Convert `proc_macro_back_compat` lint to an unconditional error.

We still check for the `rental`/`allsorts-rental` crates. But now if
they are detected we just emit a fatal error, instead of emitting a
warning and providing alternative behaviour.

The original "hack" implementing alternative behaviour was added
in #73345.

The lint was added in #83127.

The tracking issue is #83125.

The direct motivation for the change is that providing the alternative
behaviour is interfering with #125174 and follow-on work.

1 files changed, 14 insertions, 15 deletions

diff --git a/compiler/rustc_expand/src/proc_macro_server.rs b/compiler/rustc_expand/src/proc_macro_server.rs
index ec7e4416b91..c2e799abae8 100644
--- a/compiler/rustc_expand/src/proc_macro_server.rs
+++ b/compiler/rustc_expand/src/proc_macro_server.rs
@@ -276,21 +276,20 @@ impl FromInternal<(TokenStream, &mut Rustc<'_, '_>)> for Vec<TokenTree<TokenStre
 
                 Interpolated(nt) => {
                     let stream = TokenStream::from_nonterminal_ast(&nt);
-                    // A hack used to pass AST fragments to attribute and derive
-                    // macros as a single nonterminal token instead of a token
-                    // stream. Such token needs to be "unwrapped" and not
-                    // represented as a delimited group.
-                    // FIXME: It needs to be removed, but there are some
-                    // compatibility issues (see #73345).
-                    if crate::base::nt_pretty_printing_compatibility_hack(&nt, rustc.ecx.sess) {
-                        trees.extend(Self::from_internal((stream, rustc)));
-                    } else {
-                        trees.push(TokenTree::Group(Group {
-                            delimiter: pm::Delimiter::None,
-                            stream: Some(stream),
-                            span: DelimSpan::from_single(span),
-                        }))
-                    }
+                    // We used to have an alternative behaviour for crates that
+                    // needed it: a hack used to pass AST fragments to
+                    // attribute and derive macros as a single nonterminal
+                    // token instead of a token stream. Such token needs to be
+                    // "unwrapped" and not represented as a delimited group. We
+                    // had a lint for a long time, but now we just emit a hard
+                    // error. Eventually we might remove the special case hard
+                    // error check altogether. See #73345.
+                    crate::base::nt_pretty_printing_compatibility_hack(&nt, rustc.ecx.sess);
+                    trees.push(TokenTree::Group(Group {
+                        delimiter: pm::Delimiter::None,
+                        stream: Some(stream),
+                        span: DelimSpan::from_single(span),
+                    }))
                 }
 
                 OpenDelim(..) | CloseDelim(..) => unreachable!(),