Disable colors if NO_COLOR env var set (#10560)

Changes

@@ -42,6 +42,11 @@ Working version
 - #10524: Directive argument type error now shows expected and received type.
   (Wiktor Kuchta, review by Gabriel Scherer)
 
+- #10560: Disable colors if the env variable `NO_COLOR` is set.  If
+  `OCAML_COLOR` is set, its setting takes precedence over `NO_COLOR`.
+  (Nicolás Ojeda Bär, report by Gabriel Scherer, review by Daniel Bünzli,
+  Gabriel Scherer and David Allsopp)
+
 ### Manual and documentation:
 
 - #7812, #10475: reworded the description of the behaviors of

driver/compmisc.ml

@@ -68,6 +68,11 @@ let set_from_env flag Clflags.{ parse; usage; env_var } =
 
 let read_clflags_from_env () =
   set_from_env Clflags.color Clflags.color_reader;
+  if
+    Option.is_none !Clflags.color &&
+    Option.is_some (Sys.getenv_opt "NO_COLOR")
+  then
+    Clflags.color := Some Misc.Color.Never;
   set_from_env Clflags.error_style Clflags.error_style_reader;
   ()
 

man/ocaml.m

@@ -254,15 +254,17 @@ use heuristics to enable colors only if the output supports them (an
 .B never
 disable color output.
 
-The default setting is
+The environment variable "OCAML_COLOR" is considered if \-color is not
+provided. Its values are auto/always/never as above.
+
+If \-color is not provided, "OCAML_COLOR" is not set and the environment
+variable "NO_COLOR" is set, then color output is disabled. Otherwise,
+the default setting is
 .B auto,
 and the current heuristic
 checks that the "TERM" environment variable exists and is
 not empty or "dumb", and that isatty(stderr) holds.
 
-The environment variable "OCAML_COLOR" is considered if \-color is not
-provided. Its values are auto/always/never as above.
-
 .TP
 .BI \-error\-style \ mode
 Control the way error messages and warnings are printed.

man/ocamlc.m

@@ -268,15 +268,17 @@ use heuristics to enable colors only if the output supports them (an
 .B never
 disable color output.
 
-The default setting is
+The environment variable "OCAML_COLOR" is considered if \-color is not
+provided. Its values are auto/always/never as above.
+
+If \-color is not provided, "OCAML_COLOR" is not set and the environment
+variable "NO_COLOR" is set, then color output is disabled. Otherwise,
+the default setting is
 .B auto,
 and the current heuristic
 checks that the "TERM" environment variable exists and is
 not empty or "dumb", and that isatty(stderr) holds.
 
-The environment variable "OCAML_COLOR" is considered if \-color is not
-provided. Its values are auto/always/never as above.
-
 .TP
 .BI \-error\-style \ mode
 Control the way error messages and warnings are printed.

man/ocamlopt.m

@@ -224,15 +224,17 @@ use heuristics to enable colors only if the output supports them (an
 .B never
 disable color output.
 
-The default setting is
+The environment variable "OCAML_COLOR" is considered if \-color is not
+provided. Its values are auto/always/never as above.
+
+If \-color is not provided, "OCAML_COLOR" is not set and the environment
+variable "NO_COLOR" is set, then color output is disabled. Otherwise,
+the default setting is
 .B auto,
 and the current heuristic
 checks that the "TERM" environment variable exists and is
 not empty or "dumb", and that isatty(stderr) holds.
 
-The environment variable "OCAML_COLOR" is considered if \-color is not
-provided. Its values are auto/always/never as above.
-
 .TP
 .BI \-error\-style \ mode
 Control the way error messages and warnings are printed.

manual/src/cmds/unified-options.etex

@@ -126,12 +126,15 @@ The following modes are supported:
   \item["always"] enable colors unconditionally;
   \item["never"] disable color output.
 \end{description}
-The default setting is 'auto', and the current heuristic
-checks that the "TERM" environment variable exists and is
-not empty or "dumb", and that 'isatty(stderr)' holds.
 
 The environment variable "OCAML_COLOR" is considered if "-color" is not
 provided. Its values are auto/always/never as above.
+
+If "-color" is not provided, "OCAML_COLOR" is not set and the environment
+variable "NO_COLOR" is set, then color output is disabled. Otherwise,
+the default setting is 'auto', and the current heuristic
+checks that the "TERM" environment variable exists and is
+not empty or "dumb", and that 'isatty(stderr)' holds.
 }%notop
 
 \notop{%