Fix crash in inside_main_func

gdb 13.1 crashes while running the rust compiler's debugger tests.
The crash has a number of causes.

First, the rust compiler still uses the C++-like _Z mangling, but with
its own twist -- some hex digits added to the end of a symbol.  So,
while gdb finds the correct name of "main":

(top-gdb) p name
$13 = 0x292e0c0 "rustc_gdb_1031745::main"

It isn't found in the minsyms, because C++ demangling yields:

[99] t 0x90c0 _ZN17rustc_gdb_10317454main17h5b5be7fe16a97225E section .text  rustc_gdb_1031745::main::h5b5be7fe16a97225  zko06yobckx336v

This could perhaps be fixed.  I also filed a new PR to suggest
preferring the linkage name of the main program.

Next, the rust compiler emits both a DW_TAG_subprogram and a
DW_TAG_namespace for "main".  This happens because the file is named
"main.rs" -- i.e., the bug is specific to the source file name.  The
crash also seems to require the nested function inside of 'main', at
least for me.  The namespace always is generated, but perhaps this
changes the ordering in the DWARF.

When inside_main_func looks up the main symbol, it finds the namespace
symbol rather than the function.  (I filed a bug about fixing gdb's
symbol tables -- long overdue.)

Meanwhile, as I think it's important to fix this crash sooner rather
than later, this patch changes inside_main_func to check that the
symbol that is found is LOC_BLOCK.  This perhaps should have been done
in the first place, anyway.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30158

gdb/frame.c

@@ -2551,6 +2551,14 @@ inside_main_func (frame_info_ptr this_frame)
       if (bs.symbol == nullptr)
 	return false;
 
+      /* We might have found some unrelated symbol.  For example, the
+	 Rust compiler can emit both a subprogram and a namespace with
+	 the same name in the same scope; and due to how gdb's symbol
+	 tables currently work, we can't request the one we'd
+	 prefer.  */
+      if (bs.symbol->aclass () != LOC_BLOCK)
+	return false;
+
       const struct block *block = bs.symbol->value_block ();
       gdb_assert (block != nullptr);
       sym_addr = block->start ();

gdb/testsuite/gdb.rust/main-crash.exp

+# Copyright (C) 2023 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Regression test for a crash in inside_main_func.
+
+load_lib rust-support.exp
+require allow_rust_tests
+
+standard_testfile main.rs
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile \
+	 {debug rust}]} {
+    return -1
+}
+
+set line [gdb_get_line_number "BREAK"]
+# The bug was that this would crash.
+if {![runto ${srcfile}:$line]} {
+    untested "could not run to breakpoint"
+    return -1
+}
+
+# Test that gdb is alive.
+gdb_test "print 23" " = 23"

gdb/testsuite/gdb.rust/main.rs

+// Copyright (C) 2016-2023 Free Software Foundation, Inc.
+
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+#![allow(dead_code)]
+#![allow(unused_variables)]
+#![allow(unused_assignments)]
+
+fn global_fn(x: u8) {
+    // BREAK
+}
+
+fn main() {
+    fn nested(y: u8) {
+	global_fn(y)
+    }
+
+    nested(23);
+}