Give a better error message on async use in edition 2015

Author: spastorino

compiler/rustc_parse/messages.ftl

@@ -26,6 +26,8 @@ parse_async_move_block_in_2015 = `async move` blocks are only allowed in Rust 20
 parse_async_move_order_incorrect = the order of `move` and `async` is incorrect
     .suggestion = try switching the order
 
+parse_async_use_block_in_2015 = `async use` blocks are only allowed in Rust 2018 or later
+
 parse_async_use_order_incorrect = the order of `use` and `async` is incorrect
     .suggestion = try switching the order
 

compiler/rustc_parse/src/errors.rs

@@ -1689,6 +1689,13 @@ pub(crate) struct AsyncMoveBlockIn2015 {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_async_use_block_in_2015)]
+pub(crate) struct AsyncUseBlockIn2015 {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_async_bound_modifier_in_2015)]
 pub(crate) struct AsyncBoundModifierIn2015 {

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -33,11 +33,11 @@ use super::{
     SeqSep, TokenType,
 };
 use crate::errors::{
-    AddParen, AmbiguousPlus, AsyncMoveBlockIn2015, AttributeOnParamType, AwaitSuggestion,
-    BadQPathStage2, BadTypePlus, BadTypePlusSub, ColonAsSemi, ComparisonOperatorsCannotBeChained,
-    ComparisonOperatorsCannotBeChainedSugg, ConstGenericWithoutBraces,
-    ConstGenericWithoutBracesSugg, DocCommentDoesNotDocumentAnything, DocCommentOnParamType,
-    DoubleColonInBound, ExpectedIdentifier, ExpectedSemi, ExpectedSemiSugg,
+    AddParen, AmbiguousPlus, AsyncMoveBlockIn2015, AsyncUseBlockIn2015, AttributeOnParamType,
+    AwaitSuggestion, BadQPathStage2, BadTypePlus, BadTypePlusSub, ColonAsSemi,
+    ComparisonOperatorsCannotBeChained, ComparisonOperatorsCannotBeChainedSugg,
+    ConstGenericWithoutBraces, ConstGenericWithoutBracesSugg, DocCommentDoesNotDocumentAnything,
+    DocCommentOnParamType, DoubleColonInBound, ExpectedIdentifier, ExpectedSemi, ExpectedSemiSugg,
     GenericParamsWithoutAngleBrackets, GenericParamsWithoutAngleBracketsSugg,
     HelpIdentifierStartsWithNumber, HelpUseLatestEdition, InInTypo, IncorrectAwait,
     IncorrectSemicolon, IncorrectUseOfAwait, IncorrectUseOfUse, PatternMethodParamWithoutBody,
@@ -574,10 +574,17 @@ impl<'a> Parser<'a> {
             return Err(self.dcx().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.
+        if (self.token.is_keyword(kw::Move) || self.token.is_keyword(kw::Use))
+            && self.prev_token.is_keyword(kw::Async)
+        {
+            // The 2015 edition is in use because parsing of `async move` or `async use` has failed.
             let span = self.prev_token.span.to(self.token.span);
-            return Err(self.dcx().create_err(AsyncMoveBlockIn2015 { span }));
+            if self.token.is_keyword(kw::Move) {
+                return Err(self.dcx().create_err(AsyncMoveBlockIn2015 { span }));
+            } else {
+                // kw::Use
+                return Err(self.dcx().create_err(AsyncUseBlockIn2015 { span }));
+            }
         }
 
         let expect = tokens_to_string(&expected);

tests/ui/ergonomic-clones/async/edition-2015.rs

@@ -0,0 +1,6 @@
+#![feature(ergonomic_clones)]
+
+fn main() {
+    async use {};
+    //~^ ERROR `async use` blocks are only allowed in Rust 2018 or later
+}

tests/ui/ergonomic-clones/async/edition-2015.stderr

@@ -0,0 +1,8 @@
+error: `async use` blocks are only allowed in Rust 2018 or later
+  --> $DIR/edition-2015.rs:4:5
+   |
+LL |     async use {};
+   |     ^^^^^^^^^
+
+error: aborting due to 1 previous error
+