ast: Keep extern qualifiers in functions more precisely

src/librustc/hir/lowering.rs

@@ -1219,7 +1219,7 @@ impl<'a> LoweringContext<'a> {
                                     ImplTraitContext::disallowed(),
                                 ),
                                 unsafety: f.unsafety,
-                                abi: this.lower_abi(f.abi),
+                                abi: this.lower_extern(f.ext),
                                 decl: this.lower_fn_decl(&f.decl, None, false, None),
                                 param_names: this.lower_fn_params_to_names(&f.decl),
                             }))

src/librustc/hir/lowering/item.rs

@@ -735,7 +735,7 @@ impl LoweringContext<'_> {
 
     fn lower_foreign_mod(&mut self, fm: &ForeignMod) -> hir::ForeignMod {
         hir::ForeignMod {
-            abi: self.lower_abi(fm.abi),
+            abi: fm.abi.map_or(abi::Abi::C, |abi| self.lower_abi(abi)),
             items: fm.items
                 .iter()
                 .map(|x| self.lower_foreign_item(x))
@@ -1283,7 +1283,7 @@ impl LoweringContext<'_> {
             unsafety: h.unsafety,
             asyncness: self.lower_asyncness(h.asyncness.node),
             constness: h.constness.node,
-            abi: self.lower_abi(h.abi),
+            abi: self.lower_extern(h.ext),
         }
     }
 
@@ -1294,6 +1294,14 @@ impl LoweringContext<'_> {
         })
     }
 
+    pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
+        match ext {
+            Extern::None => abi::Abi::Rust,
+            Extern::Implicit => abi::Abi::C,
+            Extern::Explicit(abi) => self.lower_abi(abi),
+        }
+    }
+
     fn error_on_invalid_abi(&self, abi: Abi) {
         struct_span_err!(
             self.sess,

src/librustc_parse/parser/item.rs

@@ -114,7 +114,7 @@ impl<'a> Parser<'a> {
                     unsafety: Unsafety::Normal,
                     asyncness: respan(fn_span, IsAsync::NotAsync),
                     constness: respan(fn_span, Constness::NotConst),
-                    abi,
+                    ext: Extern::from_abi(abi),
                 };
                 return self.parse_item_fn(lo, vis, attrs, header);
             } else if self.check(&token::OpenDelim(token::Brace)) {
@@ -143,14 +143,14 @@ impl<'a> Parser<'a> {
                 if self.check_keyword(kw::Extern) {
                     self.sess.gated_spans.gate(sym::const_extern_fn, lo.to(self.token.span));
                 }
-                let abi = self.parse_extern_abi()?;
+                let ext = self.parse_extern()?;
                 self.bump(); // `fn`
 
                 let header = FnHeader {
                     unsafety,
                     asyncness: respan(const_span, IsAsync::NotAsync),
                     constness: respan(const_span, Constness::Const),
-                    abi,
+                    ext,
                 };
                 return self.parse_item_fn(lo, vis, attrs, header);
             }
@@ -193,7 +193,7 @@ impl<'a> Parser<'a> {
                     unsafety,
                     asyncness,
                     constness: respan(fn_span, Constness::NotConst),
-                    abi: Abi::new(sym::Rust, fn_span),
+                    ext: Extern::None,
                 };
                 return self.parse_item_fn(lo, vis, attrs, header);
             }
@@ -230,7 +230,7 @@ impl<'a> Parser<'a> {
                 unsafety: Unsafety::Normal,
                 asyncness: respan(fn_span, IsAsync::NotAsync),
                 constness: respan(fn_span, Constness::NotConst),
-                abi: Abi::new(sym::Rust, fn_span),
+                ext: Extern::None,
             };
             return self.parse_item_fn(lo, vis, attrs, header);
         }
@@ -242,14 +242,14 @@ impl<'a> Parser<'a> {
             self.bump(); // `unsafe`
             // `{` is also expected after `unsafe`; in case of error, include it in the diagnostic.
             self.check(&token::OpenDelim(token::Brace));
-            let abi = self.parse_extern_abi()?;
+            let ext = self.parse_extern()?;
             self.expect_keyword(kw::Fn)?;
             let fn_span = self.prev_span;
             let header = FnHeader {
                 unsafety: Unsafety::Unsafe,
                 asyncness: respan(fn_span, IsAsync::NotAsync),
                 constness: respan(fn_span, Constness::NotConst),
-                abi,
+                ext,
             };
             return self.parse_item_fn(lo, vis, attrs, header);
         }
@@ -1100,7 +1100,7 @@ impl<'a> Parser<'a> {
     fn parse_item_foreign_mod(
         &mut self,
         lo: Span,
-        abi: Abi,
+        abi: Option<Abi>,
         visibility: Visibility,
         mut attrs: Vec<Attribute>,
         extern_sp: Span,
@@ -1775,9 +1775,16 @@ impl<'a> Parser<'a> {
         attrs: Vec<Attribute>,
         header: FnHeader,
     ) -> PResult<'a, Option<P<Item>>> {
+        let is_c_abi = match header.ext {
+            ast::Extern::None => false,
+            ast::Extern::Implicit => true,
+            ast::Extern::Explicit(abi) => abi.symbol == sym::C,
+        };
         let (ident, decl, generics) = self.parse_fn_sig(ParamCfg {
             is_self_allowed: false,
-            allow_c_variadic: header.abi.symbol == sym::C && header.unsafety == Unsafety::Unsafe,
+            // FIXME: Parsing should not depend on ABI or unsafety and
+            // the variadic parameter should always be parsed.
+            allow_c_variadic: is_c_abi && header.unsafety == Unsafety::Unsafe,
             is_name_required: |_| true,
         })?;
         let (inner_attrs, body) = self.parse_inner_attrs_and_block()?;
@@ -1905,19 +1912,19 @@ impl<'a> Parser<'a> {
         }
         let asyncness = respan(self.prev_span, asyncness);
         let unsafety = self.parse_unsafety();
-        let (constness, unsafety, abi) = if is_const_fn {
-            (respan(const_span, Constness::Const), unsafety, Abi::default())
+        let (constness, unsafety, ext) = if is_const_fn {
+            (respan(const_span, Constness::Const), unsafety, Extern::None)
         } else {
-            let abi = self.parse_extern_abi()?;
-            (respan(self.prev_span, Constness::NotConst), unsafety, abi)
+            let ext = self.parse_extern()?;
+            (respan(self.prev_span, Constness::NotConst), unsafety, ext)
         };
         if !self.eat_keyword(kw::Fn) {
             // It is possible for `expect_one_of` to recover given the contents of
             // `self.expected_tokens`, therefore, do not use `self.unexpected()` which doesn't
             // account for this.
             if !self.expect_one_of(&[], &[])? { unreachable!() }
         }
-        Ok(FnHeader { constness, unsafety, asyncness, abi })
+        Ok(FnHeader { constness, unsafety, asyncness, ext })
     }
 
     /// Parse the "signature", including the identifier, parameters, and generics of a function.

src/librustc_parse/parser/mod.rs

@@ -15,7 +15,7 @@ use crate::{Directory, DirectoryOwnership};
 use crate::lexer::UnmatchedBrace;
 
 use syntax::ast::{
-    self, Abi, DUMMY_NODE_ID, AttrStyle, Attribute, CrateSugar, Ident,
+    self, Abi, DUMMY_NODE_ID, AttrStyle, Attribute, CrateSugar, Extern, Ident,
     IsAsync, MacDelimiter, Mutability, StrStyle, Visibility, VisibilityKind, Unsafety,
 };
 
@@ -1212,23 +1212,20 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses `extern string_literal?`.
-    /// If `extern` is not found, the Rust ABI is used.
-    /// If `extern` is found and a `string_literal` does not follow, the C ABI is used.
-    fn parse_extern_abi(&mut self) -> PResult<'a, Abi> {
+    fn parse_extern(&mut self) -> PResult<'a, Extern> {
         Ok(if self.eat_keyword(kw::Extern) {
-            self.parse_opt_abi()?
+            Extern::from_abi(self.parse_opt_abi()?)
         } else {
-            Abi::default()
+            Extern::None
         })
     }
 
     /// Parses a string literal as an ABI spec.
-    /// If one is not found, the "C" ABI is used.
-    fn parse_opt_abi(&mut self) -> PResult<'a, Abi> {
-        let span = if self.token.can_begin_literal_or_bool() {
+    fn parse_opt_abi(&mut self) -> PResult<'a, Option<Abi>> {
+        if self.token.can_begin_literal_or_bool() {
             let ast::Lit { span, kind, .. } = self.parse_lit()?;
             match kind {
-                ast::LitKind::Str(symbol, _) => return Ok(Abi::new(symbol, span)),
+                ast::LitKind::Str(symbol, _) => return Ok(Some(Abi { symbol, span })),
                 ast::LitKind::Err(_) => {}
                 _ => {
                     self.struct_span_err(span, "non-string ABI literal")
@@ -1241,11 +1238,8 @@ impl<'a> Parser<'a> {
                         .emit();
                 }
             }
-            span
-        } else {
-            self.prev_span
-        };
-        Ok(Abi::new(sym::C, span))
+        }
+        Ok(None)
     }
 
     /// We are parsing `async fn`. If we are on Rust 2015, emit an error.

src/librustc_parse/parser/ty.rs

@@ -287,7 +287,7 @@ impl<'a> Parser<'a> {
         */
 
         let unsafety = self.parse_unsafety();
-        let abi = self.parse_extern_abi()?;
+        let ext = self.parse_extern()?;
         self.expect_keyword(kw::Fn)?;
         let cfg = ParamCfg {
             is_self_allowed: false,
@@ -296,7 +296,7 @@ impl<'a> Parser<'a> {
         };
         let decl = self.parse_fn_decl(cfg, false)?;
         Ok(TyKind::BareFn(P(BareFnTy {
-            abi,
+            ext,
             unsafety,
             generic_params,
             decl,

src/librustc_save_analysis/sig.rs

@@ -30,9 +30,8 @@ use crate::{id_from_def_id, id_from_node_id, SaveContext};
 use rls_data::{SigElement, Signature};
 
 use rustc::hir::def::{Res, DefKind};
-use syntax::ast::{self, NodeId};
+use syntax::ast::{self, Extern, NodeId};
 use syntax::print::pprust;
-use syntax_pos::sym;
 
 pub fn item_signature(item: &ast::Item, scx: &SaveContext<'_, '_>) -> Option<Signature> {
     if !scx.config.signatures {
@@ -157,9 +156,11 @@ fn text_sig(text: String) -> Signature {
     }
 }
 
-fn push_abi(text: &mut String, abi: ast::Abi) {
-    if abi.symbol != sym::Rust {
-        text.push_str(&format!("extern \"{}\" ", abi.symbol));
+fn push_extern(text: &mut String, ext: Extern) {
+    match ext {
+        Extern::None => {}
+        Extern::Implicit => text.push_str("extern "),
+        Extern::Explicit(abi) => text.push_str(&format!("extern \"{}\" ", abi.symbol)),
     }
 }
 
@@ -237,7 +238,7 @@ impl Sig for ast::Ty {
                 if f.unsafety == ast::Unsafety::Unsafe {
                     text.push_str("unsafe ");
                 }
-                push_abi(&mut text, f.abi);
+                push_extern(&mut text, f.ext);
                 text.push_str("fn(");
 
                 let mut defs = vec![];
@@ -387,7 +388,7 @@ impl Sig for ast::Item {
                 if header.unsafety == ast::Unsafety::Unsafe {
                     text.push_str("unsafe ");
                 }
-                push_abi(&mut text, header.abi);
+                push_extern(&mut text, header.ext);
                 text.push_str("fn ");
 
                 let mut sig = name_and_generics(text, offset, generics, self.id, self.ident, scx)?;
@@ -936,7 +937,7 @@ fn make_method_signature(
     if m.header.unsafety == ast::Unsafety::Unsafe {
         text.push_str("unsafe ");
     }
-    push_abi(&mut text, m.header.abi);
+    push_extern(&mut text, m.header.ext);
     text.push_str("fn ");
 
     let mut sig = name_and_generics(text, 0, generics, id, ident, scx)?;

src/libsyntax/ast.rs

@@ -1745,7 +1745,7 @@ pub struct Ty {
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct BareFnTy {
     pub unsafety: Unsafety,
-    pub abi: Abi,
+    pub ext: Extern,
     pub generic_params: Vec<GenericParam>,
     pub decl: P<FnDecl>,
 }
@@ -2128,7 +2128,7 @@ pub struct Mod {
 /// E.g., `extern { .. }` or `extern C { .. }`.
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct ForeignMod {
-    pub abi: Abi,
+    pub abi: Option<Abi>,
     pub items: Vec<ForeignItem>,
 }
 
@@ -2420,15 +2420,20 @@ pub struct Abi {
     pub span: Span,
 }
 
-impl Abi {
-    pub fn new(symbol: Symbol, span: Span) -> Self {
-        Self { symbol, span }
-    }
+/// `extern` qualifier on a function item or function type.
+#[derive(Clone, Copy, RustcEncodable, RustcDecodable, Debug)]
+pub enum Extern {
+    None,
+    Implicit,
+    Explicit(Abi),
 }
 
-impl Default for Abi {
-    fn default() -> Self {
-        Self::new(sym::Rust, DUMMY_SP)
+impl Extern {
+    pub fn from_abi(abi: Option<Abi>) -> Extern {
+        match abi {
+            Some(abi) => Extern::Explicit(abi),
+            None => Extern::Implicit,
+        }
     }
 }
 
@@ -2441,7 +2446,7 @@ pub struct FnHeader {
     pub unsafety: Unsafety,
     pub asyncness: Spanned<IsAsync>,
     pub constness: Spanned<Constness>,
-    pub abi: Abi,
+    pub ext: Extern,
 }
 
 impl Default for FnHeader {
@@ -2450,7 +2455,7 @@ impl Default for FnHeader {
             unsafety: Unsafety::Normal,
             asyncness: dummy_spanned(IsAsync::NotAsync),
             constness: dummy_spanned(Constness::NotConst),
-            abi: Abi::default(),
+            ext: Extern::None,
         }
     }
 }

src/libsyntax/feature_gate/check.rs

@@ -258,6 +258,12 @@ impl<'a> PostExpansionVisitor<'a> {
         }
     }
 
+    fn check_extern(&self, ext: ast::Extern) {
+        if let ast::Extern::Explicit(abi) = ext {
+            self.check_abi(abi);
+        }
+    }
+
     fn maybe_report_invalid_custom_discriminants(&self, variants: &[ast::Variant]) {
         let has_fields = variants.iter().any(|variant| match variant.data {
             VariantData::Tuple(..) | VariantData::Struct(..) => true,
@@ -388,7 +394,9 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
     fn visit_item(&mut self, i: &'a ast::Item) {
         match i.kind {
             ast::ItemKind::ForeignMod(ref foreign_module) => {
-                self.check_abi(foreign_module.abi);
+                if let Some(abi) = foreign_module.abi {
+                    self.check_abi(abi);
+                }
             }
 
             ast::ItemKind::Fn(..) => {
@@ -511,7 +519,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
     fn visit_ty(&mut self, ty: &'a ast::Ty) {
         match ty.kind {
             ast::TyKind::BareFn(ref bare_fn_ty) => {
-                self.check_abi(bare_fn_ty.abi);
+                self.check_extern(bare_fn_ty.ext);
             }
             ast::TyKind::Never => {
                 gate_feature_post!(&self, never_type, ty.span,
@@ -605,7 +613,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             // Stability of const fn methods are covered in
             // `visit_trait_item` and `visit_impl_item` below; this is
             // because default methods don't pass through this point.
-            self.check_abi(header.abi);
+            self.check_extern(header.ext);
         }
 
         if fn_decl.c_variadic() {
@@ -639,7 +647,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         match ti.kind {
             ast::TraitItemKind::Method(ref sig, ref block) => {
                 if block.is_none() {
-                    self.check_abi(sig.header.abi);
+                    self.check_extern(sig.header.ext);
                 }
                 if sig.decl.c_variadic() {
                     gate_feature_post!(&self, c_variadic, ti.span,