From 3223d3a1ac0aa7a5ffb9252c2e8d871f0bf70ec0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Carl=20=C3=85stholm?= <carl@astholm.se>
Date: Mon, 2 Feb 2026 01:24:39 +0100
Subject: [PATCH] Improve debug I/O color detection/handling for WASI and
 Emscripten

When linking libc, these targets can get their environment variables
from `std.c.environ`. Additionally, it's okay for WASI to use ANSI
escape sequences; nothing in the relevant specs claim otherwise.
---
 lib/std/Io/Threaded.zig     |  7 -------
 lib/std/process/Environ.zig |  2 +-
 lib/std/start.zig           | 18 +++++++++++++++---
 3 files changed, 16 insertions(+), 11 deletions(-)

diff --git a/lib/std/Io/Threaded.zig b/lib/std/Io/Threaded.zig
index 2fcd4551104797f08b75d8ababb7833455d01590..937f75a531527f99c7b1967c66466f63d844b9b8 100644
--- a/lib/std/Io/Threaded.zig
+++ b/lib/std/Io/Threaded.zig
@@ -8729,13 +8729,6 @@ fn supportsAnsiEscapeCodes(file: File) Io.Cancelable!bool {
         }
     }
 
-    if (native_os == .wasi) {
-        // WASI sanitizes stdout when fd is a tty so ANSI escape codes will not
-        // be interpreted as actual cursor commands, and stderr is always
-        // sanitized.
-        return false;
-    }
-
     if (try isTty(file)) return true;
 
     return false;
diff --git a/lib/std/process/Environ.zig b/lib/std/process/Environ.zig
index d9ef7aec2ddf7205056b7c3ede163d7c309ae03c..57d5ea02711885f583f60c65fd26adfb3712f11b 100644
--- a/lib/std/process/Environ.zig
+++ b/lib/std/process/Environ.zig
@@ -24,7 +24,7 @@ pub const empty: Environ = .{ .block = .empty };
 /// operating system `void` is also used.
 pub const Block = switch (native_os) {
     .windows => GlobalBlock,
-    .wasi => switch (builtin.link_libc) {
+    .wasi, .emscripten => switch (builtin.link_libc) {
         false => GlobalBlock,
         true => PosixBlock,
     },
diff --git a/lib/std/start.zig b/lib/std/start.zig
index f6b4064ebdf258330870d30020e78e02a9d4be01..873504273844da12a23ddfa5d4ddd1e84342a66e 100644
--- a/lib/std/start.zig
+++ b/lib/std/start.zig
@@ -664,10 +664,22 @@ fn main(c_argc: c_int, c_argv: [*][*:0]c_char, c_envp: [*:null]?[*:0]c_char) cal
 
 fn mainWithoutEnv(c_argc: c_int, c_argv: [*][*:0]c_char) callconv(.c) c_int {
     const argv = @as([*][*:0]u8, @ptrCast(c_argv))[0..@intCast(c_argc)];
-    if (@sizeOf(std.Io.Threaded.Argv0) != 0) {
-        if (std.Options.debug_threaded_io) |t| t.argv0.value = argv[0];
+    const environ: [:null]?[*:0]u8 = switch (builtin.os.tag) {
+        .wasi, .emscripten => environ: {
+            const c_environ = std.c.environ;
+            var env_count: usize = 0;
+            while (c_environ[env_count] != null) : (env_count += 1) {}
+            break :environ c_environ[0..env_count :null];
+        },
+        else => &.{},
+    };
+    const env_block: std.process.Environ.Block = .{ .slice = environ };
+    if (std.Options.debug_threaded_io) |t| {
+        if (@sizeOf(std.Io.Threaded.Argv0) != 0) t.argv0.value = argv[0];
+        t.environ = .{ .process_environ = .{ .block = env_block } };
+        t.environ_initialized = env_block.isEmpty();
     }
-    return callMain(argv, .empty);
+    return callMain(argv, env_block);
 }
 
 /// General error message for a malformed return type
-- 
2.54.0