Auto merge of #7718 - ehuss:fix-vendor-alt-reg, r=alexcrichton

vendor: support alt registries

Adds support for alt registries to `cargo vendor`. It mostly worked before, but panicked when trying to display the `.cargo/config` instructions.

This isn't entirely elegant, as the source replacement looks like this:

```toml
[source.crates-io]
replace-with = "vendored-sources"

[source."file:///Users/eric/Proj/rust/cargo/target/cit/t0/alternative-registry"]
registry = "file:///Users/eric/Proj/rust/cargo/target/cit/t0/alternative-registry"
replace-with = "vendored-sources"

[source."file:///Users/eric/Proj/rust/cargo/target/cit/t0/gitdep"]
git = "file:///Users/eric/Proj/rust/cargo/target/cit/t0/gitdep"
branch = "master"
replace-with = "vendored-sources"

[source.vendored-sources]
directory = "vendor"
```

The duplication of the URLs is a little unfortunate.  It could use the name of the registry, but that is not readily available and is tricky to obtain.  I feel like that is a challenge for another day.

Closes #7674.

src/cargo/ops/vendor.rs

@@ -258,6 +258,12 @@ fn sync(
                 registry: None,
                 replace_with: merged_source_name.to_string(),
             }
+        } else if source_id.is_remote_registry() {
+            let registry = source_id.url().to_string();
+            VendorSource::Registry {
+                registry: Some(registry),
+                replace_with: merged_source_name.to_string(),
+            }
         } else if source_id.is_git() {
             let mut branch = None;
             let mut tag = None;

tests/testsuite/vendor.rs

@@ -1,4 +1,8 @@
 //! Tests for the `cargo vendor` command.
+//!
+//! Note that every test here uses `--respect-source-config` so that the
+//! "fake" crates.io is used. Otherwise `vendor` would download the crates.io
+//! index from the network.
 
 use cargo_test_support::git;
 use cargo_test_support::registry::Package;
@@ -584,3 +588,46 @@ fn ignore_hidden() {
         .iter()
         .all(|status| status.status() == git2::Status::CURRENT));
 }
+
+#[cargo_test]
+fn config_instructions_works() {
+    // Check that the config instructions work for all dependency kinds.
+    Package::new("dep", "0.1.0").publish();
+    Package::new("altdep", "0.1.0").alternative(true).publish();
+    let git_project = git::new("gitdep", |project| {
+        project
+            .file("Cargo.toml", &basic_lib_manifest("gitdep"))
+            .file("src/lib.rs", "")
+    });
+    let p = project()
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+
+                [dependencies]
+                dep = "0.1"
+                altdep = {{version="0.1", registry="alternative"}}
+                gitdep = {{git='{}'}}
+                "#,
+                git_project.url()
+            ),
+        )
+        .file("src/lib.rs", "")
+        .build();
+    let output = p
+        .cargo("vendor --respect-source-config")
+        .exec_with_output()
+        .unwrap();
+    let output = String::from_utf8(output.stdout).unwrap();
+    p.change_file(".cargo/config", &output);
+
+    p.cargo("check -v")
+        .with_stderr_contains("[..]foo/vendor/dep/src/lib.rs[..]")
+        .with_stderr_contains("[..]foo/vendor/altdep/src/lib.rs[..]")
+        .with_stderr_contains("[..]foo/vendor/gitdep/src/lib.rs[..]")
+        .run();
+}