Introduce all-apis feature to support abi3

Cargo.toml

@@ -33,8 +33,12 @@ trybuild = "1.0.23"
 rustversion = "1.0"
 
 [features]
-default = ["macros"]
+default = ["macros", "all-apis"]
 macros = ["ctor", "indoc", "inventory", "paste", "pyo3cls", "unindent"]
+# Enable the use of `not(Py_LIMITED_API)` s.
+# This is incompatible with abi3(PEP384). See https://www.python.org/dev/peps/pep-0384/ more.
+all-apis = []
+
 # Optimizes PyObject to Vec conversion and so on.
 nightly = []
 
@@ -46,11 +50,6 @@ python3 = []
 # so that the module can also be used with statically linked python interpreters.
 extension-module = []
 
-# The stable cpython abi as defined in PEP 384. Currently broken with
-# many compilation errors. Pull Requests working towards fixing that
-# are welcome.
-# abi3 = []
-
 [workspace]
 members = [
     "pyo3cls",

build.rs

@@ -757,7 +757,7 @@ fn configure(interpreter_config: &InterpreterConfig) -> Result<String> {
         bail!("Python 2 is not supported");
     }
 
-    if env::var_os("CARGO_FEATURE_ABI3").is_some() {
+    if env::var_os("CARGO_FEATURE_ALL_APIS").is_none() {
         println!("cargo:rustc-cfg=Py_LIMITED_API");
     }