Properly detect MinTTY when running GHCi on Windows

Before, we detecting the presence of MinTTY on Windows in a very
imprecise way: by checking if the `_` environment variable was set. Not
only is this easy to circumvent, but it also yields false positives on
terminals like ConEmu.

This changes the test to use the `GetFileInformationByHandleEx` function
instead, which provides a far more accurate check for MinTTY's presence.
I've tested this on PowerShell, MSYS2, Cygwin, ConEmu, and Git Bash, and
it does the right thing on each one.

Fixes #12958.

Test Plan: Run GHCi on many different Windows and MinTTY consoles

Reviewers: erikd, Phyx, austin, bgamari

Reviewed By: Phyx, bgamari

Subscribers: thomie, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D2878

GHC Trac Issues: #12958

1 files changed, 8 insertions, 0 deletions

diff --git a/driver/utils/isMinTTY.h b/driver/utils/isMinTTY.h
new file mode 100644
index 0000000000..22e9a473cc
--- /dev/null
+++ b/driver/utils/isMinTTY.h
@@ -0,0 +1,8 @@
+#ifndef ISMINTTY_H
+#define ISMINTTY_H
+
+#include <stdbool.h>
+
+bool isMinTTY();
+
+#endif /* ISMINTTY_H */