Merge #990

990: Add must use attribute support r=philberty a=philberty

This is a port of the CPP front-end nodiscard attribute to be used for
must_use. It contains a patch to clean up how we handle expressions vs
statements and removes more of the GCC abstraction. Its my hope that we
can leverage more and more existing code to get the static analysis where
we want it.

Fixes #856 

Co-authored-by: Philip Herron <philip.herron@embecosm.com>

1 files changed, 182 insertions, 0 deletions

diff --git a/gcc/rust/backend/rust-tree.h b/gcc/rust/backend/rust-tree.h
new file mode 100644
index 0000000..c21bf4b
--- /dev/null
+++ b/gcc/rust/backend/rust-tree.h
@@ -0,0 +1,182 @@
+// Copyright (C) 2020-2022 Free Software Foundation, Inc.
+
+// This file is part of GCC.
+
+// GCC 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, or (at your option) any later
+// version.
+
+// GCC 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 GCC; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#ifndef RUST_TREE
+#define RUST_TREE
+
+#include "rust-system.h"
+#include "coretypes.h"
+#include "tree.h"
+
+/* Returns true if NODE is a pointer.  */
+#define TYPE_PTR_P(NODE) (TREE_CODE (NODE) == POINTER_TYPE)
+
+/* Returns true if NODE is a reference.  */
+#define TYPE_REF_P(NODE) (TREE_CODE (NODE) == REFERENCE_TYPE)
+
+/* Returns true if NODE is a pointer or a reference.  */
+#define INDIRECT_TYPE_P(NODE) (TYPE_PTR_P (NODE) || TYPE_REF_P (NODE))
+
+/* [basic.fundamental]
+
+   Types  bool, char, wchar_t, and the signed and unsigned integer types
+   are collectively called integral types.
+
+   Note that INTEGRAL_TYPE_P, as defined in tree.h, allows enumeration
+   types as well, which is incorrect in C++.  Keep these checks in
+   ascending code order.  */
+#define RS_INTEGRAL_TYPE_P(TYPE)                                               \
+  (TREE_CODE (TYPE) == BOOLEAN_TYPE || TREE_CODE (TYPE) == INTEGER_TYPE)
+
+/* [basic.fundamental]
+
+   Integral and floating types are collectively called arithmetic
+   types.
+
+   As a GNU extension, we also accept complex types.
+
+   Keep these checks in ascending code order.  */
+#define ARITHMETIC_TYPE_P(TYPE)                                                \
+  (RS_INTEGRAL_TYPE_P (TYPE) || TREE_CODE (TYPE) == REAL_TYPE                  \
+   || TREE_CODE (TYPE) == COMPLEX_TYPE)
+
+/* True iff TYPE is cv decltype(nullptr).  */
+#define NULLPTR_TYPE_P(TYPE) (TREE_CODE (TYPE) == NULLPTR_TYPE)
+
+/* [basic.types]
+
+   Arithmetic types, enumeration types, pointer types,
+   pointer-to-member types, and std::nullptr_t are collectively called
+   scalar types.
+
+   Keep these checks in ascending code order.  */
+#define SCALAR_TYPE_P(TYPE)                                                    \
+  (TREE_CODE (TYPE) == ENUMERAL_TYPE || ARITHMETIC_TYPE_P (TYPE)               \
+   || TYPE_PTR_P (TYPE) || NULLPTR_TYPE_P (TYPE))
+
+/* True if NODE is an implicit INDIRECT_REF from convert_from_reference.  */
+#define REFERENCE_REF_P(NODE)                                                  \
+  (INDIRECT_REF_P (NODE) && TREE_TYPE (TREE_OPERAND (NODE, 0))                 \
+   && TYPE_REF_P (TREE_TYPE (TREE_OPERAND ((NODE), 0))))
+
+namespace Rust {
+
+// forked from gcc/cp/cvt.cc convert_to_void
+//
+// When an expression is used in a void context, its value is discarded and
+// no lvalue-rvalue and similar conversions happen [expr.static.cast/4,
+// stmt.expr/1, expr.comma/1].  This permits dereferencing an incomplete type
+// in a void context. The C++ standard does not define what an `access' to an
+// object is, but there is reason to believe that it is the lvalue to rvalue
+// conversion -- if it were not, `*&*p = 1' would violate [expr]/4 in that it
+// accesses `*p' not to calculate the value to be stored. But, dcl.type.cv/8
+// indicates that volatile semantics should be the same between C and C++
+// where ever possible. C leaves it implementation defined as to what
+// constitutes an access to a volatile. So, we interpret `*vp' as a read of
+// the volatile object `vp' points to, unless that is an incomplete type. For
+// volatile references we do not do this interpretation, because that would
+// make it impossible to ignore the reference return value from functions. We
+// issue warnings in the confusing cases.
+//
+// The IMPLICIT is ICV_CAST when the user is explicitly converting an
+// expression to void via a cast. If an expression is being implicitly
+// converted, IMPLICIT indicates the context of the implicit conversion.
+
+/* Possible cases of implicit or explicit bad conversions to void. */
+enum impl_conv_void
+{
+  ICV_CAST,	      /* (explicit) conversion to void */
+  ICV_SECOND_OF_COND, /* second operand of conditional expression */
+  ICV_THIRD_OF_COND,  /* third operand of conditional expression */
+  ICV_RIGHT_OF_COMMA, /* right operand of comma operator */
+  ICV_LEFT_OF_COMMA,  /* left operand of comma operator */
+  ICV_STATEMENT,      /* statement */
+  ICV_THIRD_IN_FOR    /* for increment expression */
+};
+
+extern tree
+convert_to_void (tree expr, impl_conv_void implicit);
+
+// The lvalue-to-rvalue conversion (7.1) is applied if and only if the
+// expression is a glvalue of volatile-qualified type and it is one of the
+// following:
+// * ( expression ), where expression is one of these expressions,
+// * id-expression (8.1.4),
+// * subscripting (8.2.1),
+// * class member access (8.2.5),
+// * indirection (8.3.1),
+// * pointer-to-member operation (8.5),
+// * conditional expression (8.16) where both the second and the third
+//   operands are one of these expressions, or
+// * comma expression (8.19) where the right operand is one of these
+//   expressions.
+extern tree
+mark_discarded_use (tree expr);
+
+// Mark EXP as read, not just set, for set but not used -Wunused warning
+// purposes.
+extern void
+mark_exp_read (tree exp);
+
+// We've seen an actual use of EXPR.  Possibly replace an outer variable
+// reference inside with its constant value or a lambda capture.
+extern tree
+mark_use (tree expr, bool rvalue_p, bool read_p, location_t loc,
+	  bool reject_builtin);
+
+// Called whenever the expression EXPR is used in an rvalue context.
+// When REJECT_BUILTIN is true the expression is checked to make sure
+// it doesn't make it possible to obtain the address of a GCC built-in
+// function with no library fallback (or any of its bits, such as in
+// a conversion to bool).
+extern tree
+mark_rvalue_use (tree e, location_t loc /* = UNKNOWN_LOCATION */,
+		 bool reject_builtin /* = true */);
+
+// Called whenever an expression is used in an lvalue context.
+extern tree
+mark_lvalue_use (tree expr);
+
+// As above, but don't consider this use a read.
+extern tree
+mark_lvalue_use_nonread (tree expr);
+
+// We are using a reference VAL for its value. Bash that reference all the way
+// down to its lowest form.
+extern tree
+convert_from_reference (tree val);
+
+// Subroutine of convert_to_void.  Warn if we're discarding something with
+// attribute [[nodiscard]].
+extern void
+maybe_warn_nodiscard (tree expr, impl_conv_void implicit);
+
+extern location_t
+expr_loc_or_loc (const_tree t, location_t or_loc);
+
+extern location_t
+expr_loc_or_input_loc (const_tree t);
+
+// FN is the callee of a CALL_EXPR or AGGR_INIT_EXPR; return the FUNCTION_DECL
+// if we can.
+extern tree
+get_fndecl_from_callee (tree fn);
+
+} // namespace Rust
+
+#endif // RUST_TREE