c_variadic: Add future-incompatibility warning for ... arguments without a pattern outside of extern blocks

compiler/rustc_lint/messages.ftl

@@ -1051,5 +1051,8 @@ lint_useless_ptr_null_checks_ref = references are not nullable, so checking them
 
 lint_uses_power_alignment = repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
 
+lint_varargs_without_pattern = missing pattern for `...` argument
+    .suggestion = name the argument, or use `_` to continue ignoring it
+
 lint_variant_size_differences =
     enum variant is more than three times larger ({$largest} bytes) than the next largest

compiler/rustc_lint/src/early/diagnostics.rs

@@ -468,5 +468,8 @@ pub fn decorate_builtin_lint(
         BuiltinLintDiag::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by } => {
             lints::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by }.decorate_lint(diag)
         }
+        BuiltinLintDiag::VarargsWithoutPattern { span } => {
+            lints::VarargsWithoutPattern { span }.decorate_lint(diag)
+        }
     }
 }

compiler/rustc_lint/src/lints.rs

@@ -3215,6 +3215,13 @@ pub(crate) struct ReservedMultihash {
     pub suggestion: Span,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_varargs_without_pattern)]
+pub(crate) struct VarargsWithoutPattern {
+    #[suggestion(code = "_: ...", applicability = "machine-applicable")]
+    pub span: Span,
+}
+
 #[derive(Debug)]
 pub(crate) struct MismatchedLifetimeSyntaxes {
     pub inputs: LifetimeSyntaxCategories<Vec<Span>>,

compiler/rustc_lint_defs/src/builtin.rs

@@ -141,6 +141,7 @@ declare_lint_pass! {
         UNUSED_UNSAFE,
         UNUSED_VARIABLES,
         USELESS_DEPRECATED,
+        VARARGS_WITHOUT_PATTERN,
         WARNINGS,
         // tidy-alphabetical-end
     ]
@@ -5099,3 +5100,50 @@ declare_lint! {
         report_in_deps: true,
     };
 }
+
+declare_lint! {
+    /// The `varargs_without_pattern` lint detects when `...` is used as an argument to a
+    /// non-foreign function without any pattern being specified.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// // Using `...` in non-foreign function definitions is unstable, however stability is
+    /// // currently only checked after attributes are expanded, so using `#[cfg(false)]` here will
+    /// // allow this to compile on stable Rust.
+    /// #[cfg(false)]
+    /// fn foo(...) {
+    ///
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Patterns are currently required for all non-`...` arguments in function definitions (with
+    /// some exceptions in the 2015 edition). Requiring `...` arguments to have patterns in
+    /// non-foreign function defitions makes the language more consistent, and removes a source of
+    /// confusion for the unstable C variadic feature. `...` arguments without a pattern are already
+    /// stable and widely used in foreign function definitions; this lint only affects non-foreign
+    /// function defitions.
+    ///
+    /// Using `...` (C varargs) in a non-foreign function definition is currently unstable. However,
+    /// stability checking for the `...` syntax in non-foreign function definitions is currently
+    /// implemented after attributes have been expanded, meaning that if the attribute removes the
+    /// use of the unstable syntax (e.g. `#[cfg(false)]`, or a procedural macro), the code will
+    /// compile on stable Rust; this is the only situtation where this lint affects code that
+    /// compiles on stable Rust.
+    ///
+    /// This is a [future-incompatible] lint to transition this to a hard error in the future.
+    ///
+    /// [future-incompatible]: ../index.md#future-incompatible-lints
+    pub VARARGS_WITHOUT_PATTERN,
+    Warn,
+    "detects usage of `...` arguments without a pattern in non-foreign items",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseError,
+        reference: "issue #FIXME <https://github.com/rust-lang/rust/issues/FIXME>",
+        report_in_deps: false,
+    };
+}

compiler/rustc_lint_defs/src/lib.rs

@@ -807,6 +807,9 @@ pub enum BuiltinLintDiag {
         cfg_name: Symbol,
         controlled_by: &'static str,
     },
+    VarargsWithoutPattern {
+        span: Span,
+    },
 }
 
 /// Lints that are buffered up early on in the `Session` before the

compiler/rustc_parse/src/parser/attr.rs

@@ -200,7 +200,7 @@ impl<'a> Parser<'a> {
             AttrWrapper::empty(),
             true,
             false,
-            FnParseMode { req_name: |_| true, req_body: true },
+            FnParseMode { req_name: |_, _| true, req_body: true },
             ForceCollect::No,
         ) {
             Ok(Some(item)) => {

compiler/rustc_parse/src/parser/item.rs

@@ -11,6 +11,8 @@ use rustc_ast::{self as ast};
 use rustc_ast_pretty::pprust;
 use rustc_errors::codes::*;
 use rustc_errors::{Applicability, PResult, StashKey, struct_span_code_err};
+use rustc_session::lint::BuiltinLintDiag;
+use rustc_session::lint::builtin::VARARGS_WITHOUT_PATTERN;
 use rustc_span::edit_distance::edit_distance;
 use rustc_span::edition::Edition;
 use rustc_span::{DUMMY_SP, ErrorGuaranteed, Ident, Span, Symbol, kw, source_map, sym};
@@ -117,7 +119,7 @@ impl<'a> Parser<'a> {
 
 impl<'a> Parser<'a> {
     pub fn parse_item(&mut self, force_collect: ForceCollect) -> PResult<'a, Option<P<Item>>> {
-        let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: true };
+        let fn_parse_mode = FnParseMode { req_name: |_, _| true, req_body: true };
         self.parse_item_(fn_parse_mode, force_collect).map(|i| i.map(P))
     }
 
@@ -949,7 +951,7 @@ impl<'a> Parser<'a> {
         &mut self,
         force_collect: ForceCollect,
     ) -> PResult<'a, Option<Option<P<AssocItem>>>> {
-        let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: true };
+        let fn_parse_mode = FnParseMode { req_name: |_, _| true, req_body: true };
         self.parse_assoc_item(fn_parse_mode, force_collect)
     }
 
@@ -958,7 +960,7 @@ impl<'a> Parser<'a> {
         force_collect: ForceCollect,
     ) -> PResult<'a, Option<Option<P<AssocItem>>>> {
         let fn_parse_mode =
-            FnParseMode { req_name: |edition| edition >= Edition::Edition2018, req_body: false };
+            FnParseMode { req_name: |edition, _| edition >= Edition::Edition2018, req_body: false };
         self.parse_assoc_item(fn_parse_mode, force_collect)
     }
 
@@ -1235,7 +1237,10 @@ impl<'a> Parser<'a> {
         &mut self,
         force_collect: ForceCollect,
     ) -> PResult<'a, Option<Option<P<ForeignItem>>>> {
-        let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: false };
+        let fn_parse_mode = FnParseMode {
+            req_name: |_, is_dot_dot_dot| is_dot_dot_dot == IsDotDotDot::No,
+            req_body: false,
+        };
         Ok(self.parse_item_(fn_parse_mode, force_collect)?.map(
             |Item { attrs, id, span, vis, kind, tokens }| {
                 let kind = match ForeignItemKind::try_from(kind) {
@@ -2107,7 +2112,7 @@ impl<'a> Parser<'a> {
                 let inherited_vis =
                     Visibility { span: DUMMY_SP, kind: VisibilityKind::Inherited, tokens: None };
                 // We use `parse_fn` to get a span for the function
-                let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: true };
+                let fn_parse_mode = FnParseMode { req_name: |_, _| true, req_body: true };
                 match self.parse_fn(
                     &mut AttrVec::new(),
                     fn_parse_mode,
@@ -2340,8 +2345,16 @@ impl<'a> Parser<'a> {
 /// The function decides if, per-parameter `p`, `p` must have a pattern or just a type.
 ///
 /// This function pointer accepts an edition, because in edition 2015, trait declarations
-/// were allowed to omit parameter names. In 2018, they became required.
-type ReqName = fn(Edition) -> bool;
+/// were allowed to omit parameter names. In 2018, they became required. It also accepts an
+/// `IsDotDotDot` parameter, as `extern` function declarations and function pointer types are
+/// allowed to omit the name of the `...` but regular function items are not.
+type ReqName = fn(Edition, IsDotDotDot) -> bool;
+
+#[derive(Copy, Clone, PartialEq)]
+pub(crate) enum IsDotDotDot {
+    Yes,
+    No,
+}
 
 /// Parsing configuration for functions.
 ///
@@ -2374,6 +2387,8 @@ pub(crate) struct FnParseMode {
     ///     to true.
     ///   * The span is from Edition 2015. In particular, you can get a
     ///     2015 span inside a 2021 crate using macros.
+    ///
+    /// Or if `IsDotDotDot::Yes`, this function will also return `false` with an `extern` block.
     pub(super) req_name: ReqName,
     /// If this flag is set to `true`, then plain, semicolon-terminated function
     /// prototypes are not allowed here.
@@ -3005,9 +3020,25 @@ impl<'a> Parser<'a> {
                 return Ok((res?, Trailing::No, UsePreAttrPos::No));
             }
 
-            let is_name_required = match this.token.kind {
-                token::DotDotDot => false,
-                _ => req_name(this.token.span.with_neighbor(this.prev_token.span).edition()),
+            let is_dot_dot_dot = if this.token.kind == token::DotDotDot {
+                IsDotDotDot::Yes
+            } else {
+                IsDotDotDot::No
+            };
+            let is_name_required = req_name(
+                this.token.span.with_neighbor(this.prev_token.span).edition(),
+                is_dot_dot_dot,
+            );
+            let is_name_required = if is_name_required && is_dot_dot_dot == IsDotDotDot::Yes {
+                this.psess.buffer_lint(
+                    VARARGS_WITHOUT_PATTERN,
+                    this.token.span,
+                    ast::CRATE_NODE_ID,
+                    BuiltinLintDiag::VarargsWithoutPattern { span: this.token.span },
+                );
+                false
+            } else {
+                is_name_required
             };
             let (pat, ty) = if is_name_required || this.is_named_param() {
                 debug!("parse_param_general parse_pat (is_name_required:{})", is_name_required);

compiler/rustc_parse/src/parser/path.rs

@@ -400,7 +400,7 @@ impl<'a> Parser<'a> {
 
                     let dcx = self.dcx();
                     let parse_params_result = self.parse_paren_comma_seq(|p| {
-                        let param = p.parse_param_general(|_| false, false, false);
+                        let param = p.parse_param_general(|_, _| false, false, false);
                         param.map(move |param| {
                             if !matches!(param.pat.kind, PatKind::Missing) {
                                 dcx.emit_err(FnPathFoundNamedParams {

compiler/rustc_parse/src/parser/stmt.rs

@@ -154,7 +154,7 @@ impl<'a> Parser<'a> {
             attrs.clone(), // FIXME: unwanted clone of attrs
             false,
             true,
-            FnParseMode { req_name: |_| true, req_body: true },
+            FnParseMode { req_name: |_, _| true, req_body: true },
             force_collect,
         )? {
             self.mk_stmt(lo.to(item.span), StmtKind::Item(P(item)))

compiler/rustc_parse/src/parser/ty.rs

@@ -750,7 +750,7 @@ impl<'a> Parser<'a> {
         if self.may_recover() && self.token == TokenKind::Lt {
             self.recover_fn_ptr_with_generics(lo, &mut params, param_insertion_point)?;
         }
-        let decl = self.parse_fn_decl(|_| false, AllowPlus::No, recover_return_sign)?;
+        let decl = self.parse_fn_decl(|_, _| false, AllowPlus::No, recover_return_sign)?;
 
         let decl_span = span_start.to(self.prev_token.span);
         Ok(TyKind::FnPtr(P(FnPtrTy { ext, safety, generic_params: params, decl, decl_span })))
@@ -1289,7 +1289,7 @@ impl<'a> Parser<'a> {
         self.bump();
         let args_lo = self.token.span;
         let snapshot = self.create_snapshot_for_diagnostic();
-        match self.parse_fn_decl(|_| false, AllowPlus::No, RecoverReturnSign::OnlyFatArrow) {
+        match self.parse_fn_decl(|_, _| false, AllowPlus::No, RecoverReturnSign::OnlyFatArrow) {
             Ok(decl) => {
                 self.dcx().emit_err(ExpectedFnPathFoundFnKeyword { fn_token_span });
                 Some(ast::Path {
@@ -1374,7 +1374,7 @@ impl<'a> Parser<'a> {
         // Parse `(T, U) -> R`.
         let inputs_lo = self.token.span;
         let inputs: ThinVec<_> =
-            self.parse_fn_params(|_| false)?.into_iter().map(|input| input.ty).collect();
+            self.parse_fn_params(|_, _| false)?.into_iter().map(|input| input.ty).collect();
         let inputs_span = inputs_lo.to(self.prev_token.span);
         let output = self.parse_ret_ty(AllowPlus::No, RecoverQPath::No, RecoverReturnSign::No)?;
         let args = ast::ParenthesizedArgs {

tests/crashes/132142.rs

@@ -1,3 +1,3 @@
 //@ known-bug: #132142
 
-async extern "cmse-nonsecure-entry" fn fun(...) {}
+async extern "cmse-nonsecure-entry" fn fun(_: ...) {}

tests/pretty/hir-fn-variadic.pp

@@ -16,12 +16,12 @@
 
 fn main() {
     fn g1(_: extern "C" fn(_: u8, va: ...)) { }
-    fn g2(_: extern "C" fn(_: u8, ...)) { }
+    fn g2(_: extern "C" fn(_: u8, _: ...)) { }
     fn g3(_: extern "C" fn(u8, va: ...)) { }
-    fn g4(_: extern "C" fn(u8, ...)) { }
+    fn g4(_: extern "C" fn(u8, _: ...)) { }
 
     fn g5(_: extern "C" fn(va: ...)) { }
-    fn g6(_: extern "C" fn(...)) { }
+    fn g6(_: extern "C" fn(_: ...)) { }
 
     {
         let _ =

tests/pretty/hir-fn-variadic.rs

@@ -14,16 +14,16 @@ pub unsafe extern "C" fn bar(_: i32, mut va2: ...) -> usize {
 
 fn main() {
     fn g1(_: extern "C" fn(_: u8, va: ...)) {}
-    fn g2(_: extern "C" fn(_: u8, ...)) {}
+    fn g2(_: extern "C" fn(_: u8, _: ...)) {}
     fn g3(_: extern "C" fn(u8, va: ...)) {}
-    fn g4(_: extern "C" fn(u8, ...)) {}
+    fn g4(_: extern "C" fn(u8, _: ...)) {}
 
     fn g5(_: extern "C" fn(va: ...)) {}
-    fn g6(_: extern "C" fn(...)) {}
+    fn g6(_: extern "C" fn(_: ...)) {}
 
     _ = { unsafe extern "C" fn f1(_: u8, va: ...) {} };
-    _ = { unsafe extern "C" fn f2(_: u8, ...) {} };
+    _ = { unsafe extern "C" fn f2(_: u8, _: ...) {} };
 
     _ = { unsafe extern "C" fn f5(va: ...) {} };
-    _ = { unsafe extern "C" fn f6(...) {} };
+    _ = { unsafe extern "C" fn f6(_: ...) {} };
 }

tests/ui/c-variadic/issue-86053-1.rs

@@ -8,7 +8,7 @@ fn ordering4 < 'a , 'b     > ( a :            ,   self , self ,   self ,
     //~| ERROR unexpected `self` parameter in function
     //~| ERROR unexpected `self` parameter in function
     //~| ERROR unexpected `self` parameter in function
-    self , ... ,   self ,   self , ... ) where F : FnOnce ( & 'a & 'b usize ) {
+    self , _: ... ,   self ,   self , _: ... ) where F : FnOnce ( & 'a & 'b usize ) {
     //~^ ERROR unexpected `self` parameter in function
     //~| ERROR unexpected `self` parameter in function
     //~| ERROR unexpected `self` parameter in function

tests/ui/c-variadic/issue-86053-1.stderr

@@ -25,46 +25,46 @@ LL | fn ordering4 < 'a , 'b     > ( a :            ,   self , self ,   self ,
 error: unexpected `self` parameter in function
   --> $DIR/issue-86053-1.rs:11:5
    |
-LL |     self , ... ,   self ,   self , ... ) where F : FnOnce ( & 'a & 'b usize ) {
+LL |     self , _: ... ,   self ,   self , _: ... ) where F : FnOnce ( & 'a & 'b usize ) {
    |     ^^^^ must be the first parameter of an associated function
 
 error: unexpected `self` parameter in function
-  --> $DIR/issue-86053-1.rs:11:20
+  --> $DIR/issue-86053-1.rs:11:23
    |
-LL |     self , ... ,   self ,   self , ... ) where F : FnOnce ( & 'a & 'b usize ) {
-   |                    ^^^^ must be the first parameter of an associated function
+LL |     self , _: ... ,   self ,   self , _: ... ) where F : FnOnce ( & 'a & 'b usize ) {
+   |                       ^^^^ must be the first parameter of an associated function
 
 error: unexpected `self` parameter in function
-  --> $DIR/issue-86053-1.rs:11:29
+  --> $DIR/issue-86053-1.rs:11:32
    |
-LL |     self , ... ,   self ,   self , ... ) where F : FnOnce ( & 'a & 'b usize ) {
-   |                             ^^^^ must be the first parameter of an associated function
+LL |     self , _: ... ,   self ,   self , _: ... ) where F : FnOnce ( & 'a & 'b usize ) {
+   |                                ^^^^ must be the first parameter of an associated function
 
 error: `...` must be the last argument of a C-variadic function
   --> $DIR/issue-86053-1.rs:11:12
    |
-LL |     self , ... ,   self ,   self , ... ) where F : FnOnce ( & 'a & 'b usize ) {
-   |            ^^^
+LL |     self , _: ... ,   self ,   self , _: ... ) where F : FnOnce ( & 'a & 'b usize ) {
+   |            ^^^^^^
 
 error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
   --> $DIR/issue-86053-1.rs:11:12
    |
-LL |     self , ... ,   self ,   self , ... ) where F : FnOnce ( & 'a & 'b usize ) {
-   |            ^^^                     ^^^
+LL |     self , _: ... ,   self ,   self , _: ... ) where F : FnOnce ( & 'a & 'b usize ) {
+   |            ^^^^^^                     ^^^^^^
 
 error[E0412]: cannot find type `F` in this scope
-  --> $DIR/issue-86053-1.rs:11:48
+  --> $DIR/issue-86053-1.rs:11:54
    |
-LL |     self , ... ,   self ,   self , ... ) where F : FnOnce ( & 'a & 'b usize ) {
-   |                                                ^
+LL |     self , _: ... ,   self ,   self , _: ... ) where F : FnOnce ( & 'a & 'b usize ) {
+   |                                                      ^
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL
    |
    = note: similarly named trait `Fn` defined here
    |
 help: a trait with a similar name exists
    |
-LL |     self , ... ,   self ,   self , ... ) where Fn : FnOnce ( & 'a & 'b usize ) {
-   |                                                 +
+LL |     self , _: ... ,   self ,   self , _: ... ) where Fn : FnOnce ( & 'a & 'b usize ) {
+   |                                                       +
 help: you might be missing a type parameter
    |
 LL | fn ordering4 < 'a , 'b, F     > ( a :            ,   self , self ,   self ,

tests/ui/c-variadic/issue-86053-2.rs

@@ -5,7 +5,7 @@
 
 trait H<T> {}
 
-unsafe extern "C" fn ordering4<'a, F: H<&'static &'a ()>>(_: (), ...) {}
+unsafe extern "C" fn ordering4<'a, F: H<&'static &'a ()>>(_: (), _: ...) {}
 //~^ ERROR: in type `&'static &'a ()`, reference has a longer lifetime than the data it references [E0491]
 
 fn main() {}