Improve error for nightly-only '--message-format' args

Previously on a stable build (`json` is only supported in `nightly`
builds):

    $ cargo fmt --message-format json
    Invalid value for `--emit` - using an unstable value without `--unstable-features`

With this change:

    $ CFG_RELEASE_CHANNEL=stable cargo make bin-build
    $ PATH="./target/debug:$PATH" cargo fmt --message-format json
    --message-format json is only supported in nightly builds
    This utility formats all bin and lib files of the current crate using rustfmt.
    <-- SNIP: the rest of the usage follows -->

This behaviour is similar to how args to `--emit` are handled in
`rustfmt`.

This change also drops displaying `json` as an option to
`--message-format` on stable builds:

    $ PATH="./target/debug:$PATH" cargo fmt --help | grep --max-count 1 --after-context 1 message-format
        --message-format <message-format>
            Specify message-format: short|human

This required rewriting the check for nightly builds (copying from the
one in `src/release_channel.rs`) to be `const` so I could rewrite the
`--message-format` flag's help string.

Issue: #6752

Author: matthewhughes934

src/cargo-fmt/main.rs

@@ -20,6 +20,19 @@ use clap::{CommandFactory, Parser};
 #[cfg(test)]
 mod cargo_fmt_tests;
 
+const fn is_nightly() -> bool {
+    match option_env!("CFG_RELEASE_CHANNEL") {
+        None => true,
+        Some(c) => matches!(c.as_bytes(), b"nightly" | b"dev"),
+    }
+}
+
+const MESSAGE_FORMATS: &str = if is_nightly() {
+    "short|json|human"
+} else {
+    "short|human"
+};
+
 #[derive(Parser)]
 #[command(
     disable_version_flag = true,
@@ -54,8 +67,11 @@ pub struct Opts {
     #[arg(long = "manifest-path", value_name = "manifest-path")]
     manifest_path: Option<String>,
 
-    /// Specify message-format: short|json|human
-    #[arg(long = "message-format", value_name = "message-format")]
+    #[arg(
+        long = "message-format",
+        value_name = "message-format",
+        help = format!("Specify message-format: {MESSAGE_FORMATS}")
+    )]
     message_format: Option<String>,
 
     /// Options passed to rustfmt
@@ -186,6 +202,11 @@ fn convert_message_format_to_rustfmt_args(
             Ok(())
         }
         "json" => {
+            if !is_nightly() {
+                return Err(String::from(
+                    "--message-format json is only supported in nightly builds",
+                ));
+            }
             if contains_emit_mode {
                 return Err(String::from(
                     "cannot include --emit arg when --message-format is set to json",
@@ -202,7 +223,8 @@ fn convert_message_format_to_rustfmt_args(
         }
         "human" => Ok(()),
         _ => Err(format!(
-            "invalid --message-format value: {message_format}. Allowed values are: short|json|human"
+            "invalid --message-format value: {message_format}. Allowed values are: \
+                {MESSAGE_FORMATS}"
         )),
     }
 }

src/cargo-fmt/test/message_format.rs

@@ -1,7 +1,10 @@
 use super::*;
 
+use rustfmt_config_proc_macro::{nightly_only_test, stable_only_test};
+
+#[nightly_only_test]
 #[test]
-fn invalid_message_format() {
+fn invalid_message_format_nightly() {
     assert_eq!(
         convert_message_format_to_rustfmt_args("awesome", &mut vec![]),
         Err(String::from(
@@ -10,6 +13,18 @@ fn invalid_message_format() {
     );
 }
 
+#[stable_only_test]
+#[test]
+fn invalid_message_format_stable() {
+    assert_eq!(
+        convert_message_format_to_rustfmt_args("awesome", &mut vec![]),
+        Err(String::from(
+            "invalid --message-format value: awesome. Allowed values are: short|human"
+        )),
+    );
+}
+
+#[nightly_only_test]
 #[test]
 fn json_message_format_and_check_arg() {
     let mut args = vec![String::from("--check")];
@@ -21,6 +36,7 @@ fn json_message_format_and_check_arg() {
     );
 }
 
+#[nightly_only_test]
 #[test]
 fn json_message_format_and_emit_arg() {
     let mut args = vec![String::from("--emit"), String::from("checkstyle")];
@@ -32,6 +48,18 @@ fn json_message_format_and_emit_arg() {
     );
 }
 
+#[stable_only_test]
+#[test]
+fn json_message_format_non_nightly() {
+    assert_eq!(
+        convert_message_format_to_rustfmt_args("json", &mut vec![]),
+        Err(String::from(
+            "--message-format json is only supported in nightly builds"
+        )),
+    );
+}
+
+#[nightly_only_test]
 #[test]
 fn json_message_format() {
     let mut args = vec![String::from("--edition"), String::from("2018")];