c++: Implement -Wrange-loop-construct [PR94695]

This new warning can be used to prevent expensive copies inside range-based
for-loops, for instance:

  struct S { char arr[128]; };
  void fn () {
    S arr[5];
    for (const auto x : arr) {  }
  }

where auto deduces to S and then we copy the big S in every iteration.
Using "const auto &x" would not incur such a copy.  With this patch the
compiler will warn:

q.C:4:19: warning: loop variable 'x' creates a copy from type 'const S' [-Wrange-loop-construct]
    4 |   for (const auto x : arr) {  }
      |                   ^
q.C:4:19: note: use reference type 'const S&' to prevent copying
    4 |   for (const auto x : arr) {  }
      |                   ^
      |                   &

As per Clang, this warning is suppressed for trivially copyable types
whose size does not exceed 64B.  The tricky part of the patch was how
to figure out if using a reference would have prevented a copy.  To
that point, I'm using the new function called ref_conv_binds_directly_p.

This warning is enabled by -Wall.  Further warnings of similar nature
should follow soon.

gcc/c-family/ChangeLog:

	PR c++/94695
	* c.opt (Wrange-loop-construct): New option.

gcc/cp/ChangeLog:

	PR c++/94695
	* call.c (ref_conv_binds_directly_p): New function.
	* cp-tree.h (ref_conv_binds_directly_p): Declare.
	* parser.c (warn_for_range_copy): New function.
	(cp_convert_range_for): Call it.

gcc/ChangeLog:

	PR c++/94695
	* doc/invoke.texi: Document -Wrange-loop-construct.

gcc/testsuite/ChangeLog:

	PR c++/94695
	* g++.dg/warn/Wrange-loop-construct.C: New test.

1 files changed, 22 insertions, 0 deletions

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 5606389..1e5fffe 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -8429,6 +8429,28 @@ conv_binds_ref_to_prvalue (conversion *c)
   return false;
 }
 
+/* True iff converting EXPR to a reference type TYPE does not involve
+   creating a temporary.  */
+
+bool
+ref_conv_binds_directly_p (tree type, tree expr)
+{
+  gcc_assert (TYPE_REF_P (type));
+
+  /* Get the high-water mark for the CONVERSION_OBSTACK.  */
+  void *p = conversion_obstack_alloc (0);
+
+  conversion *conv = implicit_conversion (type, TREE_TYPE (expr), expr,
+					  /*c_cast_p=*/false,
+					  LOOKUP_IMPLICIT, tf_none);
+  bool ret = conv && !conv->bad_p && !conv_binds_ref_to_prvalue (conv);
+
+  /* Free all the conversions we allocated.  */
+  obstack_free (&conversion_obstack, p);
+
+  return ret;
+}
+
 /* Call the trivial destructor for INSTANCE, which can be either an lvalue of
    class type or a pointer to class type.  If NO_PTR_DEREF is true and
    INSTANCE has pointer type, clobber the pointer rather than what it points