From 28457db6d6b502aa714bda8585aa3fa3708eff9f Mon Sep 17 00:00:00 2001
From: carbotaniuman <41451839+carbotaniuman@users.noreply.github.com>
Date: Tue, 23 Apr 2024 07:50:53 -0500
Subject: [PATCH] Disallow unsafe in derive

---
 compiler/rustc_builtin_macros/messages.ftl           |  3 +++
 compiler/rustc_builtin_macros/src/derive.rs          | 12 +++++++++++-
 compiler/rustc_builtin_macros/src/errors.rs          |  7 +++++++
 compiler/rustc_expand/src/expand.rs                  |  3 ++-
 .../ui/attributes/unsafe/derive-unsafe-attributes.rs |  6 ++++++
 .../unsafe/derive-unsafe-attributes.stderr           |  8 ++++++++
 6 files changed, 37 insertions(+), 2 deletions(-)
 create mode 100644 tests/ui/attributes/unsafe/derive-unsafe-attributes.rs
 create mode 100644 tests/ui/attributes/unsafe/derive-unsafe-attributes.stderr

diff --git a/compiler/rustc_builtin_macros/messages.ftl b/compiler/rustc_builtin_macros/messages.ftl
index 2a5bc58af3b3d..33324c5a4f484 100644
--- a/compiler/rustc_builtin_macros/messages.ftl
+++ b/compiler/rustc_builtin_macros/messages.ftl
@@ -110,6 +110,9 @@ builtin_macros_derive_path_args_list = traits in `#[derive(...)]` don't accept a
 builtin_macros_derive_path_args_value = traits in `#[derive(...)]` don't accept values
     .suggestion = remove the value
 
+builtin_macros_derive_unsafe_path = traits in `#[derive(...)]` don't accept `unsafe(...)`
+    .suggestion = remove the `unsafe(...)`
+
 builtin_macros_env_not_defined = environment variable `{$var}` not defined at compile time
     .cargo = Cargo sets build script variables at run time. Use `std::env::var({$var_expr})` instead
     .custom = use `std::env::var({$var_expr})` to read the variable at run time
diff --git a/compiler/rustc_builtin_macros/src/derive.rs b/compiler/rustc_builtin_macros/src/derive.rs
index 4f412cf79d91f..44cf452017af3 100644
--- a/compiler/rustc_builtin_macros/src/derive.rs
+++ b/compiler/rustc_builtin_macros/src/derive.rs
@@ -2,7 +2,7 @@ use crate::cfg_eval::cfg_eval;
 use crate::errors;
 
 use rustc_ast as ast;
-use rustc_ast::{GenericParamKind, ItemKind, MetaItemKind, NestedMetaItem, StmtKind};
+use rustc_ast::{GenericParamKind, ItemKind, MetaItemKind, NestedMetaItem, StmtKind, Unsafe};
 use rustc_expand::base::{Annotatable, ExpandResult, ExtCtxt, Indeterminate, MultiItemModifier};
 use rustc_feature::AttributeTemplate;
 use rustc_parse::validate_attr;
@@ -56,6 +56,7 @@ impl MultiItemModifier for Expander {
                                 // Reject `#[derive(Debug = "value", Debug(abc))]`, but recover the
                                 // paths.
                                 report_path_args(sess, meta);
+                                report_unsafe_args(sess, meta);
                                 meta.path.clone()
                             })
                             .map(|path| (path, dummy_annotatable(), None, self.0))
@@ -150,3 +151,12 @@ fn report_path_args(sess: &Session, meta: &ast::MetaItem) {
         }
     }
 }
+
+fn report_unsafe_args(sess: &Session, meta: &ast::MetaItem) {
+    match meta.unsafety {
+        Unsafe::Yes(span) => {
+            sess.dcx().emit_err(errors::DeriveUnsafePath { span });
+        }
+        Unsafe::No => {}
+    }
+}
diff --git a/compiler/rustc_builtin_macros/src/errors.rs b/compiler/rustc_builtin_macros/src/errors.rs
index 9078dc07a3152..4fd4b471eece7 100644
--- a/compiler/rustc_builtin_macros/src/errors.rs
+++ b/compiler/rustc_builtin_macros/src/errors.rs
@@ -295,6 +295,13 @@ pub(crate) struct DerivePathArgsValue {
     pub(crate) span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(builtin_macros_derive_unsafe_path)]
+pub(crate) struct DeriveUnsafePath {
+    #[primary_span]
+    pub(crate) span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(builtin_macros_no_default_variant)]
 #[help]
diff --git a/compiler/rustc_expand/src/expand.rs b/compiler/rustc_expand/src/expand.rs
index 0c5e767b42bb8..385f85a55c514 100644
--- a/compiler/rustc_expand/src/expand.rs
+++ b/compiler/rustc_expand/src/expand.rs
@@ -774,7 +774,8 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                     if let SyntaxExtensionKind::Derive(..) = ext {
                         self.gate_proc_macro_input(&item);
                     }
-                    // FIX THIS LATER
+                    // The `MetaItem` representing the trait to derive can't
+                    // have an unsafe around it (as of now).
                     let meta = ast::MetaItem {
                         unsafety: ast::Unsafe::No,
                         kind: MetaItemKind::Word,
diff --git a/tests/ui/attributes/unsafe/derive-unsafe-attributes.rs b/tests/ui/attributes/unsafe/derive-unsafe-attributes.rs
new file mode 100644
index 0000000000000..774ce86c0960c
--- /dev/null
+++ b/tests/ui/attributes/unsafe/derive-unsafe-attributes.rs
@@ -0,0 +1,6 @@
+#![feature(unsafe_attributes)]
+
+#[derive(unsafe(Debug))] //~ ERROR: traits in `#[derive(...)]` don't accept `unsafe(...)`
+struct Foo;
+
+fn main() {}
diff --git a/tests/ui/attributes/unsafe/derive-unsafe-attributes.stderr b/tests/ui/attributes/unsafe/derive-unsafe-attributes.stderr
new file mode 100644
index 0000000000000..fc0daf16790a8
--- /dev/null
+++ b/tests/ui/attributes/unsafe/derive-unsafe-attributes.stderr
@@ -0,0 +1,8 @@
+error: traits in `#[derive(...)]` don't accept `unsafe(...)`
+  --> $DIR/derive-unsafe-attributes.rs:3:10
+   |
+LL | #[derive(unsafe(Debug))]
+   |          ^^^^^^
+
+error: aborting due to 1 previous error
+