ast: Keep extern qualifiers in functions more precisely

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()?;
@@ -1905,11 +1912,11 @@ 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
@@ -1917,7 +1924,7 @@ impl<'a> Parser<'a> {
             // 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,