rust: Mention that RUST_OPT_LEVEL_0/1 can cause excessive stack usage

At least on ppc64le, RUST_OPT_LEVEL_0/1 causes some functions to be
generated that use up to 20KB of stack. This leads to stack overflow,
which may exhibit as a panic:

  Kernel panic - not syncing: corrupted stack end detected inside scheduler
  CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.12.0-rc3+ #1

But may also cause other crashes, depending on what is overwritten by
the overflow.

Add a note to the Kconfig for RUST_OPT_LEVEL_0/1 that this can happen.

Signed-off-by: Michael Ellerman <[email protected]>

Author: mpe

lib/Kconfig.debug

@@ -2607,12 +2607,18 @@ config RUST_OPT_LEVEL_0
 	  directly). Use the corresponding configuration options to control
 	  that instead, orthogonally.
 
+	  Note this level may cause excessive stack usage, which can lead to stack
+	  overflow and subsequent crashes.
+
 config RUST_OPT_LEVEL_1
 	bool "Basic optimizations (-Copt-level=1)"
 	help
 	  Useful for debugging without getting too lost, but without
 	  the overhead and boilerplate of no optimizations at all.
 
+	  Note this level may cause excessive stack usage, which can lead to stack
+	  overflow and subsequent crashes.
+
 config RUST_OPT_LEVEL_2
 	bool "Some optimizations (-Copt-level=2)"
 	help