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.

compiler/rustc_expand/messages.ftl

@@ -124,6 +124,9 @@ expand_not_a_meta_item =
 expand_only_one_word =
     must only be one word
 
+expand_proc_macro_back_compat = using an old version of `{$crate_name}`
+    .note = older versions of the `{$crate_name}` crate no longer compile; please update to `{$crate_name}` v{$fixed_version}, or switch to one of the `{$crate_name}` alternatives
+
 expand_proc_macro_derive_panicked =
     proc-macro derive panicked
     .help = message: {$message}

compiler/rustc_expand/src/base.rs

@@ -14,8 +14,7 @@ use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::sync::{self, Lrc};
 use rustc_errors::{DiagCtxt, ErrorGuaranteed, PResult};
 use rustc_feature::Features;
-use rustc_lint_defs::builtin::PROC_MACRO_BACK_COMPAT;
-use rustc_lint_defs::{BufferedEarlyLint, BuiltinLintDiag, RegisteredTools};
+use rustc_lint_defs::{BufferedEarlyLint, RegisteredTools};
 use rustc_parse::{parser, MACRO_ARGUMENTS};
 use rustc_session::config::CollapseMacroDebuginfo;
 use rustc_session::{parse::ParseSess, Limit, Session};
@@ -1330,13 +1329,10 @@ pub fn parse_macro_name_and_helper_attrs(
     Some((trait_ident.name, proc_attrs))
 }
 
-/// This nonterminal looks like some specific enums from
-/// `proc-macro-hack` and `procedural-masquerade` crates.
-/// We need to maintain some special pretty-printing behavior for them due to incorrect
-/// asserts in old versions of those crates and their wide use in the ecosystem.
-/// See issue #73345 for more details.
+/// If this item looks like a specific enums from `rental`, emit a fatal error.
+/// See #73345 and #83125 for more details.
 /// FIXME(#73933): Remove this eventually.
-fn pretty_printing_compatibility_hack(item: &Item, sess: &Session) -> bool {
+fn pretty_printing_compatibility_hack(item: &Item, sess: &Session) {
     let name = item.ident.name;
     if name == sym::ProceduralMasqueradeDummyType
         && let ast::ItemKind::Enum(enum_def, _) = &item.kind
@@ -1360,41 +1356,36 @@ fn pretty_printing_compatibility_hack(item: &Item, sess: &Session) -> bool {
         };
 
         if crate_matches {
-            sess.psess.buffer_lint(
-                PROC_MACRO_BACK_COMPAT,
-                item.ident.span,
-                ast::CRATE_NODE_ID,
-                BuiltinLintDiag::ProcMacroBackCompat {
-                    crate_name: "rental".to_string(),
-                    fixed_version: "0.5.6".to_string(),
-                },
-            );
-            return true;
+            // FIXME: make this translatable
+            #[allow(rustc::untranslatable_diagnostic)]
+            sess.psess.dcx.emit_fatal(errors::ProcMacroBackCompat {
+                crate_name: "rental".to_string(),
+                fixed_version: "0.5.6".to_string(),
+            });
         }
     }
-    false
 }
 
-pub(crate) fn ann_pretty_printing_compatibility_hack(ann: &Annotatable, sess: &Session) -> bool {
+pub(crate) fn ann_pretty_printing_compatibility_hack(ann: &Annotatable, sess: &Session) {
     let item = match ann {
         Annotatable::Item(item) => item,
         Annotatable::Stmt(stmt) => match &stmt.kind {
             ast::StmtKind::Item(item) => item,
-            _ => return false,
+            _ => return,
         },
-        _ => return false,
+        _ => return,
     };
     pretty_printing_compatibility_hack(item, sess)
 }
 
-pub(crate) fn nt_pretty_printing_compatibility_hack(nt: &Nonterminal, sess: &Session) -> bool {
+pub(crate) fn nt_pretty_printing_compatibility_hack(nt: &Nonterminal, sess: &Session) {
     let item = match nt {
         Nonterminal::NtItem(item) => item,
         Nonterminal::NtStmt(stmt) => match &stmt.kind {
             ast::StmtKind::Item(item) => item,
-            _ => return false,
+            _ => return,
         },
-        _ => return false,
+        _ => return,
     };
     pretty_printing_compatibility_hack(item, sess)
 }

compiler/rustc_expand/src/errors.rs

@@ -440,3 +440,13 @@ pub(crate) struct EmptyDelegationList {
     #[primary_span]
     pub span: Span,
 }
+
+// This used to be the `proc_macro_back_compat` lint (#83125). It was later
+// turned into a hard error.
+#[derive(Diagnostic)]
+#[diag(expand_proc_macro_back_compat)]
+#[note]
+pub struct ProcMacroBackCompat {
+    pub crate_name: String,
+    pub fixed_version: String,
+}

compiler/rustc_expand/src/proc_macro.rs

@@ -4,14 +4,12 @@ use crate::proc_macro_server;
 
 use rustc_ast as ast;
 use rustc_ast::ptr::P;
-use rustc_ast::token;
 use rustc_ast::tokenstream::TokenStream;
-use rustc_data_structures::sync::Lrc;
 use rustc_errors::ErrorGuaranteed;
 use rustc_parse::parser::ForceCollect;
 use rustc_session::config::ProcMacroExecutionStrategy;
 use rustc_span::profiling::SpannedEventArgRecorder;
-use rustc_span::{Span, DUMMY_SP};
+use rustc_span::Span;
 
 struct MessagePipe<T> {
     tx: std::sync::mpsc::SyncSender<T>,
@@ -120,18 +118,13 @@ impl MultiItemModifier for DeriveProcMacro {
         // We need special handling for statement items
         // (e.g. `fn foo() { #[derive(Debug)] struct Bar; }`)
         let is_stmt = matches!(item, Annotatable::Stmt(..));
-        let hack = crate::base::ann_pretty_printing_compatibility_hack(&item, &ecx.sess);
-        let input = if hack {
-            let nt = match item {
-                Annotatable::Item(item) => token::NtItem(item),
-                Annotatable::Stmt(stmt) => token::NtStmt(stmt),
-                _ => unreachable!(),
-            };
-            TokenStream::token_alone(token::Interpolated(Lrc::new(nt)), DUMMY_SP)
-        } else {
-            item.to_tokens()
-        };
 
+        // We used to have an alternative behaviour for crates that needed it.
+        // 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::ann_pretty_printing_compatibility_hack(&item, &ecx.sess);
+        let input = item.to_tokens();
         let stream = {
             let _timer =
                 ecx.sess.prof.generic_activity_with_arg_recorder("expand_proc_macro", |recorder| {

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!(),

compiler/rustc_lint/messages.ftl

@@ -629,9 +629,6 @@ lint_pattern_in_foreign = patterns aren't allowed in foreign function declaratio
 lint_private_extern_crate_reexport =
     extern crate `{$ident}` is private, and cannot be re-exported, consider declaring with `pub`
 
-lint_proc_macro_back_compat = using an old version of `{$crate_name}`
-    .note = older versions of the `{$crate_name}` crate will stop compiling in future versions of Rust; please update to `{$crate_name}` v{$fixed_version}, or switch to one of the `{$crate_name}` alternatives
-
 lint_proc_macro_derive_resolution_fallback = cannot find {$ns} `{$ident}` in this scope
     .label = names from parent modules are not accessible without an explicit import
 

compiler/rustc_lint/src/context/diagnostics.rs

@@ -191,13 +191,6 @@ pub(super) fn emit_buffered_lint(
         BuiltinLintDiag::LegacyDeriveHelpers(label_span) => {
             ctx.emit_span_lint(lint, span, lints::LegacyDeriveHelpers { span: label_span });
         }
-        BuiltinLintDiag::ProcMacroBackCompat { crate_name, fixed_version } => {
-            ctx.emit_span_lint(
-                lint,
-                span,
-                lints::ProcMacroBackCompat { crate_name, fixed_version },
-            );
-        }
         BuiltinLintDiag::OrPatternsBackCompat(suggestion_span, suggestion) => {
             ctx.emit_span_lint(
                 lint,

compiler/rustc_lint/src/lints.rs

@@ -2571,14 +2571,6 @@ pub struct LegacyDeriveHelpers {
     pub span: Span,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(lint_proc_macro_back_compat)]
-#[note]
-pub struct ProcMacroBackCompat {
-    pub crate_name: String,
-    pub fixed_version: String,
-}
-
 #[derive(LintDiagnostic)]
 #[diag(lint_or_patterns_back_compat)]
 pub struct OrPatternsBackCompat {

compiler/rustc_lint_defs/src/builtin.rs

@@ -77,7 +77,6 @@ declare_lint_pass! {
         POINTER_STRUCTURAL_MATCH,
         PRIVATE_BOUNDS,
         PRIVATE_INTERFACES,
-        PROC_MACRO_BACK_COMPAT,
         PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
         PUB_USE_OF_PRIVATE_EXTERN_CRATE,
         REDUNDANT_LIFETIMES,
@@ -3792,53 +3791,6 @@ declare_lint! {
     "detects invalid `#[doc(...)]` attributes",
 }
 
-declare_lint! {
-    /// The `proc_macro_back_compat` lint detects uses of old versions of certain
-    /// proc-macro crates, which have hardcoded workarounds in the compiler.
-    ///
-    /// ### Example
-    ///
-    /// ```rust,ignore (needs-dependency)
-    ///
-    /// use time_macros_impl::impl_macros;
-    /// struct Foo;
-    /// impl_macros!(Foo);
-    /// ```
-    ///
-    /// This will produce:
-    ///
-    /// ```text
-    /// warning: using an old version of `time-macros-impl`
-    ///   ::: $DIR/group-compat-hack.rs:27:5
-    ///    |
-    /// LL |     impl_macros!(Foo);
-    ///    |     ------------------ in this macro invocation
-    ///    |
-    ///    = note: `#[warn(proc_macro_back_compat)]` on by default
-    ///    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-    ///    = note: for more information, see issue #83125 <https://github.com/rust-lang/rust/issues/83125>
-    ///    = note: the `time-macros-impl` crate will stop compiling in futures version of Rust. Please update to the latest version of the `time` crate to avoid breakage
-    ///    = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
-    /// ```
-    ///
-    /// ### Explanation
-    ///
-    /// Eventually, the backwards-compatibility hacks present in the compiler will be removed,
-    /// causing older versions of certain crates to stop compiling.
-    /// This is a [future-incompatible] lint to ease the transition to an error.
-    /// See [issue #83125] for more details.
-    ///
-    /// [issue #83125]: https://github.com/rust-lang/rust/issues/83125
-    /// [future-incompatible]: ../index.md#future-incompatible-lints
-    pub PROC_MACRO_BACK_COMPAT,
-    Deny,
-    "detects usage of old versions of certain proc-macro crates",
-    @future_incompatible = FutureIncompatibleInfo {
-        reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
-        reference: "issue #83125 <https://github.com/rust-lang/rust/issues/83125>",
-    };
-}
-
 declare_lint! {
     /// The `rust_2021_incompatible_or_patterns` lint detects usage of old versions of or-patterns.
     ///

compiler/rustc_lint_defs/src/lib.rs

@@ -618,10 +618,6 @@ pub enum BuiltinLintDiag {
         is_foreign: bool,
     },
     LegacyDeriveHelpers(Span),
-    ProcMacroBackCompat {
-        crate_name: String,
-        fixed_version: String,
-    },
     OrPatternsBackCompat(Span, String),
     ReservedPrefix(Span, String),
     TrailingMacro(bool, Ident),