Document test.

Author: cjgillot

tests/ui/mir/static_fnptr.rs

@@ -1,3 +1,10 @@
+//! Verify that we correctly handle fn pointer provenance in MIR optimizations.
+//! By asking to inline `static_fnptr::bar`, we have two copies of `static_fnptr::foo`, one in the
+//! auxiliary crate and one in the local crate CGU.
+//! `baz` must only consider the versions from upstream crate, and not try to compare with the
+//! address of the CGU-local copy.
+//! Related issue: #123670
+
 //@ run-pass
 //@ compile-flags:-Cno-prepopulate-passes -Copt-level=0
 //@ aux-build:static_fnptr.rs