Support RUSTUP_TERM_COLOR as an override environment variable

Author: rami3l

doc/src/environment-variables.md

@@ -29,6 +29,12 @@
   determines the directory that traces will be written too. Traces are of the
   form PID.trace. Traces can be read by the Catapult project [tracing viewer].
 
+- `RUSTUP_TERM_COLOR` (default: `auto` in TTY, `never` otherwise) Controls whether colored output is used in the terminal. Possible values:
+
+  - `auto`: Automatically detect if color support is available on the terminal.
+  - `always`: Always display colors.
+  - `never`: Never display colors.
+
 - `RUSTUP_UNPACK_RAM` *unstable* (default free memory or 500MiB if unable to tell, min 210MiB) Caps the amount of
   RAM `rustup` will use for IO tasks while unpacking.
 

src/currentprocess/terminalsource.rs

@@ -9,9 +9,10 @@ use std::{
 pub(crate) use termcolor::Color;
 use termcolor::{ColorChoice, ColorSpec, StandardStream, StandardStreamLock, WriteColor};
 
+use super::{process, varsource::VarSource};
+
 #[cfg(feature = "test")]
 use super::filesource::{TestWriter, TestWriterLock};
-use super::process;
 
 /// Select what stream to make a terminal on
 pub(super) enum StreamSelector {
@@ -73,14 +74,17 @@ enum TerminalInnerLocked {
 }
 
 impl ColorableTerminal {
-    /// Construct a terminal for the selected stream. Colors written to the
-    /// terminal will be discarded if the stream is not a tty.
+    /// Construct a terminal for the selected stream.
+    /// Colors written to the terminal will be discarded if the stream is not a tty,
+    /// unless the `RUSTUP_TERM_COLOR` override environment variable is set.
     pub(super) fn new(stream: StreamSelector) -> Self {
-        let is_a_tty = stream.is_a_tty();
-        let choice = if is_a_tty {
-            ColorChoice::Auto
-        } else {
-            ColorChoice::Never
+        let env_override = process().var("RUSTUP_TERM_COLOR").ok();
+        let choice = match env_override.as_deref() {
+            Some("always") => ColorChoice::Always,
+            Some("auto") => ColorChoice::Auto,
+            Some("never") => ColorChoice::Never,
+            _ if stream.is_a_tty() => ColorChoice::Auto,
+            _ => ColorChoice::Never,
         };
         let inner = match stream {
             StreamSelector::Stdout => {