follow neovim's truecolor detection (#9577)

Cargo.lock

@@ -1387,6 +1387,7 @@ dependencies = [
  "signal-hook-tokio",
  "smallvec",
  "tempfile",
+ "termini",
  "tokio",
  "tokio-stream",
  "toml",

helix-term/Cargo.toml

@@ -42,6 +42,7 @@ signal-hook = "0.3"
 tokio-stream = "0.1"
 futures-util = { version = "0.3", features = ["std", "async-await"], default-features = false }
 arc-swap = { version = "1.6.0" }
+termini = "1"
 
 # Logging
 fern = "0.6"

helix-term/src/lib.rs

@@ -22,17 +22,30 @@ use url::Url;
 
 pub use keymap::macros::*;
 
-#[cfg(not(windows))]
-fn true_color() -> bool {
-    std::env::var("COLORTERM")
-        .map(|v| matches!(v.as_str(), "truecolor" | "24bit"))
-        .unwrap_or(false)
-}
 #[cfg(windows)]
 fn true_color() -> bool {
     true
 }
 
+#[cfg(not(windows))]
+fn true_color() -> bool {
+    if matches!(
+        std::env::var("COLORTERM").map(|v| matches!(v.as_str(), "truecolor" | "24bit")),
+        Ok(true)
+    ) {
+        return true;
+    }
+
+    match termini::TermInfo::from_env() {
+        Ok(t) => {
+            t.extended_cap("RGB").is_some()
+                || t.extended_cap("Tc").is_some()
+                || (t.extended_cap("setrgbf").is_some() && t.extended_cap("setrgbb").is_some())
+        }
+        Err(_) => false,
+    }
+}
+
 /// Function used for filtering dir entries in the various file pickers.
 fn filter_picker_entry(entry: &DirEntry, root: &Path, dedup_symlinks: bool) -> bool {
     // We always want to ignore the .git directory, otherwise if