Allow RUSTC_PROGRESS=0 to disable progress bars

Author: oli-obk

compiler/rustc_session/src/session.rs

@@ -1511,7 +1511,10 @@ pub fn build_session(
     let asm_arch =
         if target_cfg.allow_asm { InlineAsmArch::from_str(&target_cfg.arch).ok() } else { None };
 
-    let progress_bars = std::env::var_os("RUSTC_PROGRESS").map(|_| Session::init_progress_bars());
+    let progress_bars = match std::env::var_os("RUSTC_PROGRESS") {
+        Some(val) if val != "0" => Some(Session::init_progress_bars()),
+        _ => None,
+    };
 
     let sess = Session {
         target: target_cfg,