Adjust the edition2021 resolver diff report.

src/cargo/core/resolver/features.rs

@@ -351,10 +351,9 @@ impl ResolvedFeatures {
     /// Compares the result against the original resolver behavior.
     ///
     /// Used by `cargo fix --edition` to display any differences.
-    pub fn compare_legacy(&self, legacy: &ResolvedFeatures) -> FeatureDifferences {
+    pub fn compare_legacy(&self, legacy: &ResolvedFeatures) -> DiffMap {
         let legacy_features = legacy.legacy_features.as_ref().unwrap();
-        let features = self
-            .activated_features
+        self.activated_features
             .iter()
             .filter_map(|((pkg_id, for_host), new_features)| {
                 let old_features = match legacy_features.get(pkg_id) {
@@ -371,30 +370,7 @@ impl ResolvedFeatures {
                     Some(((*pkg_id, *for_host), removed_features))
                 }
             })
-            .collect();
-        let legacy_deps = legacy.legacy_dependencies.as_ref().unwrap();
-        let optional_deps = self
-            .activated_dependencies
-            .iter()
-            .filter_map(|((pkg_id, for_host), new_deps)| {
-                let old_deps = match legacy_deps.get(pkg_id) {
-                    Some(deps) => deps.iter().cloned().collect(),
-                    None => BTreeSet::new(),
-                };
-                // The new resolver should never add dependencies.
-                assert_eq!(new_deps.difference(&old_deps).next(), None);
-                let removed_deps: BTreeSet<_> = old_deps.difference(new_deps).cloned().collect();
-                if removed_deps.is_empty() {
-                    None
-                } else {
-                    Some(((*pkg_id, *for_host), removed_deps))
-                }
-            })
-            .collect();
-        FeatureDifferences {
-            features,
-            optional_deps,
-        }
+            .collect()
     }
 }
 
@@ -403,12 +379,6 @@ impl ResolvedFeatures {
 /// Key is `(pkg_id, for_host)`. Value is a set of features or dependencies removed.
 pub type DiffMap = BTreeMap<(PackageId, bool), BTreeSet<InternedString>>;
 
-/// Differences between resolvers.
-pub struct FeatureDifferences {
-    pub features: DiffMap,
-    pub optional_deps: DiffMap,
-}
-
 pub struct FeatureResolver<'a, 'cfg> {
     ws: &'a Workspace<'cfg>,
     target_data: &'a RustcTargetData<'cfg>,

src/cargo/ops/fix.rs

@@ -255,7 +255,7 @@ fn check_resolver_change(ws: &Workspace<'_>, opts: &FixOptions) -> CargoResult<(
     )?;
 
     let differences = v2_features.compare_legacy(&ws_resolve.resolved_features);
-    if differences.features.is_empty() && differences.optional_deps.is_empty() {
+    if differences.is_empty() {
         // Nothing is different, nothing to report.
         return Ok(());
     }
@@ -265,32 +265,27 @@ fn check_resolver_change(ws: &Workspace<'_>, opts: &FixOptions) -> CargoResult<(
     )?;
     drop_eprintln!(
         config,
-        "This may cause dependencies to resolve with a different set of features."
+        "This may cause some dependencies to be built with fewer features enabled than previously."
     );
     drop_eprintln!(
         config,
         "More information about the resolver changes may be found \
-         at https://doc.rust-lang.org/cargo/reference/features.html#feature-resolver-version-2"
+         at https://doc.rust-lang.org/nightly/edition-guide/rust-2021/default-cargo-resolver.html"
     );
     drop_eprintln!(
         config,
-        "The following differences were detected with the current configuration:\n"
+        "When building the following dependencies, \
+         the given features will no longer be used:\n"
     );
-    let report = |changes: crate::core::resolver::features::DiffMap, what| {
-        for ((pkg_id, for_host), removed) in changes {
-            drop_eprint!(config, "  {}", pkg_id);
-            if for_host {
-                drop_eprint!(config, " (as build dependency)");
-            }
-            if !removed.is_empty() {
-                let joined: Vec<_> = removed.iter().map(|s| s.as_str()).collect();
-                drop_eprint!(config, " removed {} `{}`", what, joined.join(","));
-            }
-            drop_eprint!(config, "\n");
+    for ((pkg_id, for_host), removed) in differences {
+        drop_eprint!(config, "  {}", pkg_id);
+        if for_host {
+            drop_eprint!(config, " (as host dependency)");
         }
-    };
-    report(differences.features, "features");
-    report(differences.optional_deps, "optional dependency");
+        drop_eprint!(config, ": ");
+        let joined: Vec<_> = removed.iter().map(|s| s.as_str()).collect();
+        drop_eprintln!(config, "{}", joined.join(", "));
+    }
     drop_eprint!(config, "\n");
     report_maybe_diesel(config, &ws_resolve.targeted_resolve)?;
     Ok(())

tests/testsuite/fix.rs

@@ -1431,8 +1431,9 @@ fn edition_v2_resolver_report() {
     }
     Package::new("common", "1.0.0")
         .feature("f1", &[])
-        .file("src/lib.rs", "")
+        .add_dep(Dependency::new("opt_dep", "1.0").optional(true))
         .publish();
+    Package::new("opt_dep", "1.0.0").publish();
 
     Package::new("bar", "1.0.0")
         .add_dep(
@@ -1454,6 +1455,9 @@ fn edition_v2_resolver_report() {
                 [dependencies]
                 common = "1.0"
                 bar = "1.0"
+
+                [build-dependencies]
+                common = { version = "1.0", features = ["opt_dep"] }
             "#,
         )
         .file("src/lib.rs", "")
@@ -1466,13 +1470,16 @@ fn edition_v2_resolver_report() {
 [DOWNLOADING] crates ...
 [DOWNLOADED] common v1.0.0 [..]
 [DOWNLOADED] bar v1.0.0 [..]
+[DOWNLOADED] opt_dep v1.0.0 [..]
 note: Switching to Edition 2021 will enable the use of the version 2 feature resolver in Cargo.
-This may cause dependencies to resolve with a different set of features.
-More information about the resolver changes may be found at https://doc.rust-lang.org/cargo/reference/features.html#feature-resolver-version-2
-The following differences were detected with the current configuration:
+This may cause some dependencies to be built with fewer features enabled than previously.
+More information about the resolver changes may be found at https://doc.rust-lang.org/nightly/edition-guide/rust-2021/default-cargo-resolver.html
+When building the following dependencies, the given features will no longer be used:
 
-  common v1.0.0 removed features `f1`
+  common v1.0.0: f1, opt_dep
+  common v1.0.0 (as host dependency): f1
 
+[CHECKING] opt_dep v1.0.0
 [CHECKING] common v1.0.0
 [CHECKING] bar v1.0.0
 [CHECKING] foo v0.1.0 [..]