parser: more friendly hints for handling async move in the 2015 edition

Author: bvanjoi

compiler/rustc_parse/messages.ftl

@@ -23,6 +23,8 @@ parse_async_block_in_2015 = `async` blocks are only allowed in Rust 2018 or late
 parse_async_fn_in_2015 = `async fn` is not permitted in Rust 2015
     .label = to use `async fn`, switch to Rust 2018 or later
 
+parse_async_move_block_in_2015 = `async move` blocks are only allowed in Rust 2018 or later
+
 parse_async_move_order_incorrect = the order of `move` and `async` is incorrect
     .suggestion = try switching the order
 

compiler/rustc_parse/src/errors.rs

@@ -1434,6 +1434,13 @@ pub(crate) struct AsyncBlockIn2015 {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_async_move_block_in_2015)]
+pub(crate) struct AsyncMoveBlockIn2015 {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_self_argument_pointer)]
 pub(crate) struct SelfArgumentPointer {

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -4,10 +4,11 @@ use super::{
     TokenExpectType, TokenType,
 };
 use crate::errors::{
-    AmbiguousPlus, AttributeOnParamType, BadQPathStage2, BadTypePlus, BadTypePlusSub, ColonAsSemi,
-    ComparisonOperatorsCannotBeChained, ComparisonOperatorsCannotBeChainedSugg,
-    ConstGenericWithoutBraces, ConstGenericWithoutBracesSugg, DocCommentDoesNotDocumentAnything,
-    DocCommentOnParamType, DoubleColonInBound, ExpectedIdentifier, ExpectedSemi, ExpectedSemiSugg,
+    AmbiguousPlus, AsyncMoveBlockIn2015, AttributeOnParamType, BadQPathStage2, BadTypePlus,
+    BadTypePlusSub, ColonAsSemi, ComparisonOperatorsCannotBeChained,
+    ComparisonOperatorsCannotBeChainedSugg, ConstGenericWithoutBraces,
+    ConstGenericWithoutBracesSugg, DocCommentDoesNotDocumentAnything, DocCommentOnParamType,
+    DoubleColonInBound, ExpectedIdentifier, ExpectedSemi, ExpectedSemiSugg,
     GenericParamsWithoutAngleBrackets, GenericParamsWithoutAngleBracketsSugg,
     HelpIdentifierStartsWithNumber, InInTypo, IncorrectAwait, IncorrectSemicolon,
     IncorrectUseOfAwait, ParenthesesInForHead, ParenthesesInForHeadSugg,
@@ -573,6 +574,12 @@ impl<'a> Parser<'a> {
             return Err(self.sess.create_err(UseEqInstead { span: self.token.span }));
         }
 
+        if self.token.is_keyword(kw::Move) && self.prev_token.is_keyword(kw::Async) {
+            // The 2015 edition is in use because parsing of `async move` has failed.
+            let span = self.prev_token.span.to(self.token.span);
+            return Err(self.sess.create_err(AsyncMoveBlockIn2015 { span }));
+        }
+
         let expect = tokens_to_string(&expected);
         let actual = super::token_descr(&self.token);
         let (msg_exp, (label_sp, label_exp)) = if expected.len() > 1 {

tests/ui/parser/issues/issue-114219.rs

@@ -0,0 +1,4 @@
+fn main() {
+    async move {};
+    //~^ ERROR `async move` blocks are only allowed in Rust 2018 or later
+}

tests/ui/parser/issues/issue-114219.stderr

@@ -0,0 +1,8 @@
+error: `async move` blocks are only allowed in Rust 2018 or later
+  --> $DIR/issue-114219.rs:2:5
+   |
+LL |     async move {};
+   |     ^^^^^^^^^^
+
+error: aborting due to previous error
+