Add -Zgc flag.

This adds the -Zgc flag in preparation to support garbage-collection.

This also factors out the `fail_if_stable_opt_custom_z` function which
will be used by the unstable gc checks.

Author: ehuss

src/cargo/core/features.rs

@@ -734,6 +734,7 @@ unstable_cli_options!(
     doctest_xcompile: bool = ("Compile and run doctests for non-host target using runner config"),
     dual_proc_macros: bool = ("Build proc-macros for both the host and the target"),
     features: Option<Vec<String>>  = (HIDDEN),
+    gc: bool = ("Track cache usage and \"garbage collect\" unused files"),
     gitoxide: Option<GitoxideFeatures> = ("Use gitoxide for the given git interactions, or all of them if no argument is given"),
     host_config: bool = ("Enable the [host] section in the .cargo/config.toml file"),
     lints: bool = ("Pass `[lints]` to the linting tools"),
@@ -1105,6 +1106,7 @@ impl CliUnstable {
             "direct-minimal-versions" => self.direct_minimal_versions = parse_empty(k, v)?,
             "doctest-xcompile" => self.doctest_xcompile = parse_empty(k, v)?,
             "dual-proc-macros" => self.dual_proc_macros = parse_empty(k, v)?,
+            "gc" => self.gc = parse_empty(k, v)?,
             "gitoxide" => {
                 self.gitoxide = v.map_or_else(
                     || Ok(Some(GitoxideFeatures::all())),
@@ -1139,7 +1141,17 @@ impl CliUnstable {
     /// Generates an error if `-Z unstable-options` was not used for a new,
     /// unstable command-line flag.
     pub fn fail_if_stable_opt(&self, flag: &str, issue: u32) -> CargoResult<()> {
-        if !self.unstable_options {
+        self.fail_if_stable_opt_custom_z(flag, issue, "unstable-options", self.unstable_options)
+    }
+
+    pub fn fail_if_stable_opt_custom_z(
+        &self,
+        flag: &str,
+        issue: u32,
+        z_name: &str,
+        enabled: bool,
+    ) -> CargoResult<()> {
+        if !enabled {
             let see = format!(
                 "See https://github.com/rust-lang/cargo/issues/{issue} for more \
                  information about the `{flag}` flag."
@@ -1148,7 +1160,7 @@ impl CliUnstable {
             let channel = channel();
             if channel == "nightly" || channel == "dev" {
                 bail!(
-                    "the `{flag}` flag is unstable, pass `-Z unstable-options` to enable it\n\
+                    "the `{flag}` flag is unstable, pass `-Z {z_name}` to enable it\n\
                      {see}"
                 );
             } else {