Support raw string literals as input for py::eval (#766)

* Support raw string literals as input for py::eval
* Dedent only when needed

docs/advanced/pycpp/utilities.rst

@@ -55,3 +55,18 @@ is always ``none``).
 
     // Evaluate the statements in an separate Python file on disk
     py::eval_file("script.py", scope);
+
+C++11 raw string literals are also supported and quite handy for this purpose.
+The only requirement is that the first statement must be on a new line following
+the raw string delimiter ``R"(``, ensuring all lines have common leading indent:
+
+.. code-block:: cpp
+
+    py::eval<py::eval_statements>(R"(
+        x = get_answer()
+        if x == 42:
+            print('Hello World!')
+        else:
+            print('Bye!')
+        )", scope
+    );

include/pybind11/eval.h

@@ -11,8 +11,6 @@
 
 #pragma once
 
-#pragma once
-
 #include "pybind11.h"
 
 NAMESPACE_BEGIN(pybind11)
@@ -56,6 +54,14 @@ object eval(str expr, object global = object(), object local = object()) {
     return reinterpret_steal<object>(result);
 }
 
+template <eval_mode mode = eval_expr, size_t N>
+object eval(const char (&s)[N], object global = object(), object local = object()) {
+    /* Support raw string literals by removing common leading whitespace */
+    auto expr = (s[0] == '\n') ? str(module::import("textwrap").attr("dedent")(s))
+                               : str(s);
+    return eval<mode>(expr, global, local);
+}
+
 template <eval_mode mode = eval_statements>
 object eval_file(str fname, object global = object(), object local = object()) {
     if (!global) {

tests/test_eval.cpp

@@ -20,11 +20,21 @@ test_initializer eval([](py::module &m) {
             return 42;
         });
 
-        auto result = py::eval<py::eval_statements>(
+        // Regular string literal
-            "print('Hello World!');\n"
+        py::eval<py::eval_statements>(
-            "x = call_test();",
+            "message = 'Hello World!'\n"
+            "x = call_test()",
             global, local
         );
+
+        // Multi-line raw string literal
+        auto result = py::eval<py::eval_statements>(R"(
+            if x == 42:
+                print(message)
+            else:
+                raise RuntimeError
+            )", global, local
+        );
         auto x = local["x"].cast<int>();
 
         return result == py::none() && x == 42;