syntax: don't keep a redundant c_variadic flag in the AST.

src/librustc/hir/lowering.rs

@@ -2168,7 +2168,7 @@ impl<'a> LoweringContext<'a> {
         P(hir::FnDecl {
             inputs,
             output,
-            c_variadic: decl.c_variadic,
+            c_variadic: decl.c_variadic(),
             implicit_self: decl.inputs.get(0).map_or(
                 hir::ImplicitSelfKind::None,
                 |arg| {

src/librustc/hir/lowering/expr.rs

@@ -450,7 +450,6 @@ impl LoweringContext<'_> {
         let ast_decl = FnDecl {
             inputs: vec![],
             output,
-            c_variadic: false
         };
         let decl = self.lower_fn_decl(&ast_decl, None, /* impl trait allowed */ false, None);
         let body_id = self.lower_fn_body(&ast_decl, |this| {
@@ -739,7 +738,6 @@ impl LoweringContext<'_> {
         let outer_decl = FnDecl {
             inputs: decl.inputs.clone(),
             output: FunctionRetTy::Default(fn_decl_span),
-            c_variadic: false,
         };
         // We need to lower the declaration outside the new scope, because we
         // have to conserve the state of being inside a loop condition for the

src/libsyntax/ast.rs

@@ -1893,7 +1893,6 @@ impl Param {
 pub struct FnDecl {
     pub inputs: Vec<Param>,
     pub output: FunctionRetTy,
-    pub c_variadic: bool,
 }
 
 impl FnDecl {
@@ -1903,6 +1902,12 @@ impl FnDecl {
     pub fn has_self(&self) -> bool {
         self.inputs.get(0).map(Param::is_self).unwrap_or(false)
     }
+    pub fn c_variadic(&self) -> bool {
+        self.inputs.last().map(|arg| match arg.ty.kind {
+            TyKind::CVarArgs => true,
+            _ => false,
+        }).unwrap_or(false)
+    }
 }
 
 /// Is the trait definition an auto trait?

src/libsyntax/ext/build.rs

@@ -562,7 +562,6 @@ impl<'a> ExtCtxt<'a> {
         P(ast::FnDecl {
             inputs,
             output,
-            c_variadic: false
         })
     }
 

src/libsyntax/feature_gate/check.rs

@@ -531,7 +531,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             self.check_abi(header.abi, span);
         }
 
-        if fn_decl.c_variadic {
+        if fn_decl.c_variadic() {
             gate_feature_post!(&self, c_variadic, span, "C-variadic functions are unstable");
         }
 
@@ -564,7 +564,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 if block.is_none() {
                     self.check_abi(sig.header.abi, ti.span);
                 }
-                if sig.decl.c_variadic {
+                if sig.decl.c_variadic() {
                     gate_feature_post!(&self, c_variadic, ti.span,
                                        "C-variadic functions are unstable");
                 }
@@ -601,7 +601,12 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         }
 
         match ii.kind {
-            ast::ImplItemKind::Method(..) => {}
+            ast::ImplItemKind::Method(ref sig, _) => {
+                if sig.decl.c_variadic() {
+                    gate_feature_post!(&self, c_variadic, ii.span,
+                                       "C-variadic functions are unstable");
+                }
+            }
             ast::ImplItemKind::OpaqueTy(..) => {
                 gate_feature_post!(
                     &self,

src/libsyntax/mut_visit.rs

@@ -717,7 +717,7 @@ pub fn noop_visit_asyncness<T: MutVisitor>(asyncness: &mut IsAsync, vis: &mut T)
 }
 
 pub fn noop_visit_fn_decl<T: MutVisitor>(decl: &mut P<FnDecl>, vis: &mut T) {
-    let FnDecl { inputs, output, c_variadic: _ } = decl.deref_mut();
+    let FnDecl { inputs, output } = decl.deref_mut();
     inputs.flat_map_in_place(|param| vis.flat_map_param(param));
     match output {
         FunctionRetTy::Default(span) => vis.visit_span(span),

src/libsyntax/parse/parser.rs

@@ -1194,7 +1194,7 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_fn_params(&mut self, named_params: bool, allow_c_variadic: bool)
-                     -> PResult<'a, (Vec<Param> , bool)> {
+                     -> PResult<'a, Vec<Param>> {
         let sp = self.token.span;
         let mut c_variadic = false;
         let (params, _): (Vec<Option<Param>>, _) = self.parse_paren_comma_seq(|p| {
@@ -1218,6 +1218,8 @@ impl<'a> Parser<'a> {
                             let span = p.token.span;
                             p.span_err(span,
                                 "`...` must be the last argument of a C-variadic function");
+                            // FIXME(eddyb) this should probably still push `CVarArgs`.
+                            // Maybe AST validation/HIR lowering should emit the above error?
                             Ok(None)
                         } else {
                             Ok(Some(param))
@@ -1245,7 +1247,7 @@ impl<'a> Parser<'a> {
                           "C-variadic function must be declared with at least one named argument");
         }
 
-        Ok((params, c_variadic))
+        Ok(params)
     }
 
     /// Returns the parsed optional self parameter and whether a self shortcut was used.
@@ -1414,7 +1416,6 @@ impl<'a> Parser<'a> {
         Ok(P(FnDecl {
             inputs: fn_inputs,
             output: self.parse_ret_ty(true)?,
-            c_variadic: false
         }))
     }
 

src/libsyntax/parse/parser/expr.rs

@@ -1176,7 +1176,6 @@ impl<'a> Parser<'a> {
         Ok(P(FnDecl {
             inputs: inputs_captures,
             output,
-            c_variadic: false
         }))
     }
 

src/libsyntax/parse/parser/item.rs

@@ -1292,13 +1292,12 @@ 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_params(true, allow_c_variadic)?;
+        let args = self.parse_fn_params(true, allow_c_variadic)?;
         let ret_ty = self.parse_ret_ty(true)?;
 
         Ok(P(FnDecl {
             inputs: args,
             output: ret_ty,
-            c_variadic,
         }))
     }
 

src/libsyntax/parse/parser/ty.rs

@@ -292,12 +292,11 @@ impl<'a> Parser<'a> {
         };
 
         self.expect_keyword(kw::Fn)?;
-        let (inputs, c_variadic) = self.parse_fn_params(false, true)?;
+        let inputs = self.parse_fn_params(false, true)?;
         let ret_ty = self.parse_ret_ty(false)?;
         let decl = P(FnDecl {
             inputs,
             output: ret_ty,
-            c_variadic,
         });
         Ok(TyKind::BareFn(P(BareFnTy {
             abi,

src/libsyntax/print/pprust/tests.rs

@@ -29,7 +29,6 @@ fn test_fun_to_string() {
         let decl = ast::FnDecl {
             inputs: Vec::new(),
             output: ast::FunctionRetTy::Default(syntax_pos::DUMMY_SP),
-            c_variadic: false
         };
         let generics = ast::Generics::default();
         assert_eq!(

src/test/ui-fulldeps/pprust-expr-roundtrip.rs

@@ -114,7 +114,6 @@ fn iter_exprs(depth: usize, f: &mut dyn FnMut(P<Expr>)) {
                 let decl = P(FnDecl {
                     inputs: vec![],
                     output: FunctionRetTy::Default(DUMMY_SP),
-                    c_variadic: false,
                 });
                 iter_exprs(depth - 1, &mut |e| g(
                         ExprKind::Closure(CaptureBy::Value,