Make "2" the default resolver in the 2021 edition.

src/cargo/util/toml/mod.rs

@@ -1132,7 +1132,13 @@ impl TomlManifest {
             project.resolver.as_ref(),
             me.workspace.as_ref().and_then(|ws| ws.resolver.as_ref()),
         ) {
-            (None, None) => None,
+            (None, None) => {
+                if edition == Edition::Edition2021 {
+                    Some(ResolveBehavior::V2)
+                } else {
+                    None
+                }
+            }
             (Some(s), None) | (None, Some(s)) => Some(ResolveBehavior::from_manifest(s)?),
             (Some(_), Some(_)) => {
                 bail!("cannot specify `resolver` field in both `[workspace]` and `[package]`")

src/doc/src/reference/unstable.md

@@ -1082,7 +1082,10 @@ This feature is very unstable, and is only intended for early testing and
 experimentation. Future nightly releases may introduce changes for the 2021
 edition that may break your build.
 
+The 2021 edition will set the default [resolver version] to "2".
+
 [edition]: ../../edition-guide/index.html
+[resolver version]: resolver.md#resolver-versions
 
 <script>
 (function() {

tests/testsuite/features2.rs

@@ -2324,3 +2324,77 @@ fn doc_proc_macro() {
     // so rustdoc can load it).
     p.cargo("doc").run();
 }
+
+#[cargo_test]
+fn edition_2021_default_2() {
+    // edition = 2021 defaults to v2 resolver.
+    Package::new("common", "1.0.0")
+        .feature("f1", &[])
+        .file("src/lib.rs", "")
+        .publish();
+
+    Package::new("bar", "1.0.0")
+        .add_dep(
+            Dependency::new("common", "1.0")
+                .target("cfg(whatever)")
+                .enable_features(&["f1"]),
+        )
+        .publish();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+
+                [dependencies]
+                common = "1.0"
+                bar = "1.0"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    // First without edition.
+    p.cargo("tree -f")
+        .arg("{p} feats:{f}")
+        .with_stdout(
+            "\
+foo v0.1.0 [..]
+├── bar v1.0.0 feats:
+└── common v1.0.0 feats:f1
+",
+        )
+        .run();
+
+    p.change_file(
+        "Cargo.toml",
+        r#"
+            cargo-features = ["edition2021"]
+
+            [package]
+            name = "foo"
+            version = "0.1.0"
+            edition = "2021"
+
+            [dependencies]
+            common = "1.0"
+            bar = "1.0"
+        "#,
+    );
+
+    // Importantly, this does not include `f1` on `common`.
+    p.cargo("tree -f")
+        .arg("{p} feats:{f}")
+        .masquerade_as_nightly_cargo()
+        .with_stdout(
+            "\
+foo v0.1.0 [..]
+├── bar v1.0.0 feats:
+└── common v1.0.0 feats:
+",
+        )
+        .run();
+}