Auto merge of #60669 - c410-f3r:attrs-fn, r=petrochenkov

Allow attributes in formal function parameters

Implements https://github.com/rust-lang/rust/issues/60406.

This is my first contribution to the compiler and since this is a large and complex project, I am not fully aware of the consequences of the changes I have made.

**TODO**

- [x] Forbid some built-in attributes.
- [x] Expand cfg/cfg_attr

13 files changed, 109 insertions, 39 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 02fbcb14fa5..68cb8c8574d 100644
--- a/src/libsyntax/ast.rs
+++ b/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,
@@ -1817,7 +1818,7 @@ 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,
@@ -1825,6 +1826,7 @@ impl Arg {
             span,
         });
         let arg = |mutbl, ty| Arg {
+            attrs,
             pat: P(Pat {
                 id: DUMMY_NODE_ID,
                 node: PatKind::Ident(BindingMode::ByValue(mutbl), eself_ident, None),
diff --git a/src/libsyntax/attr/mod.rs b/src/libsyntax/attr/mod.rs
index d7e43f645df..436620ae729 100644
--- a/src/libsyntax/attr/mod.rs
+++ b/src/libsyntax/attr/mod.rs
@@ -723,7 +723,7 @@ 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 {
diff --git a/src/libsyntax/config.rs b/src/libsyntax/config.rs
index 6123e95ccf8..1cc13ac7878 100644
--- a/src/libsyntax/config.rs
+++ b/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 {
diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index 2a03e49996b..9d4bf7d518d 100644
--- a/src/libsyntax/ext/build.rs
+++ b/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,
         }
     }
 
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 084d4fd3820..3fa96c60bff 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -1578,6 +1578,11 @@ 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> {
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 044c4b18905..004323301a2 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -557,6 +557,9 @@ 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),
@@ -2511,6 +2514,18 @@ 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,
diff --git a/src/libsyntax/mut_visit.rs b/src/libsyntax/mut_visit.rs
index 2889f8edfc6..02e2c96868d 100644
--- a/src/libsyntax/mut_visit.rs
+++ b/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);
 }
diff --git a/src/libsyntax/parse/attr.rs b/src/libsyntax/parse/attr.rs
index 77a87e26e60..b28d48b9445 100644
--- a/src/libsyntax/parse/attr.rs
+++ b/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) => {
diff --git a/src/libsyntax/parse/diagnostics.rs b/src/libsyntax/parse/diagnostics.rs
index 9d2ac5b4b51..60544cca877 100644
--- a/src/libsyntax/parse/diagnostics.rs
+++ b/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();
diff --git a/src/libsyntax/parse/lexer/mod.rs b/src/libsyntax/parse/lexer/mod.rs
index 9df2898696e..1c44155150c 100644
--- a/src/libsyntax/parse/lexer/mod.rs
+++ b/src/libsyntax/parse/lexer/mod.rs
@@ -1490,6 +1490,7 @@ 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()),
         }
     }
 
diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index dd96c63ab0e..ba5d1d009d1 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/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()),
         }
     }
 
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index d9eba3bbadb..b6388f08e73 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -1188,7 +1188,8 @@ impl<'a> Parser<'a> {
                 // definition...
 
                 // We don't allow argument names to be left off in edition 2018.
-                p.parse_arg_general(p.token.span.rust_2018(), true, false)
+                let is_name_required = p.token.span.rust_2018();
+                p.parse_arg_general(true, false, |_| is_name_required)
             })?;
             generics.where_clause = self.parse_where_clause()?;
 
@@ -1487,26 +1488,31 @@ impl<'a> Parser<'a> {
     /// Skips unexpected attributes and doc comments in this position and emits an appropriate
     /// error.
     /// This version of parse arg doesn't necessarily require identifier names.
-    fn parse_arg_general(
+    fn parse_arg_general<F>(
         &mut self,
-        require_name: bool,
         is_trait_item: bool,
         allow_c_variadic: bool,
-    ) -> PResult<'a, Arg> {
-        if let Ok(Some(arg)) = self.parse_self_arg() {
+        is_name_required: F,
+    ) -> PResult<'a, Arg>
+    where
+        F: Fn(&token::Token) -> bool
+    {
+        let attrs = self.parse_arg_attributes()?;
+        if let Ok(Some(mut arg)) = self.parse_self_arg() {
+            arg.attrs = attrs.into();
             return self.recover_bad_self_arg(arg, is_trait_item);
         }
 
-        let (pat, ty) = if require_name || self.is_named_argument() {
-            debug!("parse_arg_general parse_pat (require_name:{})", require_name);
-            self.eat_incorrect_doc_comment("method arguments");
-            let pat = self.parse_pat(Some("argument name"))?;
+        let is_name_required = is_name_required(&self.token);
+        let (pat, ty) = if is_name_required || self.is_named_argument() {
+            debug!("parse_arg_general parse_pat (is_name_required:{})", is_name_required);
 
+            let pat = self.parse_pat(Some("argument name"))?;
             if let Err(mut err) = self.expect(&token::Colon) {
                 if let Some(ident) = self.argument_without_type(
                     &mut err,
                     pat,
-                    require_name,
+                    is_name_required,
                     is_trait_item,
                 ) {
                     err.emit();
@@ -1516,12 +1522,12 @@ impl<'a> Parser<'a> {
                 }
             }
 
-            self.eat_incorrect_doc_comment("a method argument's type");
+            self.eat_incorrect_doc_comment_for_arg_type();
             (pat, self.parse_ty_common(true, true, allow_c_variadic)?)
         } else {
             debug!("parse_arg_general ident_to_pat");
             let parser_snapshot_before_ty = self.clone();
-            self.eat_incorrect_doc_comment("a method argument's type");
+            self.eat_incorrect_doc_comment_for_arg_type();
             let mut ty = self.parse_ty_common(true, true, allow_c_variadic);
             if ty.is_ok() && self.token != token::Comma &&
                self.token != token::CloseDelim(token::Paren) {
@@ -1554,11 +1560,12 @@ impl<'a> Parser<'a> {
             }
         };
 
-        Ok(Arg { ty, pat, id: ast::DUMMY_NODE_ID })
+        Ok(Arg { attrs: attrs.into(), id: ast::DUMMY_NODE_ID, pat, ty })
     }
 
     /// Parses an argument in a lambda header (e.g., `|arg, arg|`).
     fn parse_fn_block_arg(&mut self) -> PResult<'a, Arg> {
+        let attrs = self.parse_arg_attributes()?;
         let pat = self.parse_pat(Some("argument name"))?;
         let t = if self.eat(&token::Colon) {
             self.parse_ty()?
@@ -1570,6 +1577,7 @@ impl<'a> Parser<'a> {
             })
         };
         Ok(Arg {
+            attrs: attrs.into(),
             ty: t,
             pat,
             id: ast::DUMMY_NODE_ID
@@ -5413,15 +5421,19 @@ impl<'a> Parser<'a> {
                 &token::CloseDelim(token::Paren),
                 SeqSep::trailing_allowed(token::Comma),
                 |p| {
-                    // If the argument is a C-variadic argument we should not
-                    // enforce named arguments.
-                    let enforce_named_args = if p.token == token::DotDotDot {
-                        false
-                    } else {
-                        named_args
-                    };
-                    match p.parse_arg_general(enforce_named_args, false,
-                                              allow_c_variadic) {
+                    let do_not_enforce_named_arguments_for_c_variadic =
+                        |token: &token::Token| -> bool {
+                            if token == &token::DotDotDot {
+                                false
+                            } else {
+                                named_args
+                            }
+                        };
+                    match p.parse_arg_general(
+                        false,
+                        allow_c_variadic,
+                        do_not_enforce_named_arguments_for_c_variadic
+                    ) {
                         Ok(arg) => {
                             if let TyKind::CVarArgs = arg.ty.node {
                                 c_variadic = true;
@@ -5466,7 +5478,6 @@ impl<'a> Parser<'a> {
 
     /// Parses the argument list and result type of a function declaration.
     fn parse_fn_decl(&mut self, allow_c_variadic: bool) -> PResult<'a, P<FnDecl>> {
-
         let (args, c_variadic) = self.parse_fn_args(true, allow_c_variadic)?;
         let ret_ty = self.parse_ret_ty(true)?;
 
@@ -5478,6 +5489,8 @@ impl<'a> Parser<'a> {
     }
 
     /// Returns the parsed optional self argument and whether a self shortcut was used.
+    ///
+    /// See `parse_self_arg_with_attrs` to collect attributes.
     fn parse_self_arg(&mut self) -> PResult<'a, Option<Arg>> {
         let expect_ident = |this: &mut Self| match this.token.kind {
             // Preserve hygienic context.
@@ -5583,7 +5596,18 @@ impl<'a> Parser<'a> {
         };
 
         let eself = source_map::respan(eself_lo.to(eself_hi), eself);
-        Ok(Some(Arg::from_self(eself, eself_ident)))
+        Ok(Some(Arg::from_self(ThinVec::default(), eself, eself_ident)))
+    }
+
+    /// Returns the parsed optional self argument with attributes and whether a self
+    /// shortcut was used.
+    fn parse_self_arg_with_attrs(&mut self) -> PResult<'a, Option<Arg>> {
+        let attrs = self.parse_arg_attributes()?;
+        let arg_opt = self.parse_self_arg()?;
+        Ok(arg_opt.map(|mut arg| {
+            arg.attrs = attrs.into();
+            arg
+        }))
     }
 
     /// Parses the parameter list and result type of a function that may have a `self` parameter.
@@ -5593,7 +5617,7 @@ impl<'a> Parser<'a> {
         self.expect(&token::OpenDelim(token::Paren))?;
 
         // Parse optional self argument.
-        let self_arg = self.parse_self_arg()?;
+        let self_arg = self.parse_self_arg_with_attrs()?;
 
         // Parse the rest of the function parameter list.
         let sep = SeqSep::trailing_allowed(token::Comma);
@@ -5867,7 +5891,7 @@ impl<'a> Parser<'a> {
             let ident = self.parse_ident()?;
             let mut generics = self.parse_generics()?;
             let decl = self.parse_fn_decl_with_self(|p| {
-                p.parse_arg_general(true, true, false)
+                p.parse_arg_general(true, false, |_| true)
             })?;
             generics.where_clause = self.parse_where_clause()?;
             *at_end = true;
@@ -7443,7 +7467,7 @@ impl<'a> Parser<'a> {
             } else if self.look_ahead(1, |t| *t == token::OpenDelim(token::Paren)) {
                 let ident = self.parse_ident().unwrap();
                 self.bump();  // `(`
-                let kw_name = if let Ok(Some(_)) = self.parse_self_arg() {
+                let kw_name = if let Ok(Some(_)) = self.parse_self_arg_with_attrs() {
                     "method"
                 } else {
                     "function"
@@ -7494,7 +7518,7 @@ impl<'a> Parser<'a> {
                 self.eat_to_tokens(&[&token::Gt]);
                 self.bump();  // `>`
                 let (kw, kw_name, ambiguous) = if self.eat(&token::OpenDelim(token::Paren)) {
-                    if let Ok(Some(_)) = self.parse_self_arg() {
+                    if let Ok(Some(_)) = self.parse_self_arg_with_attrs() {
                         ("fn", "method", false)
                     } else {
                         ("fn", "function", false)
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 24b0c372471..8132024416a 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -550,8 +550,9 @@ pub fn walk_fn_ret_ty<'a, V: Visitor<'a>>(visitor: &mut V, ret_ty: &'a FunctionR
 
 pub fn walk_fn_decl<'a, V: Visitor<'a>>(visitor: &mut V, function_declaration: &'a FnDecl) {
     for argument in &function_declaration.inputs {
+        walk_list!(visitor, visit_attribute, argument.attrs.iter());
         visitor.visit_pat(&argument.pat);
-        visitor.visit_ty(&argument.ty)
+        visitor.visit_ty(&argument.ty);
     }
     visitor.visit_fn_ret_ty(&function_declaration.output)
 }