Fix sparse registry lockfile urls containing 'registry+sparse+'

src/cargo/core/source/source_id.rs

@@ -680,7 +680,16 @@ impl<'a> fmt::Display for SourceIdAsUrl<'a> {
                 kind: SourceKind::Registry,
                 ref url,
                 ..
-            } => write!(f, "registry+{}", url),
+            } => {
+                // For sparse http registry the URL already contains the prefix.
+                if url.scheme().starts_with("sparse+") {
+                    // e.g. sparse+http://example.com
+                    write!(f, "{url}")
+                } else {
+                    // e.g. registry+http://example.com
+                    write!(f, "registry+{url}")
+                }
+            }
             SourceIdInner {
                 kind: SourceKind::LocalRegistry,
                 ref url,

tests/testsuite/alt_registry.rs

@@ -1,6 +1,7 @@
 //! Tests for alternative registries.
 
 use cargo::util::IntoUrl;
+use cargo_test_support::compare::assert_match_exact;
 use cargo_test_support::publish::validate_alt_upload;
 use cargo_test_support::registry::{self, Package, RegistryBuilder};
 use cargo_test_support::{basic_manifest, git, paths, project};
@@ -1309,3 +1310,51 @@ fn both_index_and_registry() {
             .run();
     }
 }
+
+#[cargo_test]
+fn sparse_lockfile() {
+    let _registry = registry::RegistryBuilder::new()
+        .http_index()
+        .alternative()
+        .build();
+    Package::new("foo", "0.1.0").alternative(true).publish();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "a"
+                version = "0.5.0"
+                authors = []
+
+                [dependencies]
+                foo = { registry = 'alternative', version = '0.1.0'}
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("-Zsparse-registry generate-lockfile")
+        .masquerade_as_nightly_cargo(&["sparse-registry"])
+        .run();
+    assert_match_exact(
+        &p.read_lockfile(),
+        r#"# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "a"
+version = "0.5.0"
+dependencies = [
+ "foo",
+]
+
+[[package]]
+name = "foo"
+version = "0.1.0"
+source = "sparse+http://[..]/"
+checksum = "f6a200a9339fef960979d94d5c99cbbfd899b6f5a396a55d9775089119050203""#,
+    );
+}