Stabilize sparse-registry for crates.io

Author: arlosi

src/cargo/sources/registry/http_remote.rs

@@ -131,7 +131,7 @@ impl<'cfg> HttpRegistry<'cfg> {
         config: &'cfg Config,
         name: &str,
     ) -> CargoResult<HttpRegistry<'cfg>> {
-        if !config.cli_unstable().sparse_registry {
+        if !source_id.is_crates_io() && !config.cli_unstable().sparse_registry {
             anyhow::bail!("usage of sparse registries requires `-Z sparse-registry`");
         }
         let url = source_id.url().as_str();

src/doc/src/reference/config.md

@@ -891,6 +891,13 @@ commands like [`cargo publish`] that require authentication.
 
 Can be overridden with the `--token` command-line option.
 
+##### `registries.crates-io.protocol`
+* Type: string
+* Default: `git` (or `sparse` `-Z sparse-registry` is passed)
+* Environment: `CARGO_REGISTRIES_CRATES_IO_PROTOCOL`
+
+Specifies the protocol used to access crates.io. Allowed values are `git` or `sparse`.
+
 #### `[registry]`
 
 The `[registry]` table controls the default registry used when one is not

src/doc/src/reference/unstable.md

@@ -832,7 +832,7 @@ fn main() {
 * Tracking Issue: [9069](https://github.com/rust-lang/cargo/issues/9069)
 * RFC: [#2789](https://github.com/rust-lang/rfcs/pull/2789)
 
-The `sparse-registry` feature allows cargo to interact with remote registries served
+The `sparse-registry` feature allows Cargo to interact with remote registries served
 over plain HTTP rather than git. These registries can be identified by urls starting with
 `sparse+http://` or `sparse+https://`.
 
@@ -841,6 +841,17 @@ crates, which can save significant time and bandwidth.
 
 The format of the sparse index is identical to a checkout of a git-based index.
 
+Note that this feature is not required for accessing crates.io over the sparse protocol.
+To set the protocol used for crates.io, set in a [config file]:
+```toml
+[registries.crates-io]
+protocol = 'sparse'
+```
+Or use the environment variable `CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse`.
+
+Enabling this feature changes the default protocol for accessing crates.io to from `git`
+to `sparse`.
+
 ### credential-process
 * Tracking Issue: [#8933](https://github.com/rust-lang/cargo/issues/8933)
 * RFC: [#2730](https://github.com/rust-lang/rfcs/pull/2730)

tests/testsuite/registry.rs

@@ -2703,15 +2703,6 @@ fn http_requires_z_flag() {
         .run();
 }
 
-#[cargo_test]
-fn protocol_sparse_requires_z_flag() {
-    cargo_process("install bar")
-        .with_status(101)
-        .env("CARGO_REGISTRIES_CRATES_IO_PROTOCOL", "sparse")
-        .with_stderr("[ERROR] usage of sparse registries requires `-Z sparse-registry`")
-        .run()
-}
-
 #[cargo_test]
 fn protocol() {
     cargo_process("install bar")