feat: Add 'open-namespaces' feature

crates/cargo-util-schemas/src/manifest/mod.rs

@@ -1209,7 +1209,7 @@ str_newtype!(PackageName);
 impl<T: AsRef<str>> PackageName<T> {
     /// Validated package name
     pub fn new(name: T) -> Result<Self, NameValidationError> {
-        restricted_names::validate_package_name(name.as_ref(), "package name")?;
+        restricted_names::validate_package_name(name.as_ref())?;
         Ok(Self(name))
     }
 }
@@ -1231,7 +1231,7 @@ str_newtype!(RegistryName);
 impl<T: AsRef<str>> RegistryName<T> {
     /// Validated registry name
     pub fn new(name: T) -> Result<Self, NameValidationError> {
-        restricted_names::validate_package_name(name.as_ref(), "registry name")?;
+        restricted_names::validate_registry_name(name.as_ref())?;
         Ok(Self(name))
     }
 }

crates/cargo-util-schemas/src/restricted_names.rs

@@ -33,13 +33,18 @@ enum ErrorKind {
     FeatureNameStartsWithDepColon(String),
 }
 
-/// Check the base requirements for a package name.
-///
-/// This can be used for other things than package names, to enforce some
-/// level of sanity. Note that package names have other restrictions
-/// elsewhere. `cargo new` has a few restrictions, such as checking for
-/// reserved names. crates.io has even more restrictions.
-pub(crate) fn validate_package_name(name: &str, what: &'static str) -> Result<()> {
+pub(crate) fn validate_package_name(name: &str) -> Result<()> {
+    for part in name.split("::") {
+        validate_name(part, "package name")?;
+    }
+    Ok(())
+}
+
+pub(crate) fn validate_registry_name(name: &str) -> Result<()> {
+    validate_name(name, "registry name")
+}
+
+pub(crate) fn validate_name(name: &str, what: &'static str) -> Result<()> {
     if name.is_empty() {
         return Err(ErrorKind::Empty(what).into());
     }
@@ -84,6 +89,17 @@ pub(crate) fn validate_package_name(name: &str, what: &'static str) -> Result<()
 
 /// Ensure a package name is [valid][validate_package_name]
 pub(crate) fn sanitize_package_name(name: &str, placeholder: char) -> String {
+    let mut slug = String::new();
+    for part in name.split("::") {
+        if !slug.is_empty() {
+            slug.push_str("::");
+        }
+        slug.push_str(&sanitize_name(part, placeholder));
+    }
+    slug
+}
+
+pub(crate) fn sanitize_name(name: &str, placeholder: char) -> String {
     let mut slug = String::new();
     let mut chars = name.chars();
     while let Some(ch) = chars.next() {

src/cargo/core/features.rs

@@ -504,6 +504,9 @@ features! {
 
     /// Allow setting trim-paths in a profile to control the sanitisation of file paths in build outputs.
     (unstable, trim_paths, "", "reference/unstable.html#profile-trim-paths-option"),
+
+    /// Allow multiple packages to participate in the same API namespace
+    (unstable, open_namespaces, "", "reference/unstable.html#open-namespaces"),
 }
 
 /// Status and metadata for a single unstable feature.

src/cargo/util/toml/mod.rs

@@ -247,6 +247,16 @@ pub fn prepare_for_publish(
     package_root: &Path,
 ) -> CargoResult<manifest::TomlManifest> {
     let gctx = ws.gctx();
+
+    if me
+        .cargo_features
+        .iter()
+        .flat_map(|f| f.iter())
+        .any(|f| f == "open-namespaces")
+    {
+        anyhow::bail!("cannot publish with `open-namespaces`")
+    }
+
     let mut package = me.package().unwrap().clone();
     package.workspace = None;
     let current_resolver = package
@@ -587,6 +597,9 @@ pub fn to_real_manifest(
     };
 
     let package_name = package.name.trim();
+    if package_name.contains(':') {
+        features.require(Feature::open_namespaces())?;
+    }
 
     let resolved_path = package_root.join("Cargo.toml");
 

src/doc/src/reference/unstable.md

@@ -87,6 +87,7 @@ For the latest nightly, see the [nightly version] of this page.
     * [host-config](#host-config) --- Allows setting `[target]`-like configuration settings for host build targets.
     * [target-applies-to-host](#target-applies-to-host) --- Alters whether certain flags will be passed to host build targets.
     * [gc](#gc) --- Global cache garbage collection.
+    * [open-namespaces](#open-namespaces) --- Allow multiple packages to participate in the same API namespace
 * rustdoc
     * [rustdoc-map](#rustdoc-map) --- Provides mappings for documentation to link to external sites like [docs.rs](https://docs.rs/).
     * [scrape-examples](#scrape-examples) --- Shows examples within documentation.
@@ -1518,6 +1519,20 @@ cargo clean gc --max-download-age=1week
 cargo clean gc --max-git-size=0 --max-download-size=100MB
 ```
 
+## open-namespaces
+
+* Tracking Issue: [#13576](https://github.com/rust-lang/cargo/issues/13576)
+
+Allow multiple packages to participate in the same API namespace
+
+This can be enabled like so:
+```toml
+cargo-features = ["open-namespaces"]
+
+[package]
+# ...
+```
+
 # Stabilized and removed features
 
 ## Compile progress

tests/testsuite/build.rs

@@ -467,18 +467,18 @@ fn cargo_compile_with_empty_package_name() {
 #[cargo_test]
 fn cargo_compile_with_invalid_package_name() {
     let p = project()
-        .file("Cargo.toml", &basic_manifest("foo::bar", "0.0.0"))
+        .file("Cargo.toml", &basic_manifest("foo@bar", "0.0.0"))
         .build();
 
     p.cargo("build")
         .with_status(101)
         .with_stderr(
             "\
-[ERROR] invalid character `:` in package name: `foo::bar`, characters must be Unicode XID characters (numbers, `-`, `_`, or most letters)
+[ERROR] invalid character `@` in package name: `foo@bar`, characters must be Unicode XID characters (numbers, `-`, `_`, or most letters)
  --> Cargo.toml:3:16
   |
-3 |         name = \"foo::bar\"
-  |                ^^^^^^^^^^
+3 |         name = \"foo@bar\"
+  |                ^^^^^^^^^
   |
 ",
         )

tests/testsuite/main.rs

@@ -127,6 +127,7 @@ mod net_config;
 mod new;
 mod offline;
 mod old_cargos;
+mod open_namespaces;
 mod out_dir;
 mod owner;
 mod package;