Extend `proc_macro_back_compat` lint to `procedural-masquerade`

We now lint on *any* use of `procedural-masquerade` crate. While this
crate still exists, its main reverse dependency (`cssparser`) no longer
depends on it. Any crates still depending off should stop doing so, as
it only exists to support very old Rust versions.

If a crate actually needs to support old versions of rustc via
`procedural-masquerade`, then they'll just need to accept the warning
until we remove it entirely (at the same time as the back-compat hack).
The latest version of `procedural-masquerade` does not work with the
latest rustc, but trying to check for the version seems like more
trouble than it's worth.

While working on this, I realized that the `proc-macro-hack` check was
never actually doing anything. The corresponding enum variant in
`proc-macro-hack` is named `Value` or `Nested` - it has never been
called `Input`. Due to a strange Crater issue, the Crater run that
tested adding this did *not* end up testing it - some of the crates that
would have failed did not actually have their tests checked, making it
seem as though the `proc-macro-hack` check was working.

The Crater issue is being discussed at
https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/Nearly.20identical.20Crater.20runs.20processed.20a.20crate.20differently/near/230406661

Despite the `proc-macro-hack` check not actually doing anything, we
haven't gotten any reports from users about their build being broken.
I went ahead and removed it entirely, since it's clear that no one is
being affected by the `proc-macro-hack` regression in practice.

7 files changed, 103 insertions, 29 deletions

diff --git a/compiler/rustc_ast/src/token.rs b/compiler/rustc_ast/src/token.rs
index 093f7f2668c..7e58426d27d 100644
--- a/compiler/rustc_ast/src/token.rs
+++ b/compiler/rustc_ast/src/token.rs
@@ -784,33 +784,6 @@ impl Nonterminal {
             NtTT(tt) => tt.span(),
         }
     }
-
-    /// 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.
-    /// FIXME(#73933): Remove this eventually.
-    pub fn pretty_printing_compatibility_hack(&self) -> bool {
-        let item = match self {
-            NtItem(item) => item,
-            NtStmt(stmt) => match &stmt.kind {
-                ast::StmtKind::Item(item) => item,
-                _ => return false,
-            },
-            _ => return false,
-        };
-
-        let name = item.ident.name;
-        if name == sym::ProceduralMasqueradeDummyType || name == sym::ProcMacroHack {
-            if let ast::ItemKind::Enum(enum_def, _) = &item.kind {
-                if let [variant] = &*enum_def.variants {
-                    return variant.ident.name == sym::Input;
-                }
-            }
-        }
-        false
-    }
 }
 
 impl PartialEq for Nonterminal {
diff --git a/compiler/rustc_expand/src/base.rs b/compiler/rustc_expand/src/base.rs
index dd93fe8350e..ce657930518 100644
--- a/compiler/rustc_expand/src/base.rs
+++ b/compiler/rustc_expand/src/base.rs
@@ -10,6 +10,8 @@ use rustc_attr::{self as attr, Deprecation, Stability};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync::{self, Lrc};
 use rustc_errors::{DiagnosticBuilder, ErrorReported};
+use rustc_lint_defs::builtin::PROC_MACRO_BACK_COMPAT;
+use rustc_lint_defs::BuiltinLintDiagnostics;
 use rustc_parse::{self, nt_to_tokenstream, parser, MACRO_ARGUMENTS};
 use rustc_session::{parse::ParseSess, Limit, Session};
 use rustc_span::def_id::DefId;
@@ -1241,3 +1243,41 @@ pub fn get_exprs_from_tts(
     }
     Some(es)
 }
+
+/// 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.
+/// FIXME(#73933): Remove this eventually.
+pub(crate) fn pretty_printing_compatibility_hack(nt: &Nonterminal, sess: &ParseSess) -> bool {
+    let item = match nt {
+        Nonterminal::NtItem(item) => item,
+        Nonterminal::NtStmt(stmt) => match &stmt.kind {
+            ast::StmtKind::Item(item) => item,
+            _ => return false,
+        },
+        _ => return false,
+    };
+
+    let name = item.ident.name;
+    if name == sym::ProceduralMasqueradeDummyType {
+        if let ast::ItemKind::Enum(enum_def, _) = &item.kind {
+            if let [variant] = &*enum_def.variants {
+                if variant.ident.name == sym::Input {
+                    sess.buffer_lint_with_diagnostic(
+                        &PROC_MACRO_BACK_COMPAT,
+                        item.ident.span,
+                        ast::CRATE_NODE_ID,
+                        "using `procedural-masquerade` crate",
+                        BuiltinLintDiagnostics::ProcMacroBackCompat(
+                        "The `procedural-masquerade` crate has been unnecessary since Rust 1.30.0. \
+                        Versions of this crate below 0.1.7 will eventually stop compiling.".to_string())
+                    );
+                    return true;
+                }
+            }
+        }
+    }
+    false
+}
diff --git a/compiler/rustc_expand/src/proc_macro.rs b/compiler/rustc_expand/src/proc_macro.rs
index 8cbaa7c945a..61b776ff2d2 100644
--- a/compiler/rustc_expand/src/proc_macro.rs
+++ b/compiler/rustc_expand/src/proc_macro.rs
@@ -90,7 +90,8 @@ impl MultiItemModifier for ProcMacroDerive {
             }
             _ => unreachable!(),
         };
-        let input = if item.pretty_printing_compatibility_hack() {
+        let input = if crate::base::pretty_printing_compatibility_hack(&item, &ecx.sess.parse_sess)
+        {
             TokenTree::token(token::Interpolated(Lrc::new(item)), DUMMY_SP).into()
         } else {
             nt_to_tokenstream(&item, &ecx.sess.parse_sess, CanSynthesizeMissingTokens::Yes)
diff --git a/compiler/rustc_expand/src/proc_macro_server.rs b/compiler/rustc_expand/src/proc_macro_server.rs
index 837fad90580..67edfe19da3 100644
--- a/compiler/rustc_expand/src/proc_macro_server.rs
+++ b/compiler/rustc_expand/src/proc_macro_server.rs
@@ -187,7 +187,7 @@ impl FromInternal<(TreeAndSpacing, &'_ ParseSess, &'_ mut Vec<Self>)>
                         delimiter: Delimiter::None,
                         stream,
                         span: DelimSpan::from_single(span),
-                        flatten: nt.pretty_printing_compatibility_hack(),
+                        flatten: crate::base::pretty_printing_compatibility_hack(&nt, sess),
                     })
                 }
             }
diff --git a/src/test/ui/proc-macro/issue-73933-procedural-masquerade.rs b/src/test/ui/proc-macro/issue-73933-procedural-masquerade.rs
new file mode 100644
index 00000000000..abc3d2691a3
--- /dev/null
+++ b/src/test/ui/proc-macro/issue-73933-procedural-masquerade.rs
@@ -0,0 +1,13 @@
+// check-pass
+// aux-build:test-macros.rs
+
+#[macro_use]
+extern crate test_macros;
+
+#[derive(Print)]
+enum ProceduralMasqueradeDummyType { //~ WARN using
+//~| WARN this was previously
+    Input
+}
+
+fn main() {}
diff --git a/src/test/ui/proc-macro/issue-73933-procedural-masquerade.stderr b/src/test/ui/proc-macro/issue-73933-procedural-masquerade.stderr
new file mode 100644
index 00000000000..0b930705e35
--- /dev/null
+++ b/src/test/ui/proc-macro/issue-73933-procedural-masquerade.stderr
@@ -0,0 +1,25 @@
+warning: using `procedural-masquerade` crate
+  --> $DIR/issue-73933-procedural-masquerade.rs:8:6
+   |
+LL | enum ProceduralMasqueradeDummyType {
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = 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 `procedural-masquerade` crate has been unnecessary since Rust 1.30.0. Versions of this crate below 0.1.7 will eventually stop compiling.
+
+warning: 1 warning emitted
+
+Future incompatibility report: Future breakage date: None, diagnostic:
+warning: using `procedural-masquerade` crate
+  --> $DIR/issue-73933-procedural-masquerade.rs:8:6
+   |
+LL | enum ProceduralMasqueradeDummyType {
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = 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 `procedural-masquerade` crate has been unnecessary since Rust 1.30.0. Versions of this crate below 0.1.7 will eventually stop compiling.
+
diff --git a/src/test/ui/proc-macro/issue-73933-procedural-masquerade.stdout b/src/test/ui/proc-macro/issue-73933-procedural-masquerade.stdout
new file mode 100644
index 00000000000..8edd68f8a3b
--- /dev/null
+++ b/src/test/ui/proc-macro/issue-73933-procedural-masquerade.stdout
@@ -0,0 +1,22 @@
+PRINT-DERIVE INPUT (DISPLAY): enum ProceduralMasqueradeDummyType { Input, }
+PRINT-DERIVE RE-COLLECTED (DISPLAY): enum ProceduralMasqueradeDummyType { Input }
+PRINT-DERIVE INPUT (DEBUG): TokenStream [
+    Ident {
+        ident: "enum",
+        span: #0 bytes(100..104),
+    },
+    Ident {
+        ident: "ProceduralMasqueradeDummyType",
+        span: #0 bytes(105..134),
+    },
+    Group {
+        delimiter: Brace,
+        stream: TokenStream [
+            Ident {
+                ident: "Input",
+                span: #0 bytes(186..191),
+            },
+        ],
+        span: #0 bytes(135..193),
+    },
+]