fmt::Subscriber NO_COLOR support

tracing-subscriber/src/fmt/fmt_subscriber.rs

@@ -6,7 +6,7 @@ use crate::{
 };
 use format::{FmtSpan, TimingDisplay};
 use std::{
-    any::TypeId, cell::RefCell, fmt, io, marker::PhantomData, ops::Deref, ptr::NonNull,
+    any::TypeId, cell::RefCell, env, fmt, io, marker::PhantomData, ops::Deref, ptr::NonNull,
     time::Instant,
 };
 use tracing_core::{
@@ -680,12 +680,16 @@ impl<C, N, E, W> Subscriber<C, N, E, W> {
 
 impl<C> Default for Subscriber<C> {
     fn default() -> Self {
+        // only enable ANSI when the feature is enabled, and the NO_COLOR
+        // environment variable is unset or empty.
+        let ansi = cfg!(feature = "ansi") && env::var("NO_COLOR").map_or(true, |v| v.is_empty());
+
         Subscriber {
             fmt_fields: format::DefaultFields::default(),
             fmt_event: format::Format::default(),
             fmt_span: format::FmtSpanConfig::default(),
             make_writer: io::stdout,
-            is_ansi: cfg!(feature = "ansi"),
+            is_ansi: ansi,
             log_internal_errors: false,
             _inner: PhantomData,
         }
@@ -1505,4 +1509,60 @@ mod test {
             actual.as_str()
         );
     }
+
+    // Because we need to modify an environment variable for these test cases,
+    // we do them all in a single test.
+    #[cfg(feature = "ansi")]
+    #[test]
+    fn subscriber_no_color() {
+        // save off the existing value of NO_COLOR
+        const NO_COLOR: &str = "NO_COLOR";
+        let saved_no_color = std::env::var(NO_COLOR);
+
+        let cases: Vec<(Option<&str>, bool)> = vec![
+            (Some("0"), false),   // any non-empty value disables ansi
+            (Some("off"), false), // any non-empty value disables ansi
+            (Some("1"), false),
+            (Some(""), true), // empty value does not disable ansi
+            (None, true),
+        ];
+
+        for (var, ansi) in cases {
+            if let Some(value) = var {
+                env::set_var(NO_COLOR, value);
+            } else {
+                env::remove_var(NO_COLOR);
+            }
+
+            let subscriber: Subscriber<()> = fmt::Subscriber::default();
+            assert_eq!(
+                subscriber.is_ansi, ansi,
+                "NO_COLOR={:?}; Subscriber::default().is_ansi should be {}",
+                var, ansi
+            );
+
+            // with_ansi should override any `NO_COLOR` value
+            let subscriber: Subscriber<()> = fmt::Subscriber::default().with_ansi(true);
+            assert!(
+                subscriber.is_ansi,
+                "NO_COLOR={:?}; Subscriber::default().with_ansi(true).is_ansi should be true",
+                var
+            );
+
+            // set_ansi should override any `NO_COLOR` value
+            let mut subscriber: Subscriber<()> = fmt::Subscriber::default();
+            subscriber.set_ansi(true);
+            assert!(
+                subscriber.is_ansi,
+                "NO_COLOR={:?}; subscriber.set_ansi(true); subscriber.is_ansi should be true",
+                var
+            );
+        }
+
+        if let Ok(value) = saved_no_color {
+            env::set_var(NO_COLOR, value);
+        } else {
+            env::remove_var(NO_COLOR);
+        }
+    }
 }