Have cargo add --optional <dep> create a <dep> = "dep:<dep> feature

src/cargo/ops/cargo_add/mod.rs

@@ -7,6 +7,7 @@ use std::collections::BTreeSet;
 use std::collections::VecDeque;
 use std::fmt::Write;
 use std::path::Path;
+use std::str::FromStr;
 
 use anyhow::Context as _;
 use cargo_util::paths;
@@ -196,6 +197,20 @@ pub fn add(workspace: &Workspace<'_>, options: &AddOptions<'_>) -> CargoResult<(
         print_dep_table_msg(&mut options.config.shell(), &dep)?;
 
         manifest.insert_into_table(&dep_table, &dep)?;
+        if dep.optional == Some(true) {
+            let is_namespaced_features_supported =
+                check_rust_version_for_optional_dependency(options.spec.rust_version())?;
+            if is_namespaced_features_supported {
+                let dep_key = dep.toml_key();
+                if !manifest.is_explicit_dep_activation(dep_key) {
+                    let table = manifest.get_table_mut(&[String::from("features")])?;
+                    let dep_name = dep.rename.as_deref().unwrap_or(&dep.name);
+                    let new_feature: toml_edit::Value =
+                        [format!("dep:{dep_name}")].iter().collect();
+                    table[dep_key] = toml_edit::value(new_feature);
+                }
+            }
+        }
         manifest.gc_dep(dep.toml_key());
     }
 
@@ -469,6 +484,26 @@ fn check_invalid_ws_keys(toml_key: &str, arg: &DepOp) -> CargoResult<()> {
     Ok(())
 }
 
+/// When the `--optional` option is added using `cargo add`, we need to
+/// check the current rust-version. As the `dep:` syntax is only avaliable
+/// starting with Rust 1.60.0
+///
+/// `true` means that the rust-version is None or the rust-version is higher
+/// than the version needed.
+///
+/// Note: Previous versions can only use the implicit feature name.
+fn check_rust_version_for_optional_dependency(
+    rust_version: Option<&RustVersion>,
+) -> CargoResult<bool> {
+    match rust_version {
+        Some(version) => {
+            let syntax_support_version = RustVersion::from_str("1.60.0")?;
+            Ok(&syntax_support_version <= version)
+        }
+        None => Ok(true),
+    }
+}
+
 /// Provide the existing dependency for the target table
 ///
 /// If it doesn't exist but exists in another table, let's use that as most likely users

src/cargo/util/toml_mut/manifest.rs

@@ -420,7 +420,7 @@ impl LocalManifest {
         }
     }
 
-    fn is_explicit_dep_activation(&self, dep_key: &str) -> bool {
+    pub fn is_explicit_dep_activation(&self, dep_key: &str) -> bool {
         if let Some(toml_edit::Item::Table(feature_table)) = self.data.as_table().get("features") {
             for values in feature_table
                 .iter()

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

@@ -6,3 +6,6 @@ version = "0.0.0"
 
 [dependencies]
 some-package = { package = "my-package2", version = "99999.0.0", optional = true }
+
+[features]
+some-package = ["dep:some-package"]

tests/testsuite/cargo_add/detect_workspace_inherit_optional/out/primary/Cargo.toml

@@ -4,3 +4,6 @@ version = "0.0.0"
 
 [dependencies]
 foo = { workspace = true, optional = true }
+
+[features]
+foo = ["dep:foo"]

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

@@ -7,3 +7,7 @@ version = "0.0.0"
 [dependencies]
 my-package1 = { version = "99999.0.0", optional = true }
 my-package2 = { version = "0.4.1", optional = true }
+
+[features]
+my-package1 = ["dep:my-package1"]
+my-package2 = ["dep:my-package2"]

tests/testsuite/cargo_add/overwrite_git_with_path/out/primary/Cargo.toml

@@ -6,3 +6,6 @@ version = "0.0.0"
 
 [dependencies]
 cargo-list-test-fixture-dependency = { optional = true, path = "../dependency", version = "0.0.0" }
+
+[features]
+cargo-list-test-fixture-dependency = ["dep:cargo-list-test-fixture-dependency"]

tests/testsuite/cargo_add/overwrite_inherit_optional_noop/out/primary/Cargo.toml

@@ -4,3 +4,6 @@ version = "0.0.0"
 
 [dependencies]
 foo = { workspace = true, optional = true }
+
+[features]
+foo = ["dep:foo"]

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

@@ -7,3 +7,6 @@ version = "0.0.0"
 
 [dependencies]
 your-face = { version = "0.0.0", path = "dependency", optional = true, default-features = false, features = ["nose", "mouth"], registry = "alternative" }
+
+[features]
+your-face = ["dep:your-face"]

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

@@ -7,3 +7,7 @@ version = "0.0.0"
 [dependencies]
 my-package1 = { version = "99999.0.0", optional = true }
 my-package2 = { version = "0.4.1", optional = true }
+
+[features]
+my-package1 = ["dep:my-package1"]
+my-package2 = ["dep:my-package2"]

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

@@ -7,3 +7,7 @@ version = "0.0.0"
 [dependencies]
 my-package1 = { version = "99999.0.0", optional = true }
 my-package2 = { version = "0.4.1", optional = true }
+
+[features]
+my-package1 = ["dep:my-package1"]
+my-package2 = ["dep:my-package2"]

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

@@ -0,0 +1,11 @@
+[workspace]
+
+[package]
+name = "cargo-list-test-fixture"
+version = "0.0.0"
+
+[dependencies]
+my-package1 = { version = "99999.0.0", optional = true }
+
+[features]
+default = ["dep:my-package1"]

tests/testsuite/cargo_add/overwrite_optional_with_optional/mod.rs

@@ -0,0 +1,36 @@
+use cargo_test_support::compare::assert_ui;
+use cargo_test_support::prelude::*;
+use cargo_test_support::Project;
+
+use cargo_test_support::curr_dir;
+
+#[cargo_test]
+fn case() {
+    cargo_test_support::registry::init();
+    for ver in [
+        "0.1.1+my-package",
+        "0.2.0+my-package",
+        "0.2.3+my-package",
+        "0.4.1+my-package",
+        "20.0.0+my-package",
+        "99999.0.0+my-package",
+        "99999.0.0-alpha.1+my-package",
+    ] {
+        cargo_test_support::registry::Package::new("my-package1", ver).publish();
+    }
+
+    let project = Project::from_template(curr_dir!().join("in"));
+    let project_root = project.root();
+    let cwd = &project_root;
+
+    snapbox::cmd::Command::cargo_ui()
+        .arg("add")
+        .arg_line("my-package1 --optional")
+        .current_dir(cwd)
+        .assert()
+        .success()
+        .stdout_matches_path(curr_dir!().join("stdout.log"))
+        .stderr_matches_path(curr_dir!().join("stderr.log"));
+
+    assert_ui().subset_matches(curr_dir!().join("out"), &project_root);
+}

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

@@ -0,0 +1,11 @@
+[workspace]
+
+[package]
+name = "cargo-list-test-fixture"
+version = "0.0.0"
+
+[dependencies]
+my-package1 = { version = "99999.0.0", optional = true }
+
+[features]
+default = ["dep:my-package1"]

tests/testsuite/cargo_add/overwrite_optional_with_optional/stderr.log

@@ -0,0 +1,3 @@
+    Updating `dummy-registry` index
+      Adding my-package1 v99999.0.0 to optional dependencies.
+      Adding my-package2 v0.4.1 to optional dependencies.

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

@@ -7,3 +7,6 @@ version = "0.0.0"
 
 [dependencies]
 your-face = { version = "0.0.0", path = "dependency", optional = true, default-features = false, features = ["nose", "mouth"], registry = "alternative" }
+
+[features]
+your-face = ["dep:your-face"]

tests/testsuite/cargo_add/overwrite_path_with_version/out/primary/Cargo.toml

@@ -6,3 +6,6 @@ version = "0.0.0"
 
 [dependencies]
 cargo-list-test-fixture-dependency = { optional = true, version = "20.0" }
+
+[features]
+cargo-list-test-fixture-dependency = ["dep:cargo-list-test-fixture-dependency"]

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

@@ -6,3 +6,6 @@ version = "0.0.0"
 
 [dependencies]
 a1 = { package = "versioned-package", version = "0.1.1", optional = true }
+
+[features]
+a1 = ["dep:a1"]

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

@@ -6,3 +6,6 @@ version = "0.0.0"
 
 [dependencies]
 versioned-package = { version = "0.3.0", optional = true, git = "[ROOTURL]/versioned-package" }
+
+[features]
+versioned-package = ["dep:versioned-package"]

tests/testsuite/cargo_add/overwrite_version_with_path/out/primary/Cargo.toml

@@ -6,3 +6,6 @@ version = "0.0.0"
 
 [dependencies]
 cargo-list-test-fixture-dependency = { version = "0.0.0", optional = true, path = "../dependency" }
+
+[features]
+cargo-list-test-fixture-dependency = ["dep:cargo-list-test-fixture-dependency"]