[gcc/devel/rust/master] gccrs const folding port: continue porting potential_constant_expression_1()

[Thomas Schwinge <tschwinge@gcc.gnu.org>]

https://gcc.gnu.org/g:bb4dd8fbc857a78bb4bfb69f1bdfdcbd9b1b6cdb

commit bb4dd8fbc857a78bb4bfb69f1bdfdcbd9b1b6cdb
Author: Faisal Abbas <90.abbasfaisal@gmail.com>
Date:   Tue Jul 19 17:39:56 2022 +0100

    gccrs const folding port: continue porting potential_constant_expression_1()
    
    Following functions are ported in this changeset:
     - decl_constant_var_p
     - undeduced_auto_decl
     - require_deduced_type
    
    Signed-off-by: Faisal Abbas <90.abbasfaisal@gmail.com>

Diff:
---
 gcc/rust/backend/rust-tree.cc | 46 +++++++++++++++++++++++++++++++++++++++++++
 gcc/rust/backend/rust-tree.h  |  6 ++++++
 2 files changed, 52 insertions(+)

diff --git a/gcc/rust/backend/rust-tree.cc b/gcc/rust/backend/rust-tree.cc
index e1ab7f16074..028d88f420c 100644
--- a/gcc/rust/backend/rust-tree.cc
+++ b/gcc/rust/backend/rust-tree.cc
@@ -3969,4 +3969,50 @@ retry:
     }
 }
 
+// forked from gcc/cp/decl2.cc decl_constant_var_p
+
+/* Nonzero for a VAR_DECL whose value can be used in a constant expression.
+
+      [expr.const]
+
+      An integral constant-expression can only involve ... const
+      variables of integral or enumeration types initialized with
+      constant expressions ...
+
+      C++0x also allows constexpr variables and temporaries initialized
+      with constant expressions.  We handle the former here, but the latter
+      are just folded away in cxx_eval_constant_expression.
+
+   The standard does not require that the expression be non-volatile.
+   G++ implements the proposed correction in DR 457.  */
+
+bool
+decl_constant_var_p (tree decl)
+{
+  if (!decl_maybe_constant_var_p (decl))
+    return false;
+
+  return DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (decl);
+}
+
+// forked from gcc/cp/decl.cc undeduced_auto_decl
+
+/* Returns true iff DECL is a variable or function declared with an auto type
+   that has not yet been deduced to a real type.  */
+
+bool
+undeduced_auto_decl (tree decl)
+{
+  return false;
+}
+
+// forked from gcc/cp/decl.cc require_deduced_type
+
+/* Complain if DECL has an undeduced return type.  */
+
+bool
+require_deduced_type (tree decl, tsubst_flags_t complain)
+{
+  return true;
+}
 } // namespace Rust
diff --git a/gcc/rust/backend/rust-tree.h b/gcc/rust/backend/rust-tree.h
index fc83dc2cb8f..624f9364bed 100644
--- a/gcc/rust/backend/rust-tree.h
+++ b/gcc/rust/backend/rust-tree.h
@@ -2542,6 +2542,12 @@ extern void cxx_incomplete_type_inform (const_tree);
 
 extern tree strip_top_quals (tree);
 
+extern bool undeduced_auto_decl (tree);
+
+extern bool require_deduced_type (tree, tsubst_flags_t = tf_warning_or_error);
+
+extern bool decl_constant_var_p (tree);
+
 // forked from gcc/cp/cp-tree.h
 
 enum