Fix GenericOS.getenv returning a raw str, return an Option.t[str] instead.

src/comp/front/extenv.rs

@@ -23,9 +23,18 @@ fn expand_syntax_ext(parser.parser p,
         p.err("malformed #env call");
     }
 
+    // FIXME: if this was more thorough it would manufacture an
+    // Option.t[str] rather than just an maybe-empty string.
+
     auto var = expr_to_str(p, args.(0));
-    auto val = GenericOS.getenv(var);
-    ret make_new_str(sp, val);
+    alt (GenericOS.getenv(var)) {
+        case (Option.none[str]) {
+            ret make_new_str(sp, "");
+        }
+        case (Option.some[str](?s)) {
+            ret make_new_str(sp, s);
+        }
+    }
 }
 
 // FIXME: duplicate code copied from extfmt.

src/lib/GenericOS.rs

@@ -1,4 +1,9 @@
-fn getenv(str n) -> str {
-    ret Str.str_from_cstr(OS.libc.getenv(Str.buf(n)));
+fn getenv(str n) -> Option.t[str] {
+  auto s = OS.libc.getenv(Str.buf(n));
+  if (s == 0 as Str.sbuf) {
+    ret Option.none[str];
+  } else {
+    ret Option.some[str](Str.str_from_cstr(s));
+  }
 }
 

src/lib/Term.rs

@@ -31,7 +31,7 @@ fn reset(IO.buf_writer writer) {
 }
 
 fn color_supported() -> bool {
-    ret Str.eq(GenericOS.getenv("TERM"), "xterm-color");
+    ret GenericOS.getenv("TERM") == Option.some[str]("xterm-color");
 }
 
 fn set_color(IO.buf_writer writer, u8 first_char, u8 color) {