Detect fn with a body in an extern block

src/libsyntax/ext/expand.rs

@@ -742,7 +742,7 @@ impl<'a> Parser<'a> {
             AstFragmentKind::ForeignItems => {
                 let mut items = SmallVec::new();
                 while self.token != token::Eof {
-                    items.push(self.parse_foreign_item()?);
+                    items.push(self.parse_foreign_item(DUMMY_SP)?);
                 }
                 AstFragment::ForeignItems(items)
             }

src/libsyntax/parse/diagnostics.rs

@@ -15,6 +15,7 @@ use errors::{Applicability, DiagnosticBuilder, DiagnosticId};
 use rustc_data_structures::fx::FxHashSet;
 use syntax_pos::{Span, DUMMY_SP, MultiSpan};
 use log::{debug, trace};
+use std::mem;
 
 /// Creates a placeholder argument.
 crate fn dummy_arg(ident: Ident) -> Arg {
@@ -783,6 +784,42 @@ impl<'a> Parser<'a> {
         Err(err)
     }
 
+    crate fn parse_semi_or_incorrect_foreign_fn_body(
+        &mut self,
+        ident: &Ident,
+        extern_sp: Span,
+    ) -> PResult<'a, ()> {
+        if self.token != token::Semi {
+            // this might be an incorrect fn definition (#62109)
+            let parser_snapshot = self.clone();
+            match self.parse_inner_attrs_and_block() {
+                Ok((_, body)) => {
+                    self.struct_span_err(ident.span, "incorrect `fn` inside `extern` block")
+                        .span_label(ident.span, "can't have a body")
+                        .span_label(body.span, "this body is invalid here")
+                        .span_label(
+                            extern_sp,
+                            "`extern` blocks define existing foreign functions and `fn`s \
+                             inside of them can't have a body")
+                        .help("you might have meant to write a function accessible through ffi, \
+                               which can be done by writing `extern fn` outside of the \
+                               `extern` block")
+                        .note("for more information, visit \
+                               https://doc.rust-lang.org/std/keyword.extern.html")
+                        .emit();
+                }
+                Err(mut err) => {
+                    err.cancel();
+                    mem::replace(self, parser_snapshot);
+                    self.expect(&token::Semi)?;
+                }
+            }
+        } else {
+            self.bump();
+        }
+        Ok(())
+    }
+
     /// Consume alternative await syntaxes like `await <expr>`, `await? <expr>`, `await(<expr>)`
     /// and `await { <expr> }`.
     crate fn parse_incorrect_await_syntax(

src/libsyntax/parse/parser.rs

@@ -4615,7 +4615,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a block. Inner attributes are allowed.
-    fn parse_inner_attrs_and_block(&mut self) -> PResult<'a, (Vec<Attribute>, P<Block>)> {
+    crate fn parse_inner_attrs_and_block(&mut self) -> PResult<'a, (Vec<Attribute>, P<Block>)> {
         maybe_whole!(self, NtBlock, |x| (Vec::new(), x));
 
         let lo = self.token.span;
@@ -6700,15 +6700,20 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a function declaration from a foreign module.
-    fn parse_item_foreign_fn(&mut self, vis: ast::Visibility, lo: Span, attrs: Vec<Attribute>)
-                             -> PResult<'a, ForeignItem> {
+    fn parse_item_foreign_fn(
+        &mut self,
+        vis: ast::Visibility,
+        lo: Span,
+        attrs: Vec<Attribute>,
+        extern_sp: Span,
+    ) -> PResult<'a, ForeignItem> {
         self.expect_keyword(kw::Fn)?;
 
         let (ident, mut generics) = self.parse_fn_header()?;
         let decl = self.parse_fn_decl(true)?;
         generics.where_clause = self.parse_where_clause()?;
         let hi = self.token.span;
-        self.expect(&token::Semi)?;
+        self.parse_semi_or_incorrect_foreign_fn_body(&ident, extern_sp)?;
         Ok(ast::ForeignItem {
             ident,
             attrs,
@@ -6835,12 +6840,14 @@ impl<'a> Parser<'a> {
     /// extern "C" {}
     /// extern {}
     /// ```
-    fn parse_item_foreign_mod(&mut self,
-                              lo: Span,
-                              opt_abi: Option<Abi>,
-                              visibility: Visibility,
-                              mut attrs: Vec<Attribute>)
-                              -> PResult<'a, P<Item>> {
+    fn parse_item_foreign_mod(
+        &mut self,
+        lo: Span,
+        opt_abi: Option<Abi>,
+        visibility: Visibility,
+        mut attrs: Vec<Attribute>,
+        extern_sp: Span,
+    ) -> PResult<'a, P<Item>> {
         self.expect(&token::OpenDelim(token::Brace))?;
 
         let abi = opt_abi.unwrap_or(Abi::C);
@@ -6849,7 +6856,7 @@ impl<'a> Parser<'a> {
 
         let mut foreign_items = vec![];
         while !self.eat(&token::CloseDelim(token::Brace)) {
-            foreign_items.push(self.parse_foreign_item()?);
+            foreign_items.push(self.parse_foreign_item(extern_sp)?);
         }
 
         let prev_span = self.prev_span;
@@ -7096,6 +7103,7 @@ impl<'a> Parser<'a> {
         }
 
         if self.eat_keyword(kw::Extern) {
+            let extern_sp = self.prev_span;
             if self.eat_keyword(kw::Crate) {
                 return Ok(Some(self.parse_item_extern_crate(lo, visibility, attrs)?));
             }
@@ -7119,7 +7127,9 @@ impl<'a> Parser<'a> {
                                         maybe_append(attrs, extra_attrs));
                 return Ok(Some(item));
             } else if self.check(&token::OpenDelim(token::Brace)) {
-                return Ok(Some(self.parse_item_foreign_mod(lo, opt_abi, visibility, attrs)?));
+                return Ok(Some(
+                    self.parse_item_foreign_mod(lo, opt_abi, visibility, attrs, extern_sp)?,
+                ));
             }
 
             self.unexpected()?;
@@ -7504,7 +7514,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a foreign item.
-    crate fn parse_foreign_item(&mut self) -> PResult<'a, ForeignItem> {
+    crate fn parse_foreign_item(&mut self, extern_sp: Span) -> PResult<'a, ForeignItem> {
         maybe_whole!(self, NtForeignItem, |ni| ni);
 
         let attrs = self.parse_outer_attributes()?;
@@ -7529,7 +7539,7 @@ impl<'a> Parser<'a> {
         }
         // FOREIGN FUNCTION ITEM
         if self.check_keyword(kw::Fn) {
-            return Ok(self.parse_item_foreign_fn(visibility, lo, attrs)?);
+            return Ok(self.parse_item_foreign_fn(visibility, lo, attrs, extern_sp)?);
         }
         // FOREIGN TYPE ITEM
         if self.check_keyword(kw::Type) {

src/test/ui/extern/extern-ffi-fn-with-body.rs

@@ -0,0 +1,11 @@
+extern "C" {
+    fn foo() -> i32 { //~ ERROR incorrect `fn` inside `extern` block
+        return 0;
+    }
+}
+
+extern "C" fn bar() -> i32 {
+    return 0;
+}
+
+fn main() {}

src/test/ui/extern/extern-ffi-fn-with-body.stderr

@@ -0,0 +1,18 @@
+error: incorrect `fn` inside `extern` block
+  --> $DIR/extern-ffi-fn-with-body.rs:2:8
+   |
+LL |   extern "C" {
+   |   ------ `extern` blocks define existing foreign functions and `fn`s inside of them can't have a body
+LL |       fn foo() -> i32 {
+   |  ________^^^__________-
+   | |        |
+   | |        can't have a body
+LL | |         return 0;
+LL | |     }
+   | |_____- this body is invalid here
+   |
+   = help: you might have meant to write a function accessible through ffi, which can be done by writing `extern fn` outside of the `extern` block
+   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html
+
+error: aborting due to previous error
+