5 files changed, 18 insertions, 9 deletions
diff --git a/compiler/rustc_errors/src/diagnostic.rs b/compiler/rustc_errors/src/diagnostic.rs
index a59d91ea789..39ebd57b4b2 100644
--- a/compiler/rustc_errors/src/diagnostic.rs
+++ b/compiler/rustc_errors/src/diagnostic.rs
@@ -7,6 +7,7 @@ use crate::SuggestionStyle;
 use crate::ToolMetadata;
 use rustc_lint_defs::Applicability;
 use rustc_serialize::json::Json;
+use rustc_span::edition::LATEST_STABLE_EDITION;
 use rustc_span::{MultiSpan, Span, DUMMY_SP};
 use std::fmt;
 use std::hash::{Hash, Hasher};
@@ -342,6 +343,18 @@ impl Diagnostic {
         self
     }
 
+    /// Help the user upgrade to the latest edition.
+    /// This is factored out to make sure it does the right thing with `Cargo.toml`.
+    pub fn help_use_latest_edition(&mut self) -> &mut Self {
+        if std::env::var_os("CARGO").is_some() {
+            self.help(&format!("set `edition = \"{}\"` in `Cargo.toml`", LATEST_STABLE_EDITION));
+        } else {
+            self.help(&format!("pass `--edition {}` to `rustc`", LATEST_STABLE_EDITION));
+        }
+        self.note("for more on editions, read https://doc.rust-lang.org/edition-guide");
+        self
+    }
+
     /// Disallow attaching suggestions this diagnostic.
     /// Any suggestions attached e.g. with the `span_suggestion_*` methods
     /// (before and after the call to `disable_suggestions`) will be ignored.
diff --git a/compiler/rustc_errors/src/diagnostic_builder.rs b/compiler/rustc_errors/src/diagnostic_builder.rs
index 5dc71f16200..98b8b2a569e 100644
--- a/compiler/rustc_errors/src/diagnostic_builder.rs
+++ b/compiler/rustc_errors/src/diagnostic_builder.rs
@@ -409,6 +409,7 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         sp: impl Into<MultiSpan>,
         msg: &str,
     ) -> &mut Self);
+    forward!(pub fn help_use_latest_edition(&mut self,) -> &mut Self);
     forward!(pub fn set_is_lint(&mut self,) -> &mut Self);
 
     forward!(pub fn disable_suggestions(&mut self,) -> &mut Self);
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index df865d77b9b..b993d48c995 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -20,7 +20,6 @@ use rustc_ast_pretty::pprust;
 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, PResult};
 use rustc_session::lint::builtin::BREAK_WITH_LABEL_AND_LOOP;
 use rustc_session::lint::BuiltinLintDiagnostics;
-use rustc_span::edition::LATEST_STABLE_EDITION;
 use rustc_span::source_map::{self, Span, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{BytePos, Pos};
@@ -2712,8 +2711,7 @@ impl<'a> Parser<'a> {
         let mut async_block_err = |e: &mut Diagnostic, span: Span| {
             recover_async = true;
             e.span_label(span, "`async` blocks are only allowed in Rust 2018 or later");
-            e.help(&format!("set `edition = \"{}\"` in `Cargo.toml`", LATEST_STABLE_EDITION));
-            e.note("for more on editions, read https://doc.rust-lang.org/edition-guide");
+            e.help_use_latest_edition();
         };
 
         while self.token != token::CloseDelim(close_delim) {
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 5db1e4e0523..06460c7b1b3 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -14,7 +14,7 @@ use rustc_ast::{FnHeader, ForeignItem, Path, PathSegment, Visibility, Visibility
 use rustc_ast::{MacArgs, MacCall, MacDelimiter};
 use rustc_ast_pretty::pprust;
 use rustc_errors::{struct_span_err, Applicability, PResult, StashKey};
-use rustc_span::edition::{Edition, LATEST_STABLE_EDITION};
+use rustc_span::edition::Edition;
 use rustc_span::lev_distance::lev_distance;
 use rustc_span::source_map::{self, Span};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
@@ -2102,8 +2102,7 @@ impl<'a> Parser<'a> {
             let diag = self.diagnostic();
             struct_span_err!(diag, span, E0670, "`async fn` is not permitted in Rust 2015")
                 .span_label(span, "to use `async fn`, switch to Rust 2018 or later")
-                .help(&format!("set `edition = \"{}\"` in `Cargo.toml`", LATEST_STABLE_EDITION))
-                .note("for more on editions, read https://doc.rust-lang.org/edition-guide")
+                .help_use_latest_edition()
                 .emit();
         }
     }
diff --git a/compiler/rustc_typeck/src/check/expr.rs b/compiler/rustc_typeck/src/check/expr.rs
index 8e245beaa3d..7e7104f62fd 100644
--- a/compiler/rustc_typeck/src/check/expr.rs
+++ b/compiler/rustc_typeck/src/check/expr.rs
@@ -43,7 +43,6 @@ use rustc_middle::ty::error::TypeError::{FieldMisMatch, Sorts};
 use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::{self, AdtKind, Ty, TypeFoldable};
 use rustc_session::parse::feature_err;
-use rustc_span::edition::LATEST_STABLE_EDITION;
 use rustc_span::hygiene::DesugaringKind;
 use rustc_span::lev_distance::find_best_match_for_name;
 use rustc_span::source_map::Span;
@@ -2010,8 +2009,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // We know by construction that `<expr>.await` is either on Rust 2015
             // or results in `ExprKind::Await`. Suggest switching the edition to 2018.
             err.note("to `.await` a `Future`, switch to Rust 2018 or later");
-            err.help(&format!("set `edition = \"{}\"` in `Cargo.toml`", LATEST_STABLE_EDITION));
-            err.note("for more on editions, read https://doc.rust-lang.org/edition-guide");
+            err.help_use_latest_edition();
         }
 
         err.emit();