Don't use deprecated diagnostic library.

ouroboros_macro/Cargo.toml

@@ -15,7 +15,7 @@ proc-macro = true
 heck = "0.4.1"
 itertools = "0.12.0"
 proc-macro2 = "1.0"
-proc-macro-error = "1.0.4"
+proc-macro2-diagnostics = "0.10"
 quote = "1.0"
 syn = { version = "2.0", features = ["full"] }
 

ouroboros_macro/src/generate/with_each.rs

@@ -1,10 +1,17 @@
 use crate::info_structures::{FieldType, Options, StructInfo};
 use proc_macro2::TokenStream;
+use proc_macro2_diagnostics::Diagnostic;
 use quote::{format_ident, quote};
 use syn::Error;
 
-pub fn make_with_functions(info: &StructInfo, options: Options) -> Result<Vec<TokenStream>, Error> {
+pub enum ProcessingError {
+    Syntax(Error),
+    Covariance(Vec<Diagnostic>),
+}
+
+pub fn make_with_functions(info: &StructInfo, options: Options) -> (Vec<TokenStream>, Vec<Diagnostic>) {
     let mut users = Vec::new();
+    let mut errors = Vec::new();
     for field in &info.fields {
         let visibility = &field.vis;
         let field_name = &field.name;
@@ -50,7 +57,7 @@ pub fn make_with_functions(info: &StructInfo, options: Options) -> Result<Vec<To
                     }
                 });
             } else if field.covariant.is_none() {
-                field.covariance_error();
+                errors.push(field.covariance_error());
             }
             // If it is not borrowed at all it's safe to allow mutably borrowing it.
             let user_name = format_ident!("with_{}_mut", &field.name);
@@ -113,7 +120,7 @@ pub fn make_with_functions(info: &StructInfo, options: Options) -> Result<Vec<To
                     // Skip the other functions, they will cause compiler errors.
                     continue;
                 } else if field.covariant.is_none() {
-                    field.covariance_error();
+                    errors.push(field.covariance_error());
                 }
             }
             let borrower_name = format_ident!("borrow_{}", &field.name);
@@ -131,5 +138,5 @@ pub fn make_with_functions(info: &StructInfo, options: Options) -> Result<Vec<To
             // to get any other kinds of references to it.
         }
     }
-    Ok(users)
+    (users, errors)
 }

ouroboros_macro/src/info_structures.rs

@@ -1,9 +1,10 @@
 use crate::utils::{make_generic_arguments, make_generic_consumers, replace_this_with_lifetime};
 use proc_macro2::{Ident, TokenStream};
+use proc_macro2_diagnostics::{Diagnostic, SpanDiagnosticExt};
 use quote::{format_ident, quote, ToTokens};
 use syn::{
     punctuated::Punctuated, token::Comma, Attribute, ConstParam, Error, GenericParam, Generics,
-    LifetimeParam, Type, TypeParam, Visibility, 
+    LifetimeParam, Type, TypeParam, Visibility, spanned::Spanned, 
 };
 
 #[derive(Clone, Copy)]
@@ -213,7 +214,8 @@ impl StructFieldInfo {
 
     /// Generates an error requesting that the user explicitly specify whether or not the
     /// field's type is covariant.
-    pub fn covariance_error(&self) {
+    #[must_use]
+    pub fn covariance_error(&self) -> Diagnostic {
         let error = concat!(
             "Ouroboros cannot automatically determine if this type is covariant.\n\n",
             "As an example, a Box<&'this ()> is covariant because it can be used as a\n",
@@ -224,7 +226,7 @@ impl StructFieldInfo {
             "guarantee means the value is not covariant.\n\n",
             "To resolve this error, add #[covariant] or #[not_covariant] to the field.\n",
         );
-        proc_macro_error::emit_error!(self.typ, error);
+        self.typ.span().error(error)
     }
 
     pub fn make_constructor_arg_type_impl(

ouroboros_macro/src/lib.rs

@@ -22,10 +22,10 @@ use generate::{
 };
 use heck::ToSnakeCase;
 use info_structures::BuilderType;
+use itertools::Itertools;
 use proc_macro::TokenStream;
 use proc_macro2::TokenStream as TokenStream2;
 use proc_macro2::TokenTree;
-use proc_macro_error::proc_macro_error;
 use quote::{format_ident, quote};
 use syn::{Error, ItemStruct};
 
@@ -61,7 +61,11 @@ fn self_referencing_impl(
         async_send_try_constructor_def,
     ) = create_try_builder_and_constructor(&info, options, BuilderType::AsyncSend)?;
 
-    let with_defs = make_with_functions(&info, options)?;
+    let (with_defs, with_errors) = make_with_functions(&info, options);
+    let with_errors = with_errors
+        .into_iter()
+        .map(|err| err.emit_as_expr_tokens())
+        .collect_vec();
     let (with_all_struct_def, with_all_fn_def) = make_with_all_function(&info, options)?;
     let (with_all_mut_struct_def, with_all_mut_fn_def) =
         make_with_all_mut_function(&info, options)?;
@@ -99,6 +103,7 @@ fn self_referencing_impl(
             #async_send_try_builder_def
             #with_all_struct_def
             #with_all_mut_struct_def
+            #(#with_errors)*
             #heads_struct_def
             #impls
             impl <#generic_params> #struct_name <#(#generic_args),*> #generic_where {
@@ -125,7 +130,6 @@ fn self_referencing_impl(
     }))
 }
 
-#[proc_macro_error]
 #[proc_macro_attribute]
 pub fn self_referencing(attr: TokenStream, item: TokenStream) -> TokenStream {
     let mut options = Options {