> $DIR/issue-38821.rs:23:10