Auto merge of #11268 - arlosi:fix-sparse-publish, r=ehuss

Fix publishing with a dependency on a sparse registry

### What does this PR try to resolve?
On publishing, the `registry_index` field was being set to the registry URL without the `sparse+` prefix. During the verification build, Cargo would attempt to fetch the registry as a `git` registry. Caused by #11209.

Fixes #11263.

### How should we test and review this PR?

Added a test that fails without the change.

src/cargo/util/toml/mod.rs

@@ -1521,8 +1521,7 @@ impl TomlManifest {
                     d.rev.take();
                     // registry specifications are elaborated to the index URL
                     if let Some(registry) = d.registry.take() {
-                        let src = SourceId::alt_registry(config, &registry)?;
-                        d.registry_index = Some(src.url().to_string());
+                        d.registry_index = Some(config.get_registry_index(&registry)?.to_string());
                     }
                     Ok(TomlDependency::Detailed(d))
                 }

tests/testsuite/alt_registry.rs

@@ -1337,3 +1337,51 @@ source = "sparse+http://[..]/"
 checksum = "f6a200a9339fef960979d94d5c99cbbfd899b6f5a396a55d9775089119050203""#,
     );
 }
+
+#[cargo_test]
+fn publish_with_transitive_dep() {
+    let _alt1 = RegistryBuilder::new()
+        .http_api()
+        .http_index()
+        .alternative_named("Alt-1")
+        .build();
+    let _alt2 = RegistryBuilder::new()
+        .http_api()
+        .http_index()
+        .alternative_named("Alt-2")
+        .build();
+
+    let p1 = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "a"
+                version = "0.5.0"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+    p1.cargo("publish -Zsparse-registry --registry Alt-1")
+        .masquerade_as_nightly_cargo(&["sparse-registry"])
+        .run();
+
+    let p2 = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "b"
+                version = "0.6.0"
+                publish = ["Alt-2"]
+
+                [dependencies]
+                a = { version = "0.5.0", registry = "Alt-1" }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+    p2.cargo("publish -Zsparse-registry")
+        .masquerade_as_nightly_cargo(&["sparse-registry"])
+        .run();
+}