Allow attributes in formal function parameters

src/librustc_data_structures/thin_vec.rs

@@ -66,3 +66,9 @@ impl<T: HashStable<CTX>, CTX> HashStable<CTX> for ThinVec<T> {
         (**self).hash_stable(hcx, hasher)
     }
 }
+
+impl<T> Default for ThinVec<T> {
+    fn default() -> Self {
+        Self(None)
+    }
+}

src/librustc_passes/ast_validation.rs

@@ -14,6 +14,7 @@ use rustc::session::Session;
 use rustc_data_structures::fx::FxHashMap;
 use syntax::ast::*;
 use syntax::attr;
+use syntax::feature_gate::is_builtin_attr;
 use syntax::source_map::Spanned;
 use syntax::symbol::{kw, sym};
 use syntax::ptr::P;
@@ -365,6 +366,29 @@ impl<'a> AstValidator<'a> {
             _ => None,
         }
     }
+
+    fn check_fn_decl(&self, fn_decl: &FnDecl) {
+        fn_decl
+            .inputs
+            .iter()
+            .flat_map(|i| i.attrs.as_ref())
+            .filter(|attr| {
+                let arr = [sym::allow, sym::cfg, sym::cfg_attr, sym::deny, sym::forbid, sym::warn];
+                !arr.contains(&attr.name_or_empty()) && is_builtin_attr(attr)
+            })
+            .for_each(|attr| if attr.is_sugared_doc {
+                let mut err = self.err_handler().struct_span_err(
+                    attr.span,
+                    "documentation comments cannot be applied to function parameters"
+                );
+                err.span_label(attr.span, "doc comments are not allowed here");
+                err.emit();
+            }
+            else {
+                self.err_handler().span_err(attr.span, "allow, cfg, cfg_attr, deny, \
+                forbid, and warn are the only allowed built-in attributes in function parameters")
+            });
+    }
 }
 
 enum GenericPosition {
@@ -470,6 +494,9 @@ fn validate_generics_order<'a>(
 impl<'a> Visitor<'a> for AstValidator<'a> {
     fn visit_expr(&mut self, expr: &'a Expr) {
         match expr.node {
+            ExprKind::Closure(_, _, _, ref fn_decl, _, _) => {
+                self.check_fn_decl(fn_decl);
+            }
             ExprKind::IfLet(_, ref expr, _, _) | ExprKind::WhileLet(_, ref expr, _, _) =>
                 self.while_if_let_ambiguity(&expr),
             ExprKind::InlineAsm(..) if !self.session.target.target.options.allow_asm => {
@@ -484,6 +511,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
     fn visit_ty(&mut self, ty: &'a Ty) {
         match ty.node {
             TyKind::BareFn(ref bfty) => {
+                self.check_fn_decl(&bfty.decl);
                 self.check_decl_no_pat(&bfty.decl, |span, _| {
                     struct_span_err!(self.session, span, E0561,
                                      "patterns aren't allowed in function pointer types").emit();
@@ -601,10 +629,11 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         .note("only trait implementations may be annotated with default").emit();
                 }
             }
-            ItemKind::Fn(_, ref header, ref generics, _) => {
+            ItemKind::Fn(ref decl, ref header, ref generics, _) => {
+                self.visit_fn_header(header);
+                self.check_fn_decl(decl);
                 // We currently do not permit const generics in `const fn`, as
                 // this is tantamount to allowing compile-time dependent typing.
-                self.visit_fn_header(header);
                 if header.constness.node == Constness::Const {
                     // Look for const generics and error if we find any.
                     for param in &generics.params {
@@ -657,6 +686,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 self.no_questions_in_bounds(bounds, "supertraits", true);
                 for trait_item in trait_items {
                     if let TraitItemKind::Method(ref sig, ref block) = trait_item.node {
+                        self.check_fn_decl(&sig.decl);
                         self.check_trait_fn_not_async(trait_item.span, sig.header.asyncness.node);
                         self.check_trait_fn_not_const(sig.header.constness);
                         if block.is_none() {
@@ -711,6 +741,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
     fn visit_foreign_item(&mut self, fi: &'a ForeignItem) {
         match fi.node {
             ForeignItemKind::Fn(ref decl, _) => {
+                self.check_fn_decl(decl);
                 self.check_decl_no_pat(decl, |span, _| {
                     struct_span_err!(self.session, span, E0130,
                                      "patterns aren't allowed in foreign function declarations")
@@ -864,6 +895,16 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                              "`async fn` is not permitted in the 2015 edition").emit();
         }
     }
+
+    fn visit_impl_item(&mut self, ii: &'a ImplItem) {
+        match ii.node {
+            ImplItemKind::Method(ref sig, _) => {
+                self.check_fn_decl(&sig.decl);
+            }
+            _ => {}
+        }
+        visit::walk_impl_item(self, ii);
+    }
 }
 
 pub fn check_crate(session: &Session, krate: &Crate) -> (bool, bool) {

src/libsyntax/ast.rs

@@ -1770,6 +1770,7 @@ pub struct InlineAsm {
 /// E.g., `bar: usize` as in `fn foo(bar: usize)`.
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct Arg {
+    pub attrs: ThinVec<Attribute>,
     pub ty: P<Ty>,
     pub pat: P<Pat>,
     pub id: NodeId,
@@ -1817,14 +1818,15 @@ impl Arg {
         }
     }
 
-    pub fn from_self(eself: ExplicitSelf, eself_ident: Ident) -> Arg {
+    pub fn from_self(attrs: ThinVec<Attribute>, eself: ExplicitSelf, eself_ident: Ident) -> Arg {
         let span = eself.span.to(eself_ident.span);
         let infer_ty = P(Ty {
             id: DUMMY_NODE_ID,
             node: TyKind::ImplicitSelf,
             span,
         });
         let arg = |mutbl, ty| Arg {
+            attrs,
             pat: P(Pat {
                 id: DUMMY_NODE_ID,
                 node: PatKind::Ident(BindingMode::ByValue(mutbl), eself_ident, None),

src/libsyntax/attr/mod.rs

@@ -724,7 +724,7 @@ macro_rules! derive_has_attrs {
 
 derive_has_attrs! {
     Item, Expr, Local, ast::ForeignItem, ast::StructField, ast::ImplItem, ast::TraitItem, ast::Arm,
-    ast::Field, ast::FieldPat, ast::Variant_
+    ast::Field, ast::FieldPat, ast::Variant_, ast::Arg
 }
 
 pub fn inject(mut krate: ast::Crate, parse_sess: &ParseSess, attrs: &[String]) -> ast::Crate {

src/libsyntax/config.rs

@@ -298,6 +298,10 @@ impl<'a> StripUnconfigured<'a> {
         }
     }
 
+    pub fn configure_fn_decl(&mut self, fn_decl: &mut ast::FnDecl) {
+        fn_decl.inputs.flat_map_in_place(|arg| self.configure(arg));
+    }
+
     /// Denies `#[cfg]` on generic parameters until we decide what to do with it.
     /// See issue #51279.
     pub fn disallow_cfg_on_generic_param(&mut self, param: &ast::GenericParam) {
@@ -364,6 +368,11 @@ impl<'a> MutVisitor for StripUnconfigured<'a> {
         self.configure_pat(pat);
         noop_visit_pat(pat, self)
     }
+
+    fn visit_fn_decl(&mut self, mut fn_decl: &mut P<ast::FnDecl>) {
+        self.configure_fn_decl(&mut fn_decl);
+        noop_visit_fn_decl(fn_decl, self);
+    }
 }
 
 fn is_cfg(attr: &ast::Attribute) -> bool {

src/libsyntax/ext/build.rs

@@ -963,9 +963,10 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
     fn arg(&self, span: Span, ident: ast::Ident, ty: P<ast::Ty>) -> ast::Arg {
         let arg_pat = self.pat_ident(span, ident);
         ast::Arg {
-            ty,
+            attrs: ThinVec::default(),
+            id: ast::DUMMY_NODE_ID,
             pat: arg_pat,
-            id: ast::DUMMY_NODE_ID
+            ty,
         }
     }
 

src/libsyntax/ext/expand.rs

@@ -1616,6 +1616,11 @@ impl<'a, 'b> MutVisitor for InvocationCollector<'a, 'b> {
             *id = self.cx.resolver.next_node_id()
         }
     }
+
+    fn visit_fn_decl(&mut self, mut fn_decl: &mut P<ast::FnDecl>) {
+        self.cfg.configure_fn_decl(&mut fn_decl);
+        noop_visit_fn_decl(fn_decl, self);
+    }
 }
 
 pub struct ExpansionConfig<'feat> {

src/libsyntax/feature_gate.rs

@@ -560,6 +560,9 @@ declare_features! (
     // Allows the user of associated type bounds.
     (active, associated_type_bounds, "1.34.0", Some(52662), None),
 
+    // Attributes on formal function params
+    (active, param_attrs, "1.36.0", Some(60406), None),
+
     // Allows calling constructor functions in `const fn`
     // FIXME Create issue
     (active, const_constructor, "1.37.0", Some(61456), None),
@@ -2508,6 +2511,18 @@ pub fn check_crate(krate: &ast::Crate,
         parse_sess: sess,
         plugin_attributes,
     };
+
+    sess
+        .param_attr_spans
+        .borrow()
+        .iter()
+        .for_each(|span| gate_feature!(
+            &ctx,
+            param_attrs,
+            *span,
+            "attributes on function parameters are unstable"
+        ));
+
     let visitor = &mut PostExpansionVisitor {
         context: &ctx,
         builtin_attributes: &*BUILTIN_ATTRIBUTE_MAP,

src/libsyntax/mut_visit.rs

@@ -568,8 +568,9 @@ pub fn noop_visit_meta_item<T: MutVisitor>(mi: &mut MetaItem, vis: &mut T) {
     vis.visit_span(span);
 }
 
-pub fn noop_visit_arg<T: MutVisitor>(Arg { id, pat, ty }: &mut Arg, vis: &mut T) {
+pub fn noop_visit_arg<T: MutVisitor>(Arg { attrs, id, pat, ty }: &mut Arg, vis: &mut T) {
     vis.visit_id(id);
+    visit_thin_attrs(attrs, vis);
     vis.visit_pat(pat);
     vis.visit_ty(ty);
 }

src/libsyntax/parse/attr.rs

@@ -18,6 +18,14 @@ const DEFAULT_UNEXPECTED_INNER_ATTR_ERR_MSG: &str = "an inner attribute is not \
                                                      permitted in this context";
 
 impl<'a> Parser<'a> {
+    crate fn parse_arg_attributes(&mut self) -> PResult<'a, Vec<ast::Attribute>> {
+        let attrs = self.parse_outer_attributes()?;
+        attrs.iter().for_each(|a|
+            self.sess.param_attr_spans.borrow_mut().push(a.span)
+        );
+        Ok(attrs)
+    }
+
     /// Parse attributes that appear before an item
     crate fn parse_outer_attributes(&mut self) -> PResult<'a, Vec<ast::Attribute>> {
         let mut attrs: Vec<ast::Attribute> = Vec::new();
@@ -35,7 +43,8 @@ impl<'a> Parser<'a> {
                     };
                     let inner_parse_policy =
                         InnerAttributeParsePolicy::NotPermitted { reason: inner_error_reason };
-                    attrs.push(self.parse_attribute_with_inner_parse_policy(inner_parse_policy)?);
+                    let attr = self.parse_attribute_with_inner_parse_policy(inner_parse_policy)?;
+                    attrs.push(attr);
                     just_parsed_doc_comment = false;
                 }
                 token::DocComment(s) => {

src/libsyntax/parse/diagnostics.rs

@@ -28,7 +28,7 @@ crate fn dummy_arg(ident: Ident) -> Arg {
         span: ident.span,
         id: ast::DUMMY_NODE_ID
     };
-    Arg { ty: P(ty), pat: pat, id: ast::DUMMY_NODE_ID }
+    Arg { attrs: ThinVec::default(), id: ast::DUMMY_NODE_ID, pat, ty: P(ty) }
 }
 
 pub enum Error {
@@ -1074,11 +1074,11 @@ impl<'a> Parser<'a> {
         Err(err)
     }
 
-    crate fn eat_incorrect_doc_comment(&mut self, applied_to: &str) {
+    crate fn eat_incorrect_doc_comment_for_arg_type(&mut self) {
         if let token::DocComment(_) = self.token.kind {
             let mut err = self.diagnostic().struct_span_err(
                 self.token.span,
-                &format!("documentation comments cannot be applied to {}", applied_to),
+                "documentation comments cannot be applied to a function parameter's type",
             );
             err.span_label(self.token.span, "doc comments are not allowed here");
             err.emit();
@@ -1095,7 +1095,7 @@ impl<'a> Parser<'a> {
             self.bump();
             let mut err = self.diagnostic().struct_span_err(
                 sp,
-                &format!("attributes cannot be applied to {}", applied_to),
+                "attributes cannot be applied to a function parameter's type",
             );
             err.span_label(sp, "attributes are not allowed here");
             err.emit();

src/libsyntax/parse/lexer/mod.rs

@@ -1532,6 +1532,7 @@ mod tests {
             buffered_lints: Lock::new(vec![]),
             edition: Edition::from_session(),
             ambiguous_block_expr_parse: Lock::new(FxHashMap::default()),
+            param_attr_spans: Lock::new(Vec::new()),
         }
     }
 

src/libsyntax/parse/mod.rs

@@ -54,6 +54,7 @@ pub struct ParseSess {
     /// operation token that followed it, but that the parser cannot identify without further
     /// analysis.
     pub ambiguous_block_expr_parse: Lock<FxHashMap<Span, Span>>,
+    pub param_attr_spans: Lock<Vec<Span>>
 }
 
 impl ParseSess {
@@ -79,6 +80,7 @@ impl ParseSess {
             buffered_lints: Lock::new(vec![]),
             edition: Edition::from_session(),
             ambiguous_block_expr_parse: Lock::new(FxHashMap::default()),
+            param_attr_spans: Lock::new(Vec::new()),
         }
     }