Fix memory corruption in generation of builtin functions

When we compile normal language functions we maintain a stack of the
current function declaration and associated return addresses. This is used
while building up the GCC tree graph. When we generate builtin intrinsic
functions such as offset or size_of were missing their associated push_fn
but still performed a pop_fn on completion this resulted in a corrupt
stack which valgrind shown as bad read/writes.

This patch removes the pop_fn calls since no fncontext stack is required here for these intrinsics.

Fixes #1024

Author: philberty

gcc/rust/backend/rust-compile-intrinsic.cc

@@ -304,8 +304,6 @@ offset_intrinsic_handler (Context *ctx, TyTy::BaseType *fntype_tyty)
 
   gcc_assert (TREE_CODE (bind_tree) == BIND_EXPR);
   DECL_SAVED_TREE (fndecl) = bind_tree;
-
-  ctx->pop_fn ();
   ctx->push_function (fndecl);
 
   return fndecl;
@@ -393,8 +391,6 @@ sizeof_intrinsic_handler (Context *ctx, TyTy::BaseType *fntype_tyty)
 
   gcc_assert (TREE_CODE (bind_tree) == BIND_EXPR);
   DECL_SAVED_TREE (fndecl) = bind_tree;
-
-  ctx->pop_fn ();
   ctx->push_function (fndecl);
 
   return fndecl;

gcc/testsuite/rust/compile/torture/issue-1024.rs

@@ -0,0 +1,11 @@
+extern "rust-intrinsic" {
+    pub fn size_of<T>() -> usize;
+}
+
+fn test() -> usize {
+    unsafe { size_of::<i32>() }
+}
+
+fn main() {
+    let _a = test();
+}