parser: move ban_async_in_2015 to fn parsing & improve it.

Author: Centril

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,

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>,

src/test/ui/async-await/edition-deny-async-fns-2015.stderr

@@ -3,54 +3,90 @@ error[E0670]: `async fn` is not permitted in the 2015 edition
    |
 LL | async fn foo() {}
    | ^^^^^
+   |
+   = 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
 
 error[E0670]: `async fn` is not permitted in the 2015 edition
   --> $DIR/edition-deny-async-fns-2015.rs:5:12
    |
 LL | fn baz() { async fn foo() {} }
    |            ^^^^^
+   |
+   = 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
 
 error[E0670]: `async fn` is not permitted in the 2015 edition
   --> $DIR/edition-deny-async-fns-2015.rs:7:1
    |
 LL | async fn async_baz() {
    | ^^^^^
+   |
+   = 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
 
 error[E0670]: `async fn` is not permitted in the 2015 edition
   --> $DIR/edition-deny-async-fns-2015.rs:8:5
    |
 LL |     async fn bar() {}
    |     ^^^^^
+   |
+   = 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
 
 error[E0670]: `async fn` is not permitted in the 2015 edition
   --> $DIR/edition-deny-async-fns-2015.rs:14:5
    |
 LL |     async fn foo() {}
    |     ^^^^^
+   |
+   = 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
 
 error[E0670]: `async fn` is not permitted in the 2015 edition
   --> $DIR/edition-deny-async-fns-2015.rs:18:5
    |
 LL |     async fn foo() {}
    |     ^^^^^
+   |
+   = 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
 
 error[E0670]: `async fn` is not permitted in the 2015 edition
   --> $DIR/edition-deny-async-fns-2015.rs:36:9
    |
 LL |         async fn bar() {}
    |         ^^^^^
+   |
+   = 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
 
 error[E0670]: `async fn` is not permitted in the 2015 edition
   --> $DIR/edition-deny-async-fns-2015.rs:26:9
    |
 LL |         async fn foo() {}
    |         ^^^^^
+   |
+   = 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
 
 error[E0670]: `async fn` is not permitted in the 2015 edition
   --> $DIR/edition-deny-async-fns-2015.rs:31:13
    |
 LL |             async fn bar() {}
    |             ^^^^^
+   |
+   = 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
 
 error[E0706]: trait fns cannot be declared `async`
   --> $DIR/edition-deny-async-fns-2015.rs:18:5