Add toggle for `parse_meta_item` unsafe parsing

This makes it possible for the `unsafe(...)` syntax to only be
valid at the top level, and the `NestedMetaItem`s will automatically
reject `unsafe(...)`.

1 files changed, 17 insertions, 4 deletions

diff --git a/compiler/rustc_parse/src/parser/attr.rs b/compiler/rustc_parse/src/parser/attr.rs
index 12b9414d1f7..f34ef071e21 100644
--- a/compiler/rustc_parse/src/parser/attr.rs
+++ b/compiler/rustc_parse/src/parser/attr.rs
@@ -31,6 +31,12 @@ enum OuterAttributeType {
     Attribute,
 }
 
+#[derive(Clone, Copy, PartialEq, Eq)]
+pub enum AllowLeadingUnsafe {
+    Yes,
+    No,
+}
+
 impl<'a> Parser<'a> {
     /// Parses attributes that appear before an item.
     pub(super) fn parse_outer_attributes(&mut self) -> PResult<'a, AttrWrapper> {
@@ -332,7 +338,7 @@ impl<'a> Parser<'a> {
 
     /// Parses `cfg_attr(pred, attr_item_list)` where `attr_item_list` is comma-delimited.
     pub fn parse_cfg_attr(&mut self) -> PResult<'a, (ast::MetaItem, Vec<(ast::AttrItem, Span)>)> {
-        let cfg_predicate = self.parse_meta_item()?;
+        let cfg_predicate = self.parse_meta_item(AllowLeadingUnsafe::No)?;
         self.expect(&token::Comma)?;
 
         // Presumably, the majority of the time there will only be one attr.
@@ -368,7 +374,10 @@ impl<'a> Parser<'a> {
     /// MetaItem = SimplePath ( '=' UNSUFFIXED_LIT | '(' MetaSeq? ')' )? ;
     /// MetaSeq = MetaItemInner (',' MetaItemInner)* ','? ;
     /// ```
-    pub fn parse_meta_item(&mut self) -> PResult<'a, ast::MetaItem> {
+    pub fn parse_meta_item(
+        &mut self,
+        unsafe_allowed: AllowLeadingUnsafe,
+    ) -> PResult<'a, ast::MetaItem> {
         // We can't use `maybe_whole` here because it would bump in the `None`
         // case, which we don't want.
         if let token::Interpolated(nt) = &self.token.kind
@@ -384,7 +393,11 @@ impl<'a> Parser<'a> {
         }
 
         let lo = self.token.span;
-        let is_unsafe = self.eat_keyword(kw::Unsafe);
+        let is_unsafe = if unsafe_allowed == AllowLeadingUnsafe::Yes {
+            self.eat_keyword(kw::Unsafe)
+        } else {
+            false
+        };
         let unsafety = if is_unsafe {
             let unsafe_span = self.prev_token.span;
             self.psess.gated_spans.gate(sym::unsafe_attributes, unsafe_span);
@@ -427,7 +440,7 @@ impl<'a> Parser<'a> {
             Err(err) => err.cancel(), // we provide a better error below
         }
 
-        match self.parse_meta_item() {
+        match self.parse_meta_item(AllowLeadingUnsafe::No) {
             Ok(mi) => return Ok(ast::NestedMetaItem::MetaItem(mi)),
             Err(err) => err.cancel(), // we provide a better error below
         }