improve detection of terminals over SSH

A recurring issue with terminals over SSH is that while TERM is passed in,
COLORTERM is not by default. This means that programs running on the server
only have access to TERM.

If TERM is something like `xterm-256color` or `konsole-direct` this works fine,
but if it is something like `wezterm` or `alacritty` it doesn't, so auto
detection falls back to assuming colors aren't supported at all. This creates a
pretty bad experience.

I had a look at what some other crates do and it looks like anstyle (which is
used by Cargo itself) just checks that the terminal isn't `dumb` (or `cygwin`
on Windows). However I believe the latest version of anstyle also has a bug,
that it thinks that if TERM isn't set on Windows then ANSI colors aren't
supported. (That is fine for anstream, I believe, since that knows how to fall
back to Windows console calls.)

src/lib.rs

@@ -107,8 +107,7 @@ fn supports_color(stream: Stream) -> usize {
     {
         2
     } else if env::var("COLORTERM").is_ok()
-        || env::var("TERM").map(|term| check_ansi_color(&term)) == Ok(true)
-        || env::consts::OS == "windows"
+        || check_ansi_color(env::var("TERM").ok().as_deref())
         || env::var("CLICOLOR").map_or(false, |v| v != "0")
         || is_ci::uncached()
     {
@@ -118,16 +117,27 @@ fn supports_color(stream: Stream) -> usize {
     }
 }
 
-fn check_ansi_color(term: &str) -> bool {
-    term.starts_with("screen")
-        || term.starts_with("xterm")
-        || term.starts_with("vt100")
-        || term.starts_with("vt220")
-        || term.starts_with("rxvt")
-        || term.contains("color")
-        || term.contains("ansi")
-        || term.contains("cygwin")
-        || term.contains("linux")
+#[cfg(windows)]
+fn check_ansi_color(term: Option<&str>) -> bool {
+    if let Some(term) = term {
+        // cygwin doesn't seem to support ANSI escape sequences and instead has its own variety.
+        term != "dumb" && term != "cygwin"
+    } else {
+        // TERM is generally not set on Windows. It's reasonable to assume that all Windows
+        // terminals support ANSI escape sequences (since Windows 10 version 1511).
+        true
+    }
+}
+
+#[cfg(not(windows))]
+fn check_ansi_color(term: Option<&str>) -> bool {
+    if let Some(term) = term {
+        // dumb terminals don't support ANSI escape sequences.
+        term != "dumb"
+    } else {
+        // TERM is not set, which is really weird on Unix systems.
+        false
+    }
 }
 
 fn check_colorterm_16m(colorterm: &str) -> bool {