Warn when an edition 2021 crate is in a virtual workspace with default resolver

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 +993,24 @@ impl<'cfg> Workspace<'cfg> {
                     }
                 }
             }
+            if let MaybePackage::Virtual(vm) = self.root_maybe() {
+                if vm.resolve_behavior().is_none() {
+                    if let Some(edition) = self
+                        .members()
+                        .filter(|p| p.manifest_path() != root_manifest)
+                        .map(|p| p.manifest().edition())
+                        .filter(|&e| e >= Edition::Edition2021)
+                        .max()
+                    {
+                        let resolver = edition.default_resolve_behavior().to_manifest();
+                        self.config.shell().warn(format_args!("some crates are on edition {edition} which defaults to `resolver = \"{resolver}\"`, but virtual workspaces default to `resolver = \"1\"`"))?;
+                        self.config.shell().note(
+                            "to keep the current resolver, specify `workspace.resolver = \"1\"` in the workspace root's manifest",
+                        )?;
+                        self.config.shell().note(format_args!("to use the edition {edition} resolver, specify `workspace.resolver = \"{resolver}\"` in the workspace root's manifest"))?;
+                    }
+                }
+            }
         }
         Ok(())
     }

tests/testsuite/cargo_new/inherit_workspace_lints/in/Cargo.toml

@@ -1,4 +1,5 @@
 [workspace]
+resolver = "2"
 members = ["crates/*"]
 
 [workspace.lints.rust]

tests/testsuite/cargo_new/inherit_workspace_lints/out/Cargo.toml

@@ -1,4 +1,5 @@
 [workspace]
+resolver = "2"
 members = ["crates/*"]
 
 [workspace.lints.rust]

tests/testsuite/cargo_new/inherit_workspace_package_table.in/Cargo.toml

@@ -1,4 +1,5 @@
 [workspace]
+resolver = "2"
 members = [
   "crates/*",
 ]

tests/testsuite/cargo_new/inherit_workspace_package_table/out/Cargo.toml

@@ -1,4 +1,5 @@
 [workspace]
+resolver = "2"
 members = [
   "crates/*",
 ]

tests/testsuite/cargo_new/inherit_workspace_package_table_with_edition/out/Cargo.toml

@@ -1,4 +1,5 @@
 [workspace]
+resolver = "2"
 members = [
   "crates/*",
 ]

tests/testsuite/cargo_new/inherit_workspace_package_table_with_registry/out/Cargo.toml

@@ -1,4 +1,5 @@
 [workspace]
+resolver = "2"
 members = [
   "crates/*",
 ]

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 virtual workspaces default to `resolver = \"1\"`
+note: to keep the current resolver, specify `workspace.resolver = \"1\"` in the workspace root's manifest
+note: to use the edition 2021 resolver, specify `workspace.resolver = \"2\"` in the workspace root's manifest
+[CHECKING] a v0.1.0 [..]
+[FINISHED] [..]
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn resolver_ws_root_and_member() {
     // Check when specified in both ws root and member.