Store the sparse+ prefix in the SourceId for sparse registries

src/cargo/core/source/source_id.rs

@@ -102,17 +102,11 @@ impl SourceId {
         SourceId { inner }
     }
 
-    fn remote_source_kind(url: &Url) -> (SourceKind, Url) {
+    fn remote_source_kind(url: &Url) -> SourceKind {
         if url.as_str().starts_with("sparse+") {
-            let url = url
-                .to_string()
-                .strip_prefix("sparse+")
-                .expect("we just found that prefix")
-                .into_url()
-                .expect("a valid url without a protocol specifier should still be valid");
-            (SourceKind::SparseRegistry, url)
+            SourceKind::SparseRegistry
         } else {
-            (SourceKind::Registry, url.to_owned())
+            SourceKind::Registry
         }
     }
 
@@ -196,14 +190,14 @@ impl SourceId {
     /// Use [`SourceId::for_alt_registry`] if a name can provided, which
     /// generates better messages for cargo.
     pub fn for_registry(url: &Url) -> CargoResult<SourceId> {
-        let (kind, url) = Self::remote_source_kind(url);
-        SourceId::new(kind, url, None)
+        let kind = Self::remote_source_kind(url);
+        SourceId::new(kind, url.to_owned(), None)
     }
 
     /// Creates a `SourceId` from a remote registry URL with given name.
     pub fn for_alt_registry(url: &Url, name: &str) -> CargoResult<SourceId> {
-        let (kind, url) = Self::remote_source_kind(url);
-        SourceId::new(kind, url, Some(name))
+        let kind = Self::remote_source_kind(url);
+        SourceId::new(kind, url.to_owned(), Some(name))
     }
 
     /// Creates a SourceId from a local registry path.
@@ -263,7 +257,7 @@ impl SourceId {
             return Self::crates_io(config);
         }
         let url = config.get_registry_index(key)?;
-        let (kind, url) = Self::remote_source_kind(&url);
+        let kind = Self::remote_source_kind(&url);
         Ok(SourceId::wrap(SourceIdInner {
             kind,
             canonical_url: CanonicalUrl::new(&url)?,
@@ -428,10 +422,7 @@ impl SourceId {
         let url = self.inner.url.as_str();
         url == CRATES_IO_INDEX
             || url == CRATES_IO_HTTP_INDEX
-            || std::env::var("__CARGO_TEST_CRATES_IO_URL_DO_NOT_USE_THIS")
-                .as_deref()
-                .map(|u| u.trim_start_matches("sparse+"))
-                == Ok(url)
+            || std::env::var("__CARGO_TEST_CRATES_IO_URL_DO_NOT_USE_THIS").as_deref() == Ok(url)
     }
 
     /// Hashes `self`.
@@ -755,7 +746,7 @@ impl<'a> fmt::Display for SourceIdAsUrl<'a> {
                 ref url,
                 ..
             } => {
-                write!(f, "sparse+{url}")
+                write!(f, "{url}")
             }
             SourceIdInner {
                 kind: SourceKind::LocalRegistry,

src/cargo/sources/registry/http_remote.rs

@@ -163,15 +163,21 @@ impl<'cfg> HttpRegistry<'cfg> {
         let url = source_id.url().as_str();
         // Ensure the url ends with a slash so we can concatenate paths.
         if !url.ends_with('/') {
-            anyhow::bail!("sparse registry url must end in a slash `/`: sparse+{url}")
+            anyhow::bail!("sparse registry url must end in a slash `/`: {url}")
         }
+        assert!(source_id.is_sparse());
+        let url = url
+            .strip_prefix("sparse+")
+            .expect("sparse registry needs sparse+ prefix")
+            .into_url()
+            .expect("a url with the sparse+ stripped should still be valid");
 
         Ok(HttpRegistry {
             index_path: config.registry_index_path().join(name),
             cache_path: config.registry_cache_path().join(name),
             source_id,
             config,
-            url: source_id.url().to_owned(),
+            url: url,
             multi: Multi::new(),
             multiplexing: false,
             downloads: Downloads {

src/cargo/sources/registry/mod.rs

@@ -188,7 +188,7 @@ use crate::util::{
 
 const PACKAGE_SOURCE_LOCK: &str = ".cargo-ok";
 pub const CRATES_IO_INDEX: &str = "https://github.com/rust-lang/crates.io-index";
-pub const CRATES_IO_HTTP_INDEX: &str = "https://index.crates.io/";
+pub const CRATES_IO_HTTP_INDEX: &str = "sparse+https://index.crates.io/";
 pub const CRATES_IO_REGISTRY: &str = "crates-io";
 pub const CRATES_IO_DOMAIN: &str = "crates.io";
 const CRATE_TEMPLATE: &str = "{crate}";

tests/testsuite/vendor.rs

@@ -7,7 +7,7 @@
 use std::fs;
 
 use cargo_test_support::git;
-use cargo_test_support::registry::{self, Package};
+use cargo_test_support::registry::{self, Package, RegistryBuilder};
 use cargo_test_support::{basic_lib_manifest, paths, project, Project};
 
 #[cargo_test]
@@ -68,6 +68,41 @@ directory = "vendor"
         .run();
 }
 
+#[cargo_test]
+fn vendor_sample_config_alt_registry() {
+    let registry = RegistryBuilder::new().alternative().http_index().build();
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+
+                [dependencies]
+                log = { version = "0.3.5", registry = "alternative" }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    Package::new("log", "0.3.5").alternative(true).publish();
+
+    p.cargo("vendor --respect-source-config -Z sparse-registry")
+        .masquerade_as_nightly_cargo(&["sparse-registry"])
+        .with_stdout(format!(
+            r#"[source."{0}"]
+registry = "{0}"
+replace-with = "vendored-sources"
+
+[source.vendored-sources]
+directory = "vendor"
+"#,
+            registry.index_url()
+        ))
+        .run();
+}
+
 #[cargo_test]
 fn vendor_path_specified() {
     let p = project()