[binutils-gdb/gdb-8.0-branch] Avoid exponential behavior in rust_evaluate_subexp

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 69e9e8a0d5ae31e9869658771a7d399a53ac7833 ***

Author: Tom Tromey <tom@tromey.com>
Branch: gdb-8.0-branch
Commit: 69e9e8a0d5ae31e9869658771a7d399a53ac7833

Avoid exponential behavior in rust_evaluate_subexp

The STRUCTOP_STRUCT case in rust_evaluate_subexp would evaluate its
LHS, and then, if it did not need Rust-specific treatment, it would
back up and re-evaluate the entire STRUCTOP_STRUCT part of the
expression using evaluate_subexp_standard.  This yields exponential
behavior and causes some expressions to evaluate extremely slowly.

The fix is to simply do the needed work inline.

This is PR rust/21483.

2017-05-12  Tom Tromey  <tom@tromey.com>

	PR rust/21483:
	* rust-lang.c (rust_evaluate_subexp) <STRUCTOP_STRUCT>: Don't
	recurse, just call value_struct_elt directly.