Add hard error and migration lint for unsafe attrs

compiler/rustc_feature/src/builtin_attrs.rs

@@ -1145,10 +1145,6 @@ pub fn is_valid_for_get_attr(name: Symbol) -> bool {
     })
 }
 
-pub fn is_unsafe_attr(name: Symbol) -> bool {
-    BUILTIN_ATTRIBUTE_MAP.get(&name).is_some_and(|attr| attr.safety == AttributeSafety::Unsafe)
-}
-
 pub static BUILTIN_ATTRIBUTE_MAP: LazyLock<FxHashMap<Symbol, &BuiltinAttribute>> =
     LazyLock::new(|| {
         let mut map = FxHashMap::default();

compiler/rustc_feature/src/lib.rs

@@ -123,7 +123,7 @@ pub use accepted::ACCEPTED_FEATURES;
 pub use builtin_attrs::AttributeDuplicates;
 pub use builtin_attrs::{
     deprecated_attributes, encode_cross_crate, find_gated_cfg, is_builtin_attr_name,
-    is_unsafe_attr, is_valid_for_get_attr, AttributeGate, AttributeTemplate, AttributeType,
+    is_valid_for_get_attr, AttributeGate, AttributeSafety, AttributeTemplate, AttributeType,
     BuiltinAttribute, GatedCfg, BUILTIN_ATTRIBUTES, BUILTIN_ATTRIBUTE_MAP,
 };
 pub use removed::REMOVED_FEATURES;

compiler/rustc_lint/messages.ftl

@@ -821,6 +821,10 @@ lint_unnameable_test_items = cannot test inner items
 lint_unnecessary_qualification = unnecessary qualification
     .suggestion = remove the unnecessary path segments
 
+lint_unsafe_attr_outside_unsafe = unsafe attribute used without unsafe
+    .label = usage of unsafe attribute
+lint_unsafe_attr_outside_unsafe_suggestion = wrap the attribute in `unsafe(...)`
+
 lint_unsupported_group = `{$lint_group}` lint group is not supported with ´--force-warn´
 
 lint_untranslatable_diag = diagnostics should be created using translatable messages

compiler/rustc_lint/src/context/diagnostics.rs

@@ -319,6 +319,16 @@ pub(super) fn decorate_lint(sess: &Session, diagnostic: BuiltinLintDiag, diag: &
         BuiltinLintDiag::UnusedQualifications { removal_span } => {
             lints::UnusedQualifications { removal_span }.decorate_lint(diag);
         }
+        BuiltinLintDiag::UnsafeAttrOutsideUnsafe {
+            attribute_name_span,
+            sugg_spans: (left, right),
+        } => {
+            lints::UnsafeAttrOutsideUnsafe {
+                span: attribute_name_span,
+                suggestion: lints::UnsafeAttrOutsideUnsafeSuggestion { left, right },
+            }
+            .decorate_lint(diag);
+        }
         BuiltinLintDiag::AssociatedConstElidedLifetime { elided, span: lt_span } => {
             let lt_span = if elided { lt_span.shrink_to_hi() } else { lt_span };
             let code = if elided { "'static " } else { "'static" };

compiler/rustc_lint/src/lints.rs

@@ -2874,3 +2874,24 @@ pub struct RedundantImportVisibility {
     pub import_vis: String,
     pub max_vis: String,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(lint_unsafe_attr_outside_unsafe)]
+pub struct UnsafeAttrOutsideUnsafe {
+    #[label]
+    pub span: Span,
+    #[subdiagnostic]
+    pub suggestion: UnsafeAttrOutsideUnsafeSuggestion,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(
+    lint_unsafe_attr_outside_unsafe_suggestion,
+    applicability = "machine-applicable"
+)]
+pub struct UnsafeAttrOutsideUnsafeSuggestion {
+    #[suggestion_part(code = "unsafe(")]
+    pub left: Span,
+    #[suggestion_part(code = ")")]
+    pub right: Span,
+}

compiler/rustc_lint_defs/src/builtin.rs

@@ -114,6 +114,7 @@ declare_lint_pass! {
         UNNAMEABLE_TYPES,
         UNREACHABLE_CODE,
         UNREACHABLE_PATTERNS,
+        UNSAFE_ATTR_OUTSIDE_UNSAFE,
         UNSAFE_OP_IN_UNSAFE_FN,
         UNSTABLE_NAME_COLLISIONS,
         UNSTABLE_SYNTAX_PRE_EXPANSION,
@@ -4841,3 +4842,43 @@ declare_lint! {
         reference: "issue #123743 <https://github.com/rust-lang/rust/issues/123743>",
     };
 }
+
+declare_lint! {
+    /// The `unsafe_attr_outside_unsafe` lint detects a missing unsafe keyword
+    /// on attributes considered unsafe.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// #![feature(unsafe_attributes)]
+    /// #![warn(unsafe_attr_outside_unsafe)]
+    /// #![allow(dead_code)]
+    ///
+    /// #[unsafe(no_mangle)]
+    /// extern "C" fn foo() {}
+    ///
+    /// fn main() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Some attributes (e.g. no_mangle, export_name, link_section -- see here
+    /// for a more complete list)are considered "unsafe" attributes. An unsafe
+    /// attribute must only be used inside unsafe(...) in the attribute.
+    ///
+    /// This lint can automatically wrap the attributes in `unsafe(...)` , but this
+    /// obviously cannot verify that the preconditions of the `unsafe`
+    /// attributes are fulfilled, so that is still up to the user.
+    ///
+    /// The lint is currently "allow" by default, but that might change in the
+    /// future.
+    pub UNSAFE_ATTR_OUTSIDE_UNSAFE,
+    Allow,
+    "detects unsafe attributes outside of unsafe",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::EditionError(Edition::Edition2024),
+        reference: "issue #123757 <https://github.com/rust-lang/rust/issues/123757>",
+    };
+}

compiler/rustc_lint_defs/src/lib.rs

@@ -691,6 +691,10 @@ pub enum BuiltinLintDiag {
         /// The span of the unnecessarily-qualified path to remove.
         removal_span: Span,
     },
+    UnsafeAttrOutsideUnsafe {
+        attribute_name_span: Span,
+        sugg_spans: (Span, Span),
+    },
     AssociatedConstElidedLifetime {
         elided: bool,
         span: Span,

compiler/rustc_parse/messages.ftl

@@ -364,6 +364,10 @@ parse_inner_doc_comment_not_permitted = expected outer doc comment
     .label_does_not_annotate_this = the inner doc comment doesn't annotate this {$item}
     .sugg_change_inner_to_outer = to annotate the {$item}, change the doc comment from inner to outer style
 
+parse_invalid_attr_unsafe = `{$name}` is not an unsafe attribute
+    .suggestion = remove the `unsafe(...)`
+    .note = extraneous unsafe is not allowed in attributes
+
 parse_invalid_block_macro_segment = cannot use a `block` macro fragment here
     .label = the `block` fragment is within this context
     .suggestion = wrap this in another block
@@ -864,6 +868,11 @@ parse_unmatched_angle_brackets = {$num_extra_brackets ->
            *[other] remove extra angle brackets
         }
 
+parse_unsafe_attr_outside_unsafe = unsafe attribute used without unsafe
+    .label = usage of unsafe attribute
+parse_unsafe_attr_outside_unsafe_suggestion = wrap the attribute in `unsafe(...)`
+
+
 parse_unskipped_whitespace = whitespace symbol '{$ch}' is not skipped
     .label = {parse_unskipped_whitespace}
 

compiler/rustc_parse/src/errors.rs

@@ -2989,3 +2989,34 @@ pub(crate) struct ExprRArrowCall {
     #[suggestion(style = "short", applicability = "machine-applicable", code = ".")]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_invalid_attr_unsafe)]
+#[note]
+pub struct InvalidAttrUnsafe {
+    #[primary_span]
+    pub span: Span,
+    pub name: Path,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_unsafe_attr_outside_unsafe)]
+pub struct UnsafeAttrOutsideUnsafe {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[subdiagnostic]
+    pub suggestion: UnsafeAttrOutsideUnsafeSuggestion,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(
+    parse_unsafe_attr_outside_unsafe_suggestion,
+    applicability = "machine-applicable"
+)]
+pub struct UnsafeAttrOutsideUnsafeSuggestion {
+    #[suggestion_part(code = "unsafe(")]
+    pub left: Span,
+    #[suggestion_part(code = ")")]
+    pub right: Span,
+}

compiler/rustc_parse/src/validate_attr.rs

@@ -5,21 +5,68 @@ use crate::{errors, parse_in};
 use rustc_ast::token::Delimiter;
 use rustc_ast::tokenstream::DelimSpan;
 use rustc_ast::MetaItemKind;
-use rustc_ast::{self as ast, AttrArgs, AttrArgsEq, Attribute, DelimArgs, MetaItem};
+use rustc_ast::{self as ast, AttrArgs, AttrArgsEq, Attribute, DelimArgs, MetaItem, Safety};
 use rustc_errors::{Applicability, FatalError, PResult};
-use rustc_feature::{AttributeTemplate, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP};
+use rustc_feature::{AttributeSafety, AttributeTemplate, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP};
 use rustc_session::errors::report_lit_error;
-use rustc_session::lint::builtin::ILL_FORMED_ATTRIBUTE_INPUT;
+use rustc_session::lint::builtin::{ILL_FORMED_ATTRIBUTE_INPUT, UNSAFE_ATTR_OUTSIDE_UNSAFE};
 use rustc_session::lint::BuiltinLintDiag;
 use rustc_session::parse::ParseSess;
-use rustc_span::{sym, Span, Symbol};
+use rustc_span::{sym, BytePos, Span, Symbol};
 
 pub fn check_attr(psess: &ParseSess, attr: &Attribute) {
     if attr.is_doc_comment() {
         return;
     }
 
     let attr_info = attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name));
+    let attr_item = attr.get_normal_item();
+
+    let is_unsafe_attr =
+        attr_info.map(|attr| attr.safety == AttributeSafety::Unsafe).unwrap_or(false);
+
+    if is_unsafe_attr {
+        if let ast::Safety::Default = attr_item.unsafety {
+            let path_span = attr_item.path.span;
+
+            // If the `attr_item`'s span is not from a macro, then just suggest
+            // wrapping it in `unsafe(...)`. Otherwise, we suggest putting the
+            // `unsafe(`, `)` right after and right before the opening and closing
+            // square bracket respectively.
+            let diag_span = if attr_item.span().can_be_used_for_suggestions() {
+                attr_item.span()
+            } else {
+                attr.span.with_lo(attr.span.lo() + BytePos(2)).with_hi(attr.span.hi() - BytePos(1))
+            };
+
+            if attr.span.at_least_rust_2024() {
+                psess.dcx.emit_err(errors::UnsafeAttrOutsideUnsafe {
+                    span: path_span,
+                    suggestion: errors::UnsafeAttrOutsideUnsafeSuggestion {
+                        left: diag_span.shrink_to_lo(),
+                        right: diag_span.shrink_to_hi(),
+                    },
+                });
+            } else {
+                psess.buffer_lint(
+                    UNSAFE_ATTR_OUTSIDE_UNSAFE,
+                    path_span,
+                    ast::CRATE_NODE_ID,
+                    BuiltinLintDiag::UnsafeAttrOutsideUnsafe {
+                        attribute_name_span: path_span,
+                        sugg_spans: (diag_span.shrink_to_lo(), diag_span.shrink_to_hi()),
+                    },
+                );
+            }
+        }
+    } else {
+        if let Safety::Unsafe(unsafe_span) = attr_item.unsafety {
+            psess.dcx.emit_err(errors::InvalidAttrUnsafe {
+                span: unsafe_span,
+                name: attr_item.path.clone(),
+            });
+        }
+    }
 
     // Check input tokens for built-in and key-value attributes.
     match attr_info {
@@ -32,7 +79,7 @@ pub fn check_attr(psess: &ParseSess, attr: &Attribute) {
                 }
             }
         }
-        _ if let AttrArgs::Eq(..) = attr.get_normal_item().args => {
+        _ if let AttrArgs::Eq(..) = attr_item.args => {
             // All key-value attributes are restricted to meta-item syntax.
             match parse_meta(psess, attr) {
                 Ok(_) => {}

compiler/rustc_passes/messages.ftl

@@ -384,10 +384,6 @@ passes_invalid_attr_at_crate_level =
 passes_invalid_attr_at_crate_level_item =
     the inner attribute doesn't annotate this {$kind}
 
-passes_invalid_attr_unsafe = `{$name}` is not an unsafe attribute
-    .suggestion = remove the `unsafe(...)`
-    .note = extraneous unsafe is not allowed in attributes
-
 passes_invalid_macro_export_arguments = `{$name}` isn't a valid `#[macro_export]` argument
 
 passes_invalid_macro_export_arguments_too_many_items = `#[macro_export]` can only take 1 or 0 arguments

compiler/rustc_passes/src/check_attr.rs

@@ -10,9 +10,7 @@ use rustc_ast::{MetaItemKind, MetaItemLit, NestedMetaItem};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::StashKey;
 use rustc_errors::{Applicability, DiagCtxt, IntoDiagArg, MultiSpan};
-use rustc_feature::{
-    is_unsafe_attr, AttributeDuplicates, AttributeType, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP,
-};
+use rustc_feature::{AttributeDuplicates, AttributeType, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP};
 use rustc_hir::def_id::LocalModDefId;
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{self as hir};
@@ -116,8 +114,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         let mut seen = FxHashMap::default();
         let attrs = self.tcx.hir().attrs(hir_id);
         for attr in attrs {
-            self.check_unsafe_attr(attr);
-
             match attr.path().as_slice() {
                 [sym::diagnostic, sym::do_not_recommend] => {
                     self.check_do_not_recommend(attr.span, hir_id, target)
@@ -312,21 +308,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         true
     }
 
-    /// Checks if `unsafe()` is applied to an invalid attribute.
-    fn check_unsafe_attr(&self, attr: &Attribute) {
-        if !attr.is_doc_comment() {
-            let attr_item = attr.get_normal_item();
-            if let ast::Safety::Unsafe(unsafe_span) = attr_item.unsafety {
-                if !is_unsafe_attr(attr.name_or_empty()) {
-                    self.dcx().emit_err(errors::InvalidAttrUnsafe {
-                        span: unsafe_span,
-                        name: attr_item.path.clone(),
-                    });
-                }
-            }
-        }
-    }
-
     /// Checks if `#[diagnostic::on_unimplemented]` is applied to a trait definition
     fn check_diagnostic_on_unimplemented(
         &self,

compiler/rustc_passes/src/errors.rs

@@ -4,7 +4,7 @@ use std::{
 };
 
 use crate::fluent_generated as fluent;
-use rustc_ast::{ast, Label};
+use rustc_ast::Label;
 use rustc_errors::{
     codes::*, Applicability, Diag, DiagCtxt, DiagSymbolList, Diagnostic, EmissionGuarantee, Level,
     MultiSpan, SubdiagMessageOp, Subdiagnostic,
@@ -863,15 +863,6 @@ pub struct InvalidAttrAtCrateLevel {
     pub item: Option<ItemFollowingInnerAttr>,
 }
 
-#[derive(Diagnostic)]
-#[diag(passes_invalid_attr_unsafe)]
-#[note]
-pub struct InvalidAttrUnsafe {
-    #[primary_span]
-    pub span: Span,
-    pub name: ast::Path,
-}
-
 #[derive(Clone, Copy)]
 pub struct ItemFollowingInnerAttr {
     pub span: Span,

tests/ui/rust-2024/unsafe-attributes/in_2024_compatibility.rs

@@ -0,0 +1,8 @@
+#![deny(rust_2024_compatibility)]
+
+#[no_mangle]
+//~^ ERROR: unsafe attribute used without unsafe
+//~| WARN this is accepted in the current edition
+extern "C" fn foo() {}
+
+fn main() {}

tests/ui/rust-2024/unsafe-attributes/in_2024_compatibility.stderr

@@ -0,0 +1,21 @@
+error: unsafe attribute used without unsafe
+  --> $DIR/in_2024_compatibility.rs:3:3
+   |
+LL | #[no_mangle]
+   |   ^^^^^^^^^ usage of unsafe attribute
+   |
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123757 <https://github.com/rust-lang/rust/issues/123757>
+note: the lint level is defined here
+  --> $DIR/in_2024_compatibility.rs:1:9
+   |
+LL | #![deny(rust_2024_compatibility)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^
+   = note: `#[deny(unsafe_attr_outside_unsafe)]` implied by `#[deny(rust_2024_compatibility)]`
+help: wrap the attribute in `unsafe(...)`
+   |
+LL | #[unsafe(no_mangle)]
+   |   +++++++         +
+
+error: aborting due to 1 previous error
+

tests/ui/rust-2024/unsafe-attributes/unsafe-attribute-marked.rs

@@ -0,0 +1,12 @@
+//@ revisions: edition2021 edition2024
+//@[edition2021] edition:2021
+//@[edition2024] edition:2024
+//@[edition2024] compile-flags: -Zunstable-options
+//@ check-pass
+
+#![feature(unsafe_attributes)]
+
+#[unsafe(no_mangle)]
+extern "C" fn foo() {}
+
+fn main() {}