Allow both dashes and underscores for Z flags

This makes it a little easier to match whatever
the natural formatting is for where you're setting
unstable options -- CLI or config file.

src/cargo/core/features.rs

@@ -429,7 +429,9 @@ impl CliUnstable {
             Ok(true)
         };
 
-        match k {
+        // Permit dashes or underscores in parsing these
+        let normalized_key = k.replace("_", "-");
+        match normalized_key.as_str() {
             "print-im-a-teapot" => self.print_im_a_teapot = parse_bool(k, v)?,
             "unstable-options" => self.unstable_options = parse_empty(k, v)?,
             "no-index-update" => self.no_index_update = parse_empty(k, v)?,

src/doc/src/reference/unstable.md

@@ -2,7 +2,8 @@
 
 Experimental Cargo features are only available on the nightly channel. You
 typically use one of the `-Z` flags to enable them. Run `cargo -Z help` to
-see a list of flags available.
+see a list of flags available. All Z flags accept either dashes or underscores
+as separators.
 
 `-Z unstable-options` is a generic flag for enabling other unstable
 command-line flags. Options requiring this will be called out below.

tests/testsuite/config_cli.rs

@@ -267,6 +267,26 @@ fn rerooted_remains() {
     assert_eq!(config.get::<String>("c").unwrap(), "cli2");
 }
 
+#[cargo_test]
+fn unstable_dash_underscore_interchangable() {
+    // Confirm unstable flag parsing treats underscores and dashes
+    // interchangably when coming from the CLI.
+    let config = ConfigBuilder::new()
+        .unstable_flag("print_im_a_teapot")
+        .build();
+    assert_eq!(config.cli_unstable().print_im_a_teapot, true);
+
+    let config = ConfigBuilder::new()
+        .unstable_flag("print-im-a-teapot")
+        .build();
+    assert_eq!(config.cli_unstable().print_im_a_teapot, true);
+
+    let config = ConfigBuilder::new()
+        .unstable_flag("print_im-a_teapot")
+        .build();
+    assert_eq!(config.cli_unstable().print_im_a_teapot, true);
+}
+
 #[cargo_test]
 fn bad_parse() {
     // Fail to TOML parse.