Better diagnostics for people using nix subshell on non-NixOS.

1. Turned patch-binaries-for-nix from a boolean into a ternary flag: true,
   false, and unset.

2. When patch-binaries-for-nix is unset, we continue with the existing NixOS
   detection heuristic (look for nixos in /etc/os-release, if present), but if
   we are not atop NixOS, then issue a note if we see the IN_NIX_SHELL
   environment variable telling the user to consider setting
   patch-binaries-for-nix explicitly.

1 files changed, 19 insertions, 4 deletions

diff --git a/src/bootstrap/bootstrap.py b/src/bootstrap/bootstrap.py
index c71ecc0a139..588164e4a85 100644
--- a/src/bootstrap/bootstrap.py
+++ b/src/bootstrap/bootstrap.py
@@ -643,18 +643,33 @@ class RustBuild(object):
             if ostype != "Linux":
                 return False
 
-            # If the user has asked binaries to be patched for Nix, then
-            # don't check for NixOS.
+            # If the user has explicitly indicated whether binaries should be
+            # patched for Nix, then don't check for NixOS.
             if self.get_toml("patch-binaries-for-nix", "build") == "true":
                 return True
+            if self.get_toml("patch-binaries-for-nix", "build") == "false":
+                return False
+
+            # Assume we should fix until we see evidence that it is not NixOS
+            should_fix_retval = True
 
             # Use `/etc/os-release` instead of `/etc/NIXOS`.
             # The latter one does not exist on NixOS when using tmpfs as root.
             try:
                 with open("/etc/os-release", "r") as f:
-                    return any(ln.strip() in ("ID=nixos", "ID='nixos'", 'ID="nixos"') for ln in f)
+                    should_fix_retval = any(ln.strip() in ("ID=nixos", "ID='nixos'", 'ID="nixos"') for ln in f)
             except FileNotFoundError:
-                return False
+                should_fix_retval = False
+
+            # If not on NixOS, then warn if user seems to be atop Nix shell
+            if not should_fix_retval:
+                in_nix_shell = os.getenv('IN_NIX_SHELL')
+                if in_nix_shell:
+                    print("The IN_NIX_SHELL environment variable is set to `{}`;".format(in_nix_shell),
+                          "you may need to set `patch-binaries-for-nix=true` in your config.toml",
+                          file=sys.stderr)
+
+            return should_fix_retval
 
         answer = self._should_fix_bins_and_dylibs = get_answer()
         if answer: