Allow --unstable to be set with an environment variable

Allow the environment variable JUST_ALLOW_UNSTABLE=true to act as
equivalent to the --unstable flag. Any other value of
JUST_ALLOW_UNSTABLE is treated as false.

README.md

@@ -319,7 +319,9 @@ This does not, however, preclude fixing outright bugs, even if doing so might br
 
 There will never be a `just` 2.0. Any desirable backwards-incompatible changes will be opt-in on a per-`justfile` basis, so users may migrate at their leisure.
 
-Features that aren't yet ready for stabilization are gated behind the `--unstable` flag. Features enabled by `--unstable` may change in backwards incompatible ways at any time.
+Features that aren't yet ready for stabilization are gated behind the `--unstable` flag. Features enabled by `--unstable` may change in backwards incompatible ways at any time. The unstable
+mode can also be set using the environment variable `JUST_ALLOW_UNSTABLE=true` (any other value for the variable besides `true` is treated as false).
+
 
 Editor Support
 --------------

src/config.rs

@@ -3,6 +3,8 @@ use {
   clap::{App, AppSettings, Arg, ArgGroup, ArgMatches, ArgSettings},
 };
 
+pub(crate) const UNSTABLE_KEY: &str = "JUST_ALLOW_UNSTABLE";
+
 // These three strings should be kept in sync:
 pub(crate) const CHOOSER_DEFAULT: &str = "fzf --preview 'just --show {}'";
 pub(crate) const CHOOSER_ENVIRONMENT_KEY: &str = "JUST_CHOOSER";
@@ -568,6 +570,9 @@ impl Config {
       None
     };
 
+    let unstable = matches.is_present(arg::UNSTABLE)
+      || std::env::var_os(UNSTABLE_KEY).map_or(false, |val| val.to_string_lossy() == "true");
+
     Ok(Self {
       check: matches.is_present(arg::CHECK),
       dry_run: matches.is_present(arg::DRY_RUN),
@@ -577,7 +582,7 @@ impl Config {
       load_dotenv: !matches.is_present(arg::NO_DOTENV),
       shell_command: matches.is_present(arg::SHELL_COMMAND),
       unsorted: matches.is_present(arg::UNSORTED),
-      unstable: matches.is_present(arg::UNSTABLE),
+      unstable,
       list_heading: matches
         .value_of(arg::LIST_HEADING)
         .unwrap_or("Available recipes:\n")

tests/lib.rs

@@ -86,6 +86,7 @@ mod string;
 mod subsequents;
 mod tempdir;
 mod undefined_variables;
+mod unstable;
 #[cfg(target_family = "windows")]
 mod windows_shell;
 mod working_directory;

tests/test.rs

@@ -234,7 +234,7 @@ impl Test {
 
     if let Some(ref stdout_regex) = self.stdout_regex {
       if !stdout_regex.is_match(output_stdout) {
-        panic!("Stdout regex mismatch:\n{output_stderr:?}\n!~=\n/{stdout_regex:?}/");
+        panic!("Stdout regex mismatch:\n{output_stdout:?}\n!~=\n/{stdout_regex:?}/");
       }
     }
 

tests/unstable.rs

@@ -0,0 +1,31 @@
+use super::*;
+
+#[test]
+fn set_unstable_true_with_env_var() {
+  let justfile = r#"
+default:
+    echo 'foo'
+  "#;
+  Test::new()
+    .justfile(justfile)
+    .args(["--dump", "--dump-format", "json"])
+    .env("JUST_ALLOW_UNSTABLE", "true")
+    .status(EXIT_SUCCESS)
+    .stdout_regex("*")
+    .run();
+}
+
+#[test]
+fn set_unstable_false_with_env_var() {
+  let justfile = r#"
+default:
+    echo 'foo'
+  "#;
+  Test::new()
+    .justfile(justfile)
+    .args(["--dump", "--dump-format", "json"])
+    .env("JUST_ALLOW_UNSTABLE", "false")
+    .status(EXIT_FAILURE)
+    .stderr("error: The JSON dump format is currently unstable. Invoke `just` with the `--unstable` flag to enable unstable features.\n")
+    .run();
+}