Utilize cfg(bootstrap) over cfg(stage0)

Also removes stage1, stage2 cfgs being passed to rustc to ensure that
stage1 and stage2 are only differentiated as a group (i.e., only through
not bootstrap).

Author: Mark-Simulacrum

src/bootstrap/bin/rustc.rs

@@ -89,12 +89,16 @@ fn main() {
 
     let mut cmd = Command::new(rustc);
     cmd.args(&args)
-        .arg("--cfg")
-        .arg(format!("stage{}", stage))
         .env(bootstrap::util::dylib_path_var(),
              env::join_paths(&dylib_path).unwrap());
     let mut maybe_crate = None;
 
+    // Non-zero stages must all be treated uniformly to avoid problems when attempting to uplift
+    // compiler libraries and such from stage 1 to 2.
+    if stage == "0" {
+        cmd.arg("--cfg").arg("bootstrap");
+    }
+
     // Print backtrace in case of ICE
     if env::var("RUSTC_BACKTRACE_ON_ICE").is_ok() && env::var("RUST_BACKTRACE").is_err() {
         cmd.env("RUST_BACKTRACE", "1");

src/bootstrap/compile.rs

@@ -341,7 +341,7 @@ impl Step for StartupObjects {
             if !up_to_date(src_file, dst_file) {
                 let mut cmd = Command::new(&builder.initial_rustc);
                 builder.run(cmd.env("RUSTC_BOOTSTRAP", "1")
-                            .arg("--cfg").arg("stage0")
+                            .arg("--cfg").arg("bootstrap")
                             .arg("--target").arg(target)
                             .arg("--emit=obj")
                             .arg("-o").arg(dst_file)

src/libcore/num/mod.rs

@@ -50,7 +50,7 @@ assert_eq!(size_of::<Option<core::num::", stringify!($Ty), ">>(), size_of::<", s
                 #[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
                 #[repr(transparent)]
                 #[rustc_layout_scalar_valid_range_start(1)]
-                #[cfg_attr(not(stage0), rustc_nonnull_optimization_guaranteed)]
+                #[cfg_attr(not(bootstrap), rustc_nonnull_optimization_guaranteed)]
                 pub struct $Ty($Int);
             }
 

src/libcore/ptr/non_null.rs

@@ -38,7 +38,7 @@ use crate::cmp::Ordering;
 #[stable(feature = "nonnull", since = "1.25.0")]
 #[repr(transparent)]
 #[rustc_layout_scalar_valid_range_start(1)]
-#[cfg_attr(not(stage0), rustc_nonnull_optimization_guaranteed)]
+#[cfg_attr(not(bootstrap), rustc_nonnull_optimization_guaranteed)]
 pub struct NonNull<T: ?Sized> {
     pointer: *const T,
 }