Unhide nightly-only flags.

I changed my mind on how these should work. I think it is useful to
"advertise" the existence of these options on stable. The error message
if you try to use it should help guide on what to do.

Author: ehuss

src/bin/cargo/cli.rs

@@ -327,9 +327,12 @@ See 'cargo help <command>' for more information on a specific command.\n",
         .arg(opt("locked", "Require Cargo.lock is up to date").global(true))
         .arg(opt("offline", "Run without accessing the network").global(true))
         .arg(
-            multi_opt("config", "KEY=VALUE", "Override a configuration value")
-                .global(true)
-                .hidden(true),
+            multi_opt(
+                "config",
+                "KEY=VALUE",
+                "Override a configuration value (unstable)",
+            )
+            .global(true),
         )
         .arg(
             Arg::with_name("unstable-features")

src/cargo/util/command_prelude.rs

@@ -169,7 +169,7 @@ pub trait AppExt: Sized {
     }
 
     fn arg_unit_graph(self) -> Self {
-        self._arg(opt("unit-graph", "Output build graph in JSON (unstable)").hidden(true))
+        self._arg(opt("unit-graph", "Output build graph in JSON (unstable)"))
     }
 
     fn arg_new_opts(self) -> Self {
@@ -214,13 +214,10 @@ pub trait AppExt: Sized {
     }
 
     fn arg_ignore_rust_version(self) -> Self {
-        self._arg(
-            opt(
-                "ignore-rust-version",
-                "Ignore `rust-version` specification in packages",
-            )
-            .hidden(true), // nightly only (`rust-version` feature)
-        )
+        self._arg(opt(
+            "ignore-rust-version",
+            "Ignore `rust-version` specification in packages (unstable)",
+        ))
     }
 }