Don't add the new package to workspace.members if there is no existing workspace in Cargo.toml.

src/cargo/ops/cargo_new.rs

@@ -970,33 +970,34 @@ fn update_manifest_with_new_member(
     // in the array already includes the new package's relative path.
     // - Add the relative path if the members don't match the new package's path.
     // - Create a new members array if there are no members element in the workspace yet.
-    if let Some(members) = workspace_document
-        .get_mut("workspace")
-        .and_then(|workspace| workspace.get_mut("members"))
-        .and_then(|members| members.as_array_mut())
-    {
-        for member in members.iter() {
-            let pat = member
-                .as_str()
-                .with_context(|| format!("invalid non-string member `{}`", member))?;
-            let pattern = glob::Pattern::new(pat)
-                .with_context(|| format!("cannot build glob pattern from `{}`", pat))?;
+    if let Some(workspace) = workspace_document.get_mut("workspace") {
+        if let Some(members) = workspace
+            .get_mut("members")
+            .and_then(|members| members.as_array_mut())
+        {
+            for member in members.iter() {
+                let pat = member
+                    .as_str()
+                    .with_context(|| format!("invalid non-string member `{}`", member))?;
+                let pattern = glob::Pattern::new(pat)
+                    .with_context(|| format!("cannot build glob pattern from `{}`", pat))?;
+
+                if pattern.matches(&display_path) {
+                    return Ok(());
+                }
+            }
 
-            if pattern.matches(&display_path) {
-                return Ok(());
+            let was_sorted = is_sorted(members.iter().map(Value::as_str));
+            members.push(display_path);
+            if was_sorted {
+                members.sort_by(|lhs, rhs| lhs.as_str().cmp(&rhs.as_str()));
             }
-        }
+        } else {
+            let mut array = Array::new();
+            array.push(display_path);
 
-        let was_sorted = is_sorted(members.iter().map(Value::as_str));
-        members.push(display_path);
-        if was_sorted {
-            members.sort_by(|lhs, rhs| lhs.as_str().cmp(&rhs.as_str()));
+            workspace["members"] = toml_edit::value(array);
         }
-    } else {
-        let mut array = Array::new();
-        array.push(display_path);
-
-        workspace_document["workspace"]["members"] = toml_edit::value(array);
     }
 
     write_atomic(

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

@@ -0,0 +1,6 @@
+[package]
+name = "foo"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

tests/testsuite/cargo_new/add_members_to_non_workspace/in/src/main.rs

@@ -0,0 +1 @@
+fn main() {}

tests/testsuite/cargo_new/add_members_to_non_workspace/mod.rs

@@ -0,0 +1,22 @@
+use cargo_test_support::compare::assert_ui;
+use cargo_test_support::curr_dir;
+use cargo_test_support::CargoCommand;
+use cargo_test_support::Project;
+
+#[cargo_test]
+fn case() {
+    let project = Project::from_template(curr_dir!().join("in"));
+    let project_root = project.root();
+    let cwd = &project_root;
+
+    snapbox::cmd::Command::cargo_ui()
+        .arg("new")
+        .args(["bar", "--lib"])
+        .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_new/add_members_to_non_workspace/out/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "foo"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

tests/testsuite/cargo_new/add_members_to_non_workspace/out/bar/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "bar"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

tests/testsuite/cargo_new/add_members_to_non_workspace/out/bar/src/lib.rs

@@ -0,0 +1,14 @@
+pub fn add(left: usize, right: usize) -> usize {
+    left + right
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        let result = add(2, 2);
+        assert_eq!(result, 4);
+    }
+}

tests/testsuite/cargo_new/add_members_to_non_workspace/out/src/main.rs

@@ -0,0 +1 @@
+fn main() {}

tests/testsuite/cargo_new/add_members_to_non_workspace/stderr.log

@@ -0,0 +1,2 @@
+    Creating library `bar` package
+note: see more `Cargo.toml` keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

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

@@ -0,0 +1,6 @@
+[workspace]
+
+[package]
+name = "foo"
+version = "0.1.0"
+edition = "2021"

tests/testsuite/cargo_new/add_members_to_workspace_without_members/in/src/main.rs

@@ -0,0 +1 @@
+fn main() {}

tests/testsuite/cargo_new/add_members_to_workspace_without_members/mod.rs

@@ -0,0 +1,22 @@
+use cargo_test_support::compare::assert_ui;
+use cargo_test_support::curr_dir;
+use cargo_test_support::CargoCommand;
+use cargo_test_support::Project;
+
+#[cargo_test]
+fn case() {
+    let project = Project::from_template(curr_dir!().join("in"));
+    let project_root = project.root();
+    let cwd = &project_root;
+
+    snapbox::cmd::Command::cargo_ui()
+        .arg("new")
+        .args(["bar", "--lib"])
+        .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_new/add_members_to_workspace_without_members/out/Cargo.toml

@@ -0,0 +1,7 @@
+[workspace]
+members = ["bar"]
+
+[package]
+name = "foo"
+version = "0.1.0"
+edition = "2021"

tests/testsuite/cargo_new/add_members_to_workspace_without_members/out/bar/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "bar"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

tests/testsuite/cargo_new/add_members_to_workspace_without_members/out/bar/src/lib.rs

@@ -0,0 +1,14 @@
+pub fn add(left: usize, right: usize) -> usize {
+    left + right
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        let result = add(2, 2);
+        assert_eq!(result, 4);
+    }
+}

tests/testsuite/cargo_new/add_members_to_workspace_without_members/out/src/main.rs

@@ -0,0 +1 @@
+fn main() {}

tests/testsuite/cargo_new/add_members_to_workspace_without_members/stderr.log

@@ -0,0 +1,2 @@
+    Creating library `bar` package
+note: see more `Cargo.toml` keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

tests/testsuite/cargo_new/mod.rs

@@ -1,9 +1,11 @@
+mod add_members_to_non_workspace;
 mod add_members_to_workspace_format_previous_items;
 mod add_members_to_workspace_format_sorted;
 mod add_members_to_workspace_with_absolute_package_path;
 mod add_members_to_workspace_with_empty_members;
 mod add_members_to_workspace_with_exclude_list;
 mod add_members_to_workspace_with_members_glob;
+mod add_members_to_workspace_without_members;
 mod empty_name;
 mod help;
 mod inherit_workspace_lints;