parser: move `ban_async_in_2015` to `fn` parsing & improve it.

2 files changed, 12 insertions, 13 deletions

diff --git a/src/librustc_parse/parser/item.rs b/src/librustc_parse/parser/item.rs
index 8ba957c6b3b..a851ec2124f 100644
--- a/src/librustc_parse/parser/item.rs
+++ b/src/librustc_parse/parser/item.rs
@@ -1717,6 +1717,18 @@ impl<'a> Parser<'a> {
         Ok(FnHeader { constness, unsafety, asyncness, ext })
     }
 
+    /// We are parsing `async fn`. If we are on Rust 2015, emit an error.
+    fn ban_async_in_2015(&self, span: Span) {
+        if span.rust_2015() {
+            let diag = self.diagnostic();
+            struct_span_err!(diag, span, E0670, "`async fn` is not permitted in the 2015 edition")
+                .note("to use `async fn`, switch to Rust 2018")
+                .help("set `edition = \"2018\"` in `Cargo.toml`")
+                .note("for more on editions, read https://doc.rust-lang.org/edition-guide")
+                .emit();
+        }
+    }
+
     /// Parses the parameter list and result type of a function declaration.
     pub(super) fn parse_fn_decl(
         &mut self,
diff --git a/src/librustc_parse/parser/mod.rs b/src/librustc_parse/parser/mod.rs
index 2f2f2f8f176..4716d722778 100644
--- a/src/librustc_parse/parser/mod.rs
+++ b/src/librustc_parse/parser/mod.rs
@@ -1273,19 +1273,6 @@ impl<'a> Parser<'a> {
         }
     }
 
-    /// We are parsing `async fn`. If we are on Rust 2015, emit an error.
-    fn ban_async_in_2015(&self, async_span: Span) {
-        if async_span.rust_2015() {
-            struct_span_err!(
-                self.diagnostic(),
-                async_span,
-                E0670,
-                "`async fn` is not permitted in the 2015 edition",
-            )
-            .emit();
-        }
-    }
-
     fn collect_tokens<R>(
         &mut self,
         f: impl FnOnce(&mut Self) -> PResult<'a, R>,