Add configuration option for controlling crates.io protocol

`registries.crates-io.protocol` can be set to either `sparse` or `git`.
The default is `git` unless `-Z sparse-registry` is passed.

Author: arlosi

src/cargo/core/source/source_id.rs

@@ -2,7 +2,7 @@ use crate::core::PackageId;
 use crate::sources::registry::CRATES_IO_HTTP_INDEX;
 use crate::sources::{DirectorySource, CRATES_IO_DOMAIN, CRATES_IO_INDEX, CRATES_IO_REGISTRY};
 use crate::sources::{GitSource, PathSource, RegistrySource};
-use crate::util::{CanonicalUrl, CargoResult, Config, IntoUrl};
+use crate::util::{config, CanonicalUrl, CargoResult, Config, IntoUrl};
 use log::trace;
 use serde::de;
 use serde::ser;
@@ -215,7 +215,7 @@ impl SourceId {
     /// Returns the `SourceId` corresponding to the main repository, using the
     /// sparse HTTP index if allowed.
     pub fn crates_io_maybe_sparse_http(config: &Config) -> CargoResult<SourceId> {
-        if config.cli_unstable().sparse_registry {
+        if Self::crates_io_is_sparse(config)? {
             config.check_registry_index_not_set()?;
             let url = CRATES_IO_HTTP_INDEX.into_url().unwrap();
             SourceId::new(SourceKind::Registry, url, Some(CRATES_IO_REGISTRY))
@@ -224,6 +224,21 @@ impl SourceId {
         }
     }
 
+    /// Returns whether to access crates.io over the sparse protocol.
+    pub fn crates_io_is_sparse(config: &Config) -> CargoResult<bool> {
+        let proto: Option<config::Value<String>> = config.get("registries.crates-io.protocol")?;
+        let is_sparse = match proto.as_ref().map(|v| v.val.as_str()) {
+            Some("sparse") => true,
+            Some("git") => false,
+            Some(unknown) => anyhow::bail!(
+                "unsupported registry protocol `{unknown}` (defined in {})",
+                proto.as_ref().unwrap().definition
+            ),
+            None => config.cli_unstable().sparse_registry,
+        };
+        Ok(is_sparse)
+    }
+
     /// Gets the `SourceId` associated with given name of the remote registry.
     pub fn alt_registry(config: &Config, key: &str) -> CargoResult<SourceId> {
         if key == CRATES_IO_REGISTRY {

src/cargo/sources/config.rs

@@ -91,7 +91,7 @@ impl<'cfg> SourceConfigMap<'cfg> {
                 replace_with: None,
             },
         )?;
-        if config.cli_unstable().sparse_registry {
+        if SourceId::crates_io_is_sparse(config)? {
             base.add(
                 CRATES_IO_REGISTRY,
                 SourceConfig {

tests/testsuite/registry.rs

@@ -2703,6 +2703,24 @@ 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")
+        .with_status(101)
+        .env("CARGO_REGISTRIES_CRATES_IO_PROTOCOL", "invalid")
+        .with_stderr("[ERROR] unsupported registry protocol `invalid` (defined in environment variable `CARGO_REGISTRIES_CRATES_IO_PROTOCOL`)")
+        .run()
+}
+
 #[cargo_test]
 fn http_requires_trailing_slash() {
     cargo_process("-Z sparse-registry install bar --index sparse+https://index.crates.io")