feat(toml): allow overriding inherited default-features in 2024

doc/book/src/reference/specifying-dependencies.md

@@ -637,11 +637,16 @@ dependency in the workspace's [`[workspace.dependencies]`][workspace.dependencie
 After that, add it to the `[dependencies]` table with `workspace = true`.
 
 Along with the `workspace` key, dependencies can also include these keys:
-- [`optional`][optional]: Note that the`[workspace.dependencies]` table is not allowed to specify `optional`.
+- [`optional`][optional]: Note that the `[workspace.dependencies]` table is not allowed to specify `optional`.
 - [`features`][features]: These are additive with the features declared in the `[workspace.dependencies]`
+- [`default-features`][default-features] (Edition 2024 packages, requires Rust 1.99+):
+  Overrides the value set in `[workspace.dependencies]`.
+  If neither the package nor the workspace specifies `default-features`, it defaults to `true`.
+  Before Rust 1.99, or in earlier editions, package-level `default-features = false`
+  may be ignored or rejected unless the workspace dependency also disables default features.
 
-Other than `optional` and `features`, inherited dependencies cannot use any other
-dependency key (such as `version` or `default-features`).
+Other than `optional`, `features`, and `default-features`, inherited dependencies
+cannot use any other dependency key (such as `version`).
 
 Dependencies in the `[dependencies]`, `[dev-dependencies]`, `[build-dependencies]`, and
 `[target."...".dependencies]` sections support the ability to reference the
@@ -651,9 +656,11 @@ Dependencies in the `[dependencies]`, `[dev-dependencies]`, `[build-dependencies
 [package]
 name = "bar"
 version = "0.2.0"
+edition = "2024"
 
 [dependencies]
 regex = { workspace = true, features = ["unicode"] }
+serde = { workspace = true, default-features = false }
 
 [build-dependencies]
 cc.workspace = true
@@ -669,3 +676,4 @@ rand = { workspace = true, optional = true }
 [workspace.dependencies]: workspaces.md#the-dependencies-table
 [optional]: features.md#optional-dependencies
 [features]: features.md
+[default-features]: features.md#dependency-features

doc/book/src/reference/workspaces.md

@@ -203,6 +203,10 @@ inherited by members of a workspace.
 Specifying a workspace dependency is similar to [package dependencies][specifying-dependencies] except:
 - Dependencies from this table cannot be declared as `optional`
 - [`features`][features] declared in this table are additive with the `features` from `[dependencies]`
+- `default-features` declared on a package dependency overrides the value set
+  here in Edition 2024 packages (requires Rust 1.99+). Before Rust 1.99, or in
+  earlier editions, package-level `default-features = false` may be ignored or
+  rejected unless the workspace dependency also disables default features.
 
 You can then [inherit the workspace dependency as a package dependency][inheriting-a-dependency-from-a-workspace]
 
@@ -216,16 +220,19 @@ members = ["bar"]
 cc = "1.0.73"
 rand = "0.8.5"
 regex = { version = "1.6.0", default-features = false, features = ["std"] }
+serde = { version = "1.0.190", default-features = true }
 ```
 
 ```toml
 # [PROJECT_DIR]/bar/Cargo.toml
 [package]
 name = "bar"
 version = "0.2.0"
+edition = "2024"
 
 [dependencies]
 regex = { workspace = true, features = ["unicode"] }
+serde = { workspace = true, default-features = false }
 
 [build-dependencies]
 cc.workspace = true

src/ops/cargo_add/mod.rs

@@ -28,6 +28,7 @@ use crate::util::OptVersionReq;
 use crate::util::cache_lock::CacheLockMode;
 use crate::util::edit_distance;
 use crate::util::style;
+use crate::workspace::Edition;
 use crate::workspace::Feature;
 use crate::workspace::FeatureValue;
 use crate::workspace::Features;
@@ -503,7 +504,20 @@ fn resolve_dependency(
     }
 
     if let Some(Source::Workspace(_)) = dependency.source() {
-        check_invalid_ws_keys(dependency.toml_key(), arg)?;
+        check_invalid_ws_keys(dependency.toml_key(), arg, spec.manifest().edition())?;
+        if spec.manifest().edition() >= Edition::Edition2024 && arg.default_features == Some(true) {
+            let ws_dep = find_workspace_dep(
+                dependency.toml_key(),
+                ws,
+                ws.root_manifest(),
+                ws.unstable_features(),
+            )?;
+            // Only write `default-features = true` when the workspace dependency
+            // explicitly disables default features.
+            if ws_dep.default_features() == Some(false) {
+                dependency.default_features = Some(true);
+            }
+        }
     }
 
     let version_required = dependency.source().and_then(|s| s.as_registry()).is_some();
@@ -703,14 +717,16 @@ fn fuzzy_lookup(
 
 /// When { workspace = true } you cannot define other keys that configure
 /// the source of the dependency such as `version`, `registry`, `registry-index`,
-/// `path`, `git`, `branch`, `tag`, `rev`, or `package`. You can also not define
-/// `default-features`.
+/// `path`, `git`, `branch`, `tag`, `rev`, or `package`.
+/// Prior to Edition 2024 (RFC 3945) `default-features` was also forbidden;
+/// from Edition 2024 onwards a package-level `default-features` overrides the
+/// workspace value, so it is allowed.
 ///
 /// Only `default-features`, `registry` and `rename` need to be checked
 ///  for currently. This is because `git` and its associated keys, `path`, and
 /// `version`  should all bee checked before this is called. `rename` is checked
 /// for as it turns into `package`
-fn check_invalid_ws_keys(toml_key: &str, arg: &DepOp) -> CargoResult<()> {
+fn check_invalid_ws_keys(toml_key: &str, arg: &DepOp, edition: Edition) -> CargoResult<()> {
     fn err_msg(toml_key: &str, flag: &str, field: &str) -> String {
         format!(
             "cannot override workspace dependency with `{flag}`, \
@@ -719,7 +735,7 @@ fn check_invalid_ws_keys(toml_key: &str, arg: &DepOp) -> CargoResult<()> {
         )
     }
 
-    if arg.default_features.is_some() {
+    if arg.default_features.is_some() && edition < Edition::Edition2024 {
         anyhow::bail!(
             "{}",
             err_msg(toml_key, "--default-features", "default-features")

src/workspace/editor/dependency.rs

@@ -392,8 +392,8 @@ impl Dependency {
     ///
     /// Returns a tuple with the dependency's name and either the version as a
     /// `String` or the path/git repository as an `InlineTable`.
-    /// (If the dependency is set as `optional` or `default-features` is set to
-    /// `false`, an `InlineTable` is returned in any case.)
+    /// (If the dependency is set as `optional` or `default-features` is
+    /// explicitly set, an `InlineTable` is returned in any case.)
     ///
     /// # Panic
     ///
@@ -414,7 +414,7 @@ impl Dependency {
             self.public.unwrap_or(false),
             self.optional.unwrap_or(false),
             self.features.as_ref(),
-            self.default_features.unwrap_or(true),
+            self.default_features,
             self.source.as_ref(),
             self.registry.as_ref(),
             self.rename.as_ref(),
@@ -424,12 +424,12 @@ impl Dependency {
                 false,
                 false,
                 None,
-                true,
+                None,
                 Some(Source::Registry(RegistrySource { version: v })),
                 None,
                 None,
             ) => toml_edit::value(v),
-            (false, false, None, true, Some(Source::Workspace(WorkspaceSource {})), None, None) => {
+            (false, false, None, None, Some(Source::Workspace(WorkspaceSource {})), None, None) => {
                 let mut table = toml_edit::InlineTable::default();
                 table.set_dotted(true);
                 table.insert("workspace", true.into());

src/workspace/parser/mod.rs

@@ -1223,25 +1223,31 @@ fn inner_dependency_inherit_with<'a>(
     } = &pkg_dep;
     let default_features = default_features.or(*default_features2);
 
-    match (default_features, merged_dep.default_features()) {
-        // member: default-features = true and
-        // workspace: default-features = false should turn on
-        // default-features
-        (Some(true), Some(false)) => {
-            merged_dep.default_features = Some(true);
-        }
-        // member: default-features = false and
-        // workspace: default-features = true should ignore member
-        // default-features
-        (Some(false), Some(true)) => {
-            deprecated_ws_default_features(name, Some(true), edition, warnings)?;
-        }
-        // member: default-features = false and
-        // workspace: dep = "1.0" should ignore member default-features
-        (Some(false), None) => {
-            deprecated_ws_default_features(name, None, edition, warnings)?;
+    // RFC 3945: Allow workspace members to override the workspace dependency's
+    // `default-features` setting.
+    if edition >= Edition::Edition2024 {
+        merged_dep.default_features = default_features.or(merged_dep.default_features);
+    } else {
+        match (default_features, merged_dep.default_features()) {
+            // member: default-features = true and
+            // workspace: default-features = false should turn on
+            // default-features
+            (Some(true), Some(false)) => {
+                merged_dep.default_features = Some(true);
+            }
+            // member: default-features = false and
+            // workspace: default-features = true should ignore member
+            // default-features
+            (Some(false), Some(true)) => {
+                deprecated_ws_default_features(name, Some(true), warnings);
+            }
+            // member: default-features = false and
+            // workspace: dep = "1.0" should ignore member default-features
+            (Some(false), None) => {
+                deprecated_ws_default_features(name, None, warnings);
+            }
+            _ => {}
         }
-        _ => {}
     }
     merged_dep.features = match (merged_dep.features.clone(), features.clone()) {
         (Some(dep_feat), Some(inherit_feat)) => Some(
@@ -1262,24 +1268,19 @@ fn inner_dependency_inherit_with<'a>(
 fn deprecated_ws_default_features(
     label: &str,
     ws_def_feat: Option<bool>,
-    edition: Edition,
     warnings: &mut Vec<String>,
-) -> CargoResult<()> {
+) {
     let ws_def_feat = match ws_def_feat {
         Some(true) => "true",
         Some(false) => "false",
         None => "not specified",
     };
-    if Edition::Edition2024 <= edition {
-        anyhow::bail!("`default-features = false` cannot override workspace's `default-features`");
-    } else {
-        warnings.push(format!(
-            "`default-features` is ignored for {label}, since `default-features` was \
-                {ws_def_feat} for `workspace.dependencies.{label}`, \
-                this could become a hard error in the future"
-        ));
-    }
-    Ok(())
+    warnings.push(format!(
+        "`default-features` is ignored for {label}, since `default-features` was \
+                {ws_def_feat} for `workspace.dependencies.{label}`; \
+                overriding workspace `default-features` to false requires Rust 1.99+ \
+                and the 2024 edition"
+    ));
 }
 
 #[tracing::instrument(skip_all)]

tests/testsuite/cargo_add/inherit_default_features_2024_edition/in/Cargo.toml

@@ -0,0 +1,6 @@
+[workspace]
+members = ["foo", "bar"]
+resolver = "3"
+
+[workspace.dependencies]
+foo = { version = "0.0.0", path = "./foo", default-features = false }

tests/testsuite/cargo_add/inherit_default_features_2024_edition/in/bar/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "bar"
+version = "0.0.0"
+edition = "2024"
+
+[dependencies]

tests/testsuite/cargo_add/inherit_default_features_2024_edition/in/bar/src/lib.rs

@@ -0,0 +1 @@
+

tests/testsuite/cargo_add/inherit_default_features_2024_edition/in/foo/Cargo.toml

@@ -0,0 +1,4 @@
+[package]
+name = "foo"
+version = "0.0.0"
+edition = "2024"

tests/testsuite/cargo_add/inherit_default_features_2024_edition/in/foo/src/lib.rs

@@ -0,0 +1 @@
+

tests/testsuite/cargo_add/inherit_default_features_2024_edition/mod.rs

@@ -0,0 +1,25 @@
+use crate::prelude::*;
+use cargo_test_support::Project;
+use cargo_test_support::compare::assert_ui;
+use cargo_test_support::current_dir;
+use cargo_test_support::file;
+use cargo_test_support::str;
+
+#[cargo_test]
+fn case() {
+    cargo_test_support::registry::init();
+    let project = Project::from_template(current_dir!().join("in"));
+    let project_root = project.root();
+    let cwd = &project_root;
+
+    snapbox::cmd::Command::cargo_ui()
+        .arg("add")
+        .args(["foo", "--default-features", "-p", "bar"])
+        .current_dir(cwd)
+        .assert()
+        .success()
+        .stdout_eq(str![""])
+        .stderr_eq(file!["stderr.term.svg"]);
+
+    assert_ui().subset_matches(current_dir!().join("out"), &project_root);
+}

tests/testsuite/cargo_add/inherit_default_features_2024_edition/out/Cargo.toml

@@ -0,0 +1,6 @@
+[workspace]
+members = ["foo", "bar"]
+resolver = "3"
+
+[workspace.dependencies]
+foo = { version = "0.0.0", path = "./foo", default-features = false }

tests/testsuite/cargo_add/inherit_default_features_2024_edition/out/bar/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "bar"
+version = "0.0.0"
+edition = "2024"
+
+[dependencies]
+foo = { workspace = true, default-features = true }

tests/testsuite/cargo_add/inherit_default_features_2024_edition/out/bar/src/lib.rs

@@ -0,0 +1 @@
+

tests/testsuite/cargo_add/inherit_default_features_2024_edition/out/foo/Cargo.toml

@@ -0,0 +1,4 @@
+[package]
+name = "foo"
+version = "0.0.0"
+edition = "2024"

tests/testsuite/cargo_add/inherit_default_features_2024_edition/out/foo/src/lib.rs

@@ -0,0 +1 @@
+

tests/testsuite/cargo_add/inherit_no_default_features_2024_edition/in/Cargo.toml

@@ -0,0 +1,6 @@
+[workspace]
+members = ["foo", "bar"]
+resolver = "3"
+
+[workspace.dependencies]
+foo = { version = "0.0.0", path = "./foo", default-features = true }

tests/testsuite/cargo_add/inherit_no_default_features_2024_edition/in/bar/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "bar"
+version = "0.0.0"
+edition = "2024"
+
+[dependencies]

tests/testsuite/cargo_add/inherit_no_default_features_2024_edition/in/foo/Cargo.toml

@@ -0,0 +1,4 @@
+[package]
+name = "foo"
+version = "0.0.0"
+edition = "2024"

tests/testsuite/cargo_add/inherit_no_default_features_2024_edition/mod.rs

@@ -0,0 +1,25 @@
+use crate::prelude::*;
+use cargo_test_support::Project;
+use cargo_test_support::compare::assert_ui;
+use cargo_test_support::current_dir;
+use cargo_test_support::file;
+use cargo_test_support::str;
+
+#[cargo_test]
+fn case() {
+    cargo_test_support::registry::init();
+    let project = Project::from_template(current_dir!().join("in"));
+    let project_root = project.root();
+    let cwd = &project_root;
+
+    snapbox::cmd::Command::cargo_ui()
+        .arg("add")
+        .args(["foo", "--no-default-features", "-p", "bar"])
+        .current_dir(cwd)
+        .assert()
+        .success()
+        .stdout_eq(str![""])
+        .stderr_eq(file!["stderr.term.svg"]);
+
+    assert_ui().subset_matches(current_dir!().join("out"), &project_root);
+}

tests/testsuite/cargo_add/inherit_no_default_features_2024_edition/out/Cargo.toml

@@ -0,0 +1,6 @@
+[workspace]
+members = ["foo", "bar"]
+resolver = "3"
+
+[workspace.dependencies]
+foo = { version = "0.0.0", path = "./foo", default-features = true }