Suggest using raw identifiers in 2018 edition when using keywords

src/libsyntax/parse/parser.rs

@@ -798,7 +798,18 @@ impl<'a> Parser<'a> {
         let mut err = self.struct_span_err(self.span,
                                            &format!("expected identifier, found {}",
                                                     self.this_token_descr()));
-        if let Some(token_descr) = self.token_descr() {
+        if let (true, token::Ident(ref s, false), true) = (
+            self.span.rust_2018(),
+            &self.token,
+            self.token.is_used_keyword() || self.token.is_unused_keyword(),
+        ) {
+            err.span_suggestion_with_applicability(
+                self.span,
+                "you can escape reserved keywords to use them as identifiers",
+                format!("r#{}", s.to_string()),
+                Applicability::MaybeIncorrect,
+            );
+        } else if let Some(token_descr) = self.token_descr() {
             err.span_label(self.span, format!("expected identifier, found {}", token_descr));
         } else {
             err.span_label(self.span, "expected identifier");

src/test/ui/editions/edition-keywords-2015-2018-expansion.stderr

@@ -2,9 +2,13 @@ error: expected identifier, found reserved keyword `async`
   --> $DIR/edition-keywords-2015-2018-expansion.rs:8:5
    |
 LL |     produces_async! {} //~ ERROR expected identifier, found reserved keyword
-   |     ^^^^^^^^^^^^^^^^^^ expected identifier, found reserved keyword
+   |     ^^^^^^^^^^^^^^^^^^
    |
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+help: you can escape reserved keywords to use them as identifiers
+   |
+LL | (  ) => ( pub fn r#async (  ) {  } )
+   |                  ^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/editions/edition-keywords-2018-2015-parsing.stderr

@@ -2,13 +2,21 @@ error: expected identifier, found reserved keyword `async`
   --> $DIR/edition-keywords-2018-2015-parsing.rs:8:13
    |
 LL |     let mut async = 1; //~ ERROR expected identifier, found reserved keyword `async`
-   |             ^^^^^ expected identifier, found reserved keyword
+   |             ^^^^^
+help: you can escape reserved keywords to use them as identifiers
+   |
+LL |     let mut r#async = 1; //~ ERROR expected identifier, found reserved keyword `async`
+   |             ^^^^^^^
 
 error: expected identifier, found reserved keyword `async`
   --> $DIR/edition-keywords-2018-2015-parsing.rs:18:13
    |
 LL |     module::async(); //~ ERROR expected identifier, found reserved keyword `async`
-   |             ^^^^^ expected identifier, found reserved keyword
+   |             ^^^^^
+help: you can escape reserved keywords to use them as identifiers
+   |
+LL |     module::r#async(); //~ ERROR expected identifier, found reserved keyword `async`
+   |             ^^^^^^^
 
 error: no rules expected the token `r#async`
   --> $DIR/edition-keywords-2018-2015-parsing.rs:12:31

src/test/ui/editions/edition-keywords-2018-2018-expansion.stderr

@@ -2,9 +2,13 @@ error: expected identifier, found reserved keyword `async`
   --> $DIR/edition-keywords-2018-2018-expansion.rs:8:5
    |
 LL |     produces_async! {} //~ ERROR expected identifier, found reserved keyword `async`
-   |     ^^^^^^^^^^^^^^^^^^ expected identifier, found reserved keyword
+   |     ^^^^^^^^^^^^^^^^^^
    |
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+help: you can escape reserved keywords to use them as identifiers
+   |
+LL | (  ) => ( pub fn r#async (  ) {  } )
+   |                  ^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/editions/edition-keywords-2018-2018-parsing.stderr

@@ -2,13 +2,21 @@ error: expected identifier, found reserved keyword `async`
   --> $DIR/edition-keywords-2018-2018-parsing.rs:8:13
    |
 LL |     let mut async = 1; //~ ERROR expected identifier, found reserved keyword `async`
-   |             ^^^^^ expected identifier, found reserved keyword
+   |             ^^^^^
+help: you can escape reserved keywords to use them as identifiers
+   |
+LL |     let mut r#async = 1; //~ ERROR expected identifier, found reserved keyword `async`
+   |             ^^^^^^^
 
 error: expected identifier, found reserved keyword `async`
   --> $DIR/edition-keywords-2018-2018-parsing.rs:18:13
    |
 LL |     module::async(); //~ ERROR expected identifier, found reserved keyword `async`
-   |             ^^^^^ expected identifier, found reserved keyword
+   |             ^^^^^
+help: you can escape reserved keywords to use them as identifiers
+   |
+LL |     module::r#async(); //~ ERROR expected identifier, found reserved keyword `async`
+   |             ^^^^^^^
 
 error: no rules expected the token `r#async`
   --> $DIR/edition-keywords-2018-2018-parsing.rs:12:31

src/test/ui/rust-2018/dyn-trait-compatibility.stderr

@@ -2,7 +2,11 @@ error: expected identifier, found keyword `dyn`
   --> $DIR/dyn-trait-compatibility.rs:4:16
    |
 LL | type A1 = dyn::dyn; //~ERROR expected identifier, found keyword `dyn`
-   |                ^^^ expected identifier, found keyword
+   |                ^^^
+help: you can escape reserved keywords to use them as identifiers
+   |
+LL | type A1 = dyn::r#dyn; //~ERROR expected identifier, found keyword `dyn`
+   |                ^^^^^
 
 error: expected identifier, found `<`
   --> $DIR/dyn-trait-compatibility.rs:5:14