stop inlining config into pyo3-build-config; generate directly in pyo3-ffi (#5862)

* stop inlining config into `pyo3-build-config`; generate directly in `pyo3-ffi`

* enable `pyo3-ffi-check` macro to see pyo3 config

* fixup benches `abi3` dependency

* fix `pyo3-build-config` test

* correct path

* fixup call

* fix linux-only use of `resolve_build_config`

* add newsfragment

* fixup ordering

* directly resolve host config in test

* more fixups

* workaround sysconfig pain point

* migration notes, bump coverage

* fix typo in migration guide

* review feedback

Author: davidhewitt

Cargo.toml

@@ -22,6 +22,13 @@ exclude = [
     "/tests/ui",
 ]
 edition = "2021"
+# This allows PyO3 to forward build-config from `pyo3-ffi` to avoid downstream crates
+# needing to depend on `pyo3-ffi` directly to use `pyo3-build-config` functionality.
+#
+# See also https://github.com/rust-lang/cargo/issues/3544 and
+# https://github.com/rust-lang/cargo/pull/16436 which may allow this to be
+# reverted in the future.
+links = "pyo3-python"
 rust-version.workspace = true
 
 [dependencies]
@@ -82,7 +89,7 @@ uuid = { version = "1.10.0", features = ["v4"] }
 parking_lot = { version = "0.12.3", features = ["arc_lock"] }
 
 [build-dependencies]
-pyo3-build-config = { path = "pyo3-build-config", version = "=0.28.3", features = ["resolve-config"] }
+pyo3-build-config = { path = "pyo3-build-config", version = "=0.28.3" }
 
 [features]
 default = ["macros"]
@@ -106,16 +113,16 @@ multiple-pymethods = ["inventory", "pyo3-macros/multiple-pymethods"]
 extension-module = ["pyo3-ffi/extension-module"]
 
 # Use the Python limited API. See https://www.python.org/dev/peps/pep-0384/ for more.
-abi3 = ["pyo3-build-config/abi3", "pyo3-ffi/abi3"]
+abi3 = ["pyo3-ffi/abi3"]
 
 # With abi3, we can manually set the minimum Python version.
-abi3-py38 = ["abi3-py39", "pyo3-build-config/abi3-py38", "pyo3-ffi/abi3-py38"]
-abi3-py39 = ["abi3-py310", "pyo3-build-config/abi3-py39", "pyo3-ffi/abi3-py39"]
-abi3-py310 = ["abi3-py311", "pyo3-build-config/abi3-py310", "pyo3-ffi/abi3-py310"]
-abi3-py311 = ["abi3-py312", "pyo3-build-config/abi3-py311", "pyo3-ffi/abi3-py311"]
-abi3-py312 = ["abi3-py313", "pyo3-build-config/abi3-py312", "pyo3-ffi/abi3-py312"]
-abi3-py313 = ["abi3-py314", "pyo3-build-config/abi3-py313", "pyo3-ffi/abi3-py313"]
-abi3-py314 = ["abi3", "pyo3-build-config/abi3-py314", "pyo3-ffi/abi3-py314"]
+abi3-py38 = ["abi3-py39", "pyo3-ffi/abi3-py38"]
+abi3-py39 = ["abi3-py310", "pyo3-ffi/abi3-py39"]
+abi3-py310 = ["abi3-py311", "pyo3-ffi/abi3-py310"]
+abi3-py311 = ["abi3-py312", "pyo3-ffi/abi3-py311"]
+abi3-py312 = ["abi3-py313", "pyo3-ffi/abi3-py312"]
+abi3-py313 = ["abi3-py314", "pyo3-ffi/abi3-py313"]
+abi3-py314 = ["abi3", "pyo3-ffi/abi3-py314"]
 
 # deprecated: no longer needed, raw-dylib is used instead
 generate-import-lib = ["pyo3-ffi/generate-import-lib"]

build.rs

@@ -43,6 +43,10 @@ fn configure_pyo3() -> Result<()> {
 
     print_feature_cfgs();
 
+    // Forwards interpreter config under the links = "pyo3-python" configuration,
+    // which allows consumers of `pyo3-build-config` APIs to depend on pyo3 instead of pyo3-ffi.
+    interpreter_config.to_cargo_dep_env()?;
+
     // Make `cargo test` etc work on macOS with Xcode bundled Python
     add_python_framework_link_args();
 

guide/src/building-and-distribution/multiple-python-versions.md

@@ -36,11 +36,11 @@ To see a full reference of all the `#[cfg]` flags provided, see the [`pyo3-build
 
 You can use the `#[cfg]` flags in just two steps:
 
-1. Add `pyo3-build-config` with the [`resolve-config`](../features.md#resolve-config) feature enabled to your crate's build dependencies in `Cargo.toml`:
+1. Add `pyo3-build-config` to your crate's build dependencies in `Cargo.toml`:
 
    ```toml
    [build-dependencies]
-   pyo3-build-config = { {{#PYO3_CRATE_VERSION}}, features = ["resolve-config"] }
+   pyo3-build-config = {{#PYO3_CRATE_VERSION}}
    ```
 
 2. Add a [`build.rs`](https://doc.rust-lang.org/cargo/reference/build-scripts.html) file to your crate with the following contents:

guide/src/features.md

@@ -105,12 +105,6 @@ See [the `#[pyclass]` implementation details](class.md#implementation-details) f
 The `nightly` feature needs the nightly Rust compiler.
 This allows PyO3 to use the `auto_traits` and `negative_impls` features to fix the `Python::detach` function.
 
-### `resolve-config`
-
-The `resolve-config` feature of the `pyo3-build-config` crate controls whether that crate's build script automatically resolves a Python interpreter / build configuration.
-This feature is primarily useful when building PyO3 itself.
-By default this feature is not enabled, meaning you can freely use `pyo3-build-config` as a standalone library to read or write PyO3 build configuration files or resolve metadata about a Python interpreter.
-
 ## Optional Dependencies
 
 These features enable conversions between Python types and types from other Rust crates, enabling easy access to the rest of the Rust ecosystem.

guide/src/migration.md

@@ -36,6 +36,17 @@ fn bytes_to_str<'a>(py: Python<'_>, bytes: &'a [u8]) -> PyResult<&'a str> {
 For `string::FromUtf16Error` and `char::DecodeUtf16Error` the Rust error types do not contain any of the information required to construct a `UnicodeDecodeError`.
 To raise a Python `UnicodeDecodeError` a new error should be manually constructed by calling `PyUnicodeDecodeError::new_err(...)`.
 
+### `pyo3_build_config` APIs now require a direct dependency on `pyo3` or `pyo3-ffi`
+
+Prior to PyO3 0.29, `pyo3-build-config` would inline part of the build configuration into the crate in its own build script.
+This worked for simple builds but not for cross-compiling; `pyo3-ffi`'s build script would need to re-implement a lot of the same machinery to correctly configure the build for cross-compilation.
+There were also edge cases where `pyo3-build-config` would use this inlined configuration incorrectly and misconfigure the build - e.g. [when cross compiling with `buck` or `bazel` build systems](https://github.com/PyO3/pyo3/issues/4579).
+
+In PyO3 0.29, `pyo3-build-config` no longer inlines any configuration into itself and instead requires a direct dependency on either `pyo3-ffi` or `pyo3` to load the configuration resolved from `pyo3-ffi`'s build script.
+
+This has upside of faster compilation - `pyo3-build-config` no longer has a build script and never recompiles when changing Python version.
+It also guarantees that all builds are based on the single fully-configured build configuration resolved by `pyo3-ffi`.
+
 ## from 0.27.* to 0.28
 
 ### Default to supporting free-threaded Python

newsfragments/5862.changed.md

@@ -0,0 +1 @@
+Deprecate `pyo3-build-config/resolve-config` feature.

pyo3-benches/Cargo.toml

@@ -20,7 +20,7 @@ rust_decimal = { version = "1.0.0", default-features = false }
 hashbrown = "0.17"
 
 [features]
-abi3 = ["pyo3-build-config/abi3"]
+abi3 = ["pyo3/abi3"]
 
 [[bench]]
 name = "bench_any"

pyo3-build-config/Cargo.toml

@@ -20,25 +20,11 @@ target-lexicon = "0.13.3"
 [features]
 default = []
 
-# Attempt to resolve a Python interpreter config for building in the build
-# script. If this feature isn't enabled, the build script no-ops.
+# deprecated
 resolve-config = []
 
 # deprecated
 extension-module = []
 
 # deprecated: no longer needed, raw-dylib is used instead
 generate-import-lib = []
-
-# These features are enabled by pyo3 when building Stable ABI extension modules.
-abi3 = []
-abi3-py38 = ["abi3-py39"]
-abi3-py39 = ["abi3-py310"]
-abi3-py310 = ["abi3-py311"]
-abi3-py311 = ["abi3-py312"]
-abi3-py312 = ["abi3-py313"]
-abi3-py313 = ["abi3-py314"]
-abi3-py314 = ["abi3"]
-
-[package.metadata.docs.rs]
-features = ["resolve-config"]