Work around deprecation warning on generated impl for deprecated type

impl/src/expand.rs

@@ -2,7 +2,7 @@ use crate::ast::{Enum, Field, Input, Struct};
 use crate::attr::Trait;
 use crate::generics::InferredBounds;
 use crate::unraw::MemberUnraw;
-use proc_macro2::{Ident, TokenStream};
+use proc_macro2::{Ident, Span, TokenStream};
 use quote::{format_ident, quote, quote_spanned, ToTokens};
 use std::collections::BTreeSet as Set;
 use syn::{DeriveInput, GenericArgument, PathArguments, Result, Token, Type};
@@ -27,7 +27,7 @@ fn try_expand(input: &DeriveInput) -> Result<TokenStream> {
 }
 
 fn fallback(input: &DeriveInput, error: syn::Error) -> TokenStream {
-    let ty = &input.ident;
+    let ty = call_site_ident(&input.ident);
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
 
     let error = error.to_compile_error();
@@ -55,7 +55,7 @@ fn fallback(input: &DeriveInput, error: syn::Error) -> TokenStream {
 }
 
 fn impl_struct(input: Struct) -> TokenStream {
-    let ty = &input.ident;
+    let ty = call_site_ident(&input.ident);
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
     let mut error_inferred_bounds = InferredBounds::new();
 
@@ -228,7 +228,7 @@ fn impl_struct(input: Struct) -> TokenStream {
 }
 
 fn impl_enum(input: Enum) -> TokenStream {
-    let ty = &input.ident;
+    let ty = call_site_ident(&input.ident);
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
     let mut error_inferred_bounds = InferredBounds::new();
 
@@ -492,6 +492,14 @@ fn impl_enum(input: Enum) -> TokenStream {
     }
 }
 
+// Create an ident with which we can expand `impl Trait for #ident {}` on a
+// deprecated type without triggering deprecation warning on the generated impl.
+fn call_site_ident(ident: &Ident) -> Ident {
+    let mut ident = ident.clone();
+    ident.set_span(Span::call_site());
+    ident
+}
+
 fn fields_pat(fields: &[Field]) -> TokenStream {
     let mut members = fields.iter().map(|field| &field.member).peekable();
     match members.peek() {

tests/ui/missing-display.stderr

@@ -1,8 +1,8 @@
 error[E0277]: `MyError` doesn't implement `std::fmt::Display`
- --> tests/ui/missing-display.rs:4:10
+ --> tests/ui/missing-display.rs:3:10
   |
-4 | pub enum MyError {
-  |          ^^^^^^^ `MyError` cannot be formatted with the default formatter
+3 | #[derive(Error, Debug)]
+  |          ^^^^^ `MyError` cannot be formatted with the default formatter
   |
   = help: the trait `std::fmt::Display` is not implemented for `MyError`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
@@ -11,3 +11,4 @@ note: required by a bound in `std::error::Error`
   |
   | pub trait Error: Debug + Display {
   |                          ^^^^^^^ required by this bound in `Error`
+  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)