termwiz: support NO_COLOR environment variable (#5020)

* termwiz: support NO_COLOR environment variable

* style: update formatting

* refactor: use capabilities for enabling no-color

termwiz/src/caps/mod.rs

@@ -61,6 +61,10 @@ use terminfo::{self, capability as cap};
 
 pub mod probed;
 
+/// Environment variable name indicating that color output should be disabled.
+/// See <https://no-color.org>
+const NO_COLOR_ENV: &str = "NO_COLOR";
+
 builder! {
     /// Use the `ProbeHints` to configure an instance of
     /// the `ProbeHints` struct.  `ProbeHints` are passed to the `Capabilities`
@@ -121,12 +125,21 @@ builder! {
 
 impl ProbeHints {
     pub fn new_from_env() -> Self {
-        ProbeHints::default()
+        let mut probe_hints = ProbeHints::default()
             .term(var("TERM").ok())
             .colorterm(var("COLORTERM").ok())
             .colorterm_bce(var("COLORTERM_BCE").ok())
             .term_program(var("TERM_PROGRAM").ok())
-            .term_program_version(var("TERM_PROGRAM_VERSION").ok())
+            .term_program_version(var("TERM_PROGRAM_VERSION").ok());
+
+        if !std::env::var(NO_COLOR_ENV)
+            .unwrap_or("".to_string())
+            .is_empty()
+        {
+            probe_hints.color_level = Some(ColorLevel::MonoChrome);
+        }
+
+        probe_hints
     }
 }
 
@@ -146,6 +159,9 @@ pub enum ColorLevel {
     /// What we care about here is whether the terminal supports the escape
     /// sequence to specify RGB values rather than a palette index.
     TrueColor,
+    /// Describes monochrome (black and white) color support.
+    /// Enabled via NO_COLOR environment variable.
+    MonoChrome,
 }
 
 /// `Capabilities` holds information about the capabilities of a terminal.

termwiz/src/render/terminfo.rs

@@ -149,7 +149,9 @@ impl TerminfoRenderer {
                 None => 0,
             };
 
-            if attr.foreground() != current_foreground {
+            if attr.foreground() != current_foreground
+                && self.caps.color_level() != ColorLevel::MonoChrome
+            {
                 match (has_true_color, attr.foreground()) {
                     (true, ColorAttribute::TrueColorWithPaletteFallback(tc, _))
                     | (true, ColorAttribute::TrueColorWithDefaultFallback(tc)) => {
@@ -183,7 +185,9 @@ impl TerminfoRenderer {
                 }
             }
 
-            if attr.background() != current_background {
+            if attr.background() != current_background
+                && self.caps.color_level() != ColorLevel::MonoChrome
+            {
                 match (has_true_color, attr.background()) {
                     (true, ColorAttribute::TrueColorWithPaletteFallback(tc, _))
                     | (true, ColorAttribute::TrueColorWithDefaultFallback(tc)) => {

termwiz/src/render/windows.rs

@@ -1,6 +1,7 @@
 //! A Renderer for windows consoles
 
 use crate::caps::Capabilities;
+use crate::caps::ColorLevel;
 use crate::cell::{AttributeChange, CellAttributes, Underline};
 use crate::color::{AnsiColor, ColorAttribute};
 use crate::surface::{Change, Position};
@@ -17,12 +18,14 @@ use winapi::um::wincon::{
 
 pub struct WindowsConsoleRenderer {
     pending_attr: CellAttributes,
+    capabilities: Capabilities,
 }
 
 impl WindowsConsoleRenderer {
-    pub fn new(_caps: Capabilities) -> Self {
+    pub fn new(capabilities: Capabilities) -> Self {
         Self {
             pending_attr: CellAttributes::default(),
+            capabilities,
         }
     }
 }
@@ -69,6 +72,21 @@ fn to_attr_word(attr: &CellAttributes) -> u16 {
         };
     }
 
+    let reverse = if attr.reverse() {
+        COMMON_LVB_REVERSE_VIDEO
+    } else {
+        0
+    };
+    let underline = if attr.underline() != Underline::None {
+        COMMON_LVB_UNDERSCORE
+    } else {
+        0
+    };
+
+    if attr.capabilities.color_level() == ColorLevel::MonoChrome {
+        return reverse | underline;
+    }
+
     let fg = match attr.foreground() {
         ColorAttribute::TrueColorWithDefaultFallback(_) | ColorAttribute::Default => {
             FOREGROUND_BLUE | FOREGROUND_RED | FOREGROUND_GREEN
@@ -98,17 +116,6 @@ fn to_attr_word(attr: &CellAttributes) -> u16 {
         ),
     };
 
-    let reverse = if attr.reverse() {
-        COMMON_LVB_REVERSE_VIDEO
-    } else {
-        0
-    };
-    let underline = if attr.underline() != Underline::None {
-        COMMON_LVB_UNDERSCORE
-    } else {
-        0
-    };
-
     bg | fg | reverse | underline
 }