Warn when an edition 2021 crate is in a virtual workspace with defaul…

…t 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

@@ -16,7 +16,7 @@ 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};
@@ -994,6 +994,23 @@ 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(())
     }

tests/testsuite/features2.rs

@@ -1398,6 +1398,41 @@ 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.