Enable yanked dependencies with an unstable flag

src/bin/cargo/cli.rs

@@ -28,13 +28,14 @@ pub fn main(config: &mut Config) -> CliResult {
             "
 Available unstable (nightly-only) flags:
 
-    -Z avoid-dev-deps   -- Avoid installing dev-dependencies if possible
-    -Z minimal-versions -- Install minimal dependency versions instead of maximum
-    -Z no-index-update  -- Do not update the registry, avoids a network request for benchmarking
-    -Z offline          -- Offline mode that does not perform network requests
-    -Z unstable-options -- Allow the usage of unstable options such as --registry
-    -Z config-profile   -- Read profiles from .cargo/config files
-    -Z compile-progress -- Display a progress bar while compiling
+    -Z avoid-dev-deps    -- Avoid installing dev-dependencies if possible
+    -Z minimal-versions  -- Install minimal dependency versions instead of maximum
+    -Z no-index-update   -- Do not update the registry, avoids a network request for benchmarking
+    -Z offline           -- Offline mode that does not perform network requests
+    -Z unstable-options  -- Allow the usage of unstable options such as --registry
+    -Z config-profile    -- Read profiles from .cargo/config files
+    -Z compile-progress  -- Display a progress bar while compiling
+    -Z allow-yanked-deps -- Allow depending on yanked crates
 
 Run with 'cargo -Z [FLAG] [SUBCOMMAND]'"
         );

src/cargo/core/features.rs

@@ -318,6 +318,7 @@ pub struct CliUnstable {
     pub advanced_env: bool,
     pub config_profile: bool,
     pub compile_progress: bool,
+    pub allow_yanked_deps: bool,
 }
 
 impl CliUnstable {
@@ -355,6 +356,7 @@ impl CliUnstable {
             "advanced-env" => self.advanced_env = true,
             "config-profile" => self.config_profile = true,
             "compile-progress" => self.compile_progress = true,
+            "allow-yanked-deps" => self.allow_yanked_deps = true,
             _ => bail!("unknown `-Z` flag specified: {}", k),
         }
 

src/cargo/sources/registry/index.rs

@@ -268,12 +268,15 @@ impl<'cfg> RegistryIndex<'cfg> {
         load: &mut RegistryData,
         f: &mut FnMut(Summary),
     ) -> CargoResult<()> {
+        let allow_yanked_deps = self.config.cli_unstable().allow_yanked_deps;
         let source_id = self.source_id.clone();
         let name = dep.package_name().as_str();
         let summaries = self.summaries(name, load)?;
         let summaries = summaries
             .iter()
-            .filter(|&&(_, yanked)| dep.source_id().precise().is_some() || !yanked)
+            .filter(|&&(_, yanked)| {
+                dep.source_id().precise().is_some() || allow_yanked_deps || !yanked
+            })
             .map(|s| s.0.clone());
 
         // Handle `cargo update --precise` here. If specified, our own source

src/doc/src/reference/unstable.md

@@ -198,6 +198,18 @@ directory.  Example:
 cargo +nightly build --out-dir=out -Z unstable-options
 ```
 
+## allow-yanked-deps
+* Original Issue: [#4425](https://github.com/rust-lang/cargo/issues/4225)
+
+This feature allows your project to depend on yanked crates.
+
+The flag can be useful if you need to build an old project that relies on a
+yanked dependency, and you can't update it. The flag should not be used on new
+or maintained projects: remember that yanked crates are not meant to be used.
+
+```
+cargo +nightly build -Z allow-yanked-deps
+```
 
 ### Edition
 * Tracking Issue: [rust-lang/rust#44581](https://github.com/rust-lang/rust/issues/44581)

tests/testsuite/cargo_command.rs

@@ -315,6 +315,6 @@ fn explain() {
 fn z_flags_help() {
     cargo_process("-Z help")
         .with_stdout_contains(
-            "    -Z unstable-options -- Allow the usage of unstable options such as --registry",
+            "    -Z unstable-options  -- Allow the usage of unstable options such as --registry",
         ).run();
 }

tests/testsuite/registry.rs

@@ -523,6 +523,42 @@ required by package `bar v0.0.1`
         ).run();
 }
 
+#[test]
+fn yanks_are_used_with_z_flag() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [project]
+            name = "foo"
+            version = "0.0.1"
+            authors = []
+
+            [dependencies]
+            bar = "*"
+        "#,
+        ).file("src/main.rs", "fn main() {}")
+        .build();
+
+    Package::new("baz", "0.0.1").publish();
+    Package::new("baz", "0.0.2").yanked(true).publish();
+    Package::new("bar", "0.0.1").dep("baz", "=0.0.2").publish();
+
+    p.cargo("build -Zallow-yanked-deps")
+        .masquerade_as_nightly_cargo()
+        .with_stderr(&format!(
+            "\
+[UPDATING] registry `[..]`
+[DOWNLOADING] [..] (registry `file://[..]`)
+[DOWNLOADING] [..] (registry `file://[..]`)
+[COMPILING] baz v0.0.2
+[COMPILING] bar v0.0.1
+[COMPILING] foo v0.0.1 (CWD)
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]s
+",
+        )).run();
+}
+
 #[test]
 fn yanks_in_lockfiles_are_ok() {
     let p = project()