From 2e2b3c1f1ec2ba3d47d0633cffdd9816552b2941 Mon Sep 17 00:00:00 2001
From: Wim Looman <git@nemo157.com>
Date: Thu, 28 Jul 2022 17:42:20 +0200
Subject: [PATCH] Warn when an edition 2021 crate is in a virtual workspace
 with default resolver

Edition 2021 updates the default resolver to version "2", but developers
using virtual workspaces commonly don't get this update because the
virtual workspace defaults to version "1". Warn when this situation
occurs so those developers can explicitly configure their workspace and
will be more likely to know that they will need to update it in the
future.
---
 src/cargo/core/workspace.rs  | 19 ++++++++++++++++++-
 tests/testsuite/features2.rs | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/src/cargo/core/workspace.rs b/src/cargo/core/workspace.rs
index 9922d6d33..c4dc5899a 100644
--- a/src/cargo/core/workspace.rs
+++ b/src/cargo/core/workspace.rs
@@ -15,7 +15,7 @@ use crate::core::features::Features;
 use crate::core::registry::PackageRegistry;
 use crate::core::resolver::features::CliFeatures;
 use crate::core::resolver::ResolveBehavior;
-use crate::core::{Dependency, FeatureValue, PackageId, PackageIdSpec};
+use crate::core::{Dependency, Edition, FeatureValue, PackageId, PackageIdSpec};
 use crate::core::{EitherManifest, Package, SourceId, VirtualManifest};
 use crate::ops;
 use crate::sources::{PathSource, CRATES_IO_INDEX, CRATES_IO_REGISTRY};
@@ -993,6 +993,23 @@ impl<'cfg> Workspace<'cfg> {
                     }
                 }
             }
+            if let MaybePackage::Virtual(vm) = self.root_maybe() {
+                if vm.resolve_behavior().is_none() {
+                    if self
+                        .members()
+                        .filter(|p| p.manifest_path() != root_manifest)
+                        .any(|p| p.manifest().edition() >= Edition::Edition2021)
+                    {
+                        self.config.shell().warn(
+                            "\
+                        some crates are on edition 2021 which defaults to `resolver = \"2\"`,\n\
+                     \x20   but a virtual workspace defaults to `resolver = \"1\"`\n\
+                     \x20   specify the desired resolver version explicitly at the workspace root\
+                            ",
+                        )?;
+                    }
+                }
+            }
         }
         Ok(())
     }
diff --git a/tests/testsuite/features2.rs b/tests/testsuite/features2.rs
index 494c83f1e..188f5bb07 100644
--- a/tests/testsuite/features2.rs
+++ b/tests/testsuite/features2.rs
@@ -1406,6 +1406,41 @@ workspace: [..]/foo/Cargo.toml
         .run();
 }
 
+#[cargo_test]
+fn edition_2021_workspace_member() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [workspace]
+            members = ["a"]
+            "#,
+        )
+        .file(
+            "a/Cargo.toml",
+            r#"
+            [package]
+            name = "a"
+            version = "0.1.0"
+            edition = "2021"
+            "#,
+        )
+        .file("a/src/lib.rs", "")
+        .build();
+
+    p.cargo("check")
+        .with_stderr(
+            "\
+warning: some crates are on edition 2021 which defaults to `resolver = \"2\"`,
+    but a virtual workspace defaults to `resolver = \"1\"`
+    specify the desired resolver version explicitly at the workspace root
+[CHECKING] a v0.1.0 [..]
+[FINISHED] [..]
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn resolver_ws_root_and_member() {
     // Check when specified in both ws root and member.