rust: allow using build-root bindings.rs from cargo

Right now, using cargo with QEMU requires copying by hand the bindings.rs to the
source tree.  Instead, we can use an include file to escape the cage of cargo's
mandated source directory structure.

By running cargo within meson's "devenv" and adding a MESON_BUILD_ROOT
environment variable, it is easy for build.rs to find the file.  However, the
file must be symlinked into cargo's output directory for rust-analyzer to find
it.

Suggested-by: Junjie Mao <junjie.mao@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

7 files changed, 60 insertions, 34 deletions

diff --git a/rust/hw/char/pl011/.gitignore b/rust/hw/char/pl011/.gitignore
deleted file mode 100644
index 71eaff2..0000000
--- a/rust/hw/char/pl011/.gitignore
+++ /dev/null
@@ -1,2 +0,0 @@
-# Ignore generated bindings file overrides.
-src/bindings.rs.inc
diff --git a/rust/qemu-api/.gitignore b/rust/qemu-api/.gitignore
index b9e7e00..df6c216 100644
--- a/rust/qemu-api/.gitignore
+++ b/rust/qemu-api/.gitignore
@@ -1,2 +1,2 @@
 # Ignore generated bindings file overrides.
-src/bindings.rs
+/src/bindings.inc.rs
diff --git a/rust/qemu-api/README.md b/rust/qemu-api/README.md
index 7588fa2..53810f4 100644
--- a/rust/qemu-api/README.md
+++ b/rust/qemu-api/README.md
@@ -5,7 +5,7 @@ This library exports helper Rust types, Rust macros and C FFI bindings for inter
 The C bindings can be generated with `bindgen`, using this build target:
 
 ```console
-$ ninja bindings.rs
+$ ninja bindings.inc.rs
 ```
 
 ## Generate Rust documentation
@@ -13,5 +13,5 @@ $ ninja bindings.rs
 To generate docs for this crate, including private items:
 
 ```sh
-cargo doc --no-deps --document-private-items
+pyvenv/bin/meson devenv -w ../rust cargo doc --no-deps --document-private-items
 ```
diff --git a/rust/qemu-api/build.rs b/rust/qemu-api/build.rs
index 20f8f71..40644d5 100644
--- a/rust/qemu-api/build.rs
+++ b/rust/qemu-api/build.rs
@@ -2,17 +2,36 @@
 // Author(s): Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
 // SPDX-License-Identifier: GPL-2.0-or-later
 
-use std::path::Path;
+#[cfg(unix)]
+use std::os::unix::fs::symlink as symlink_file;
+#[cfg(windows)]
+use std::os::windows::fs::symlink_file;
+use std::{env, fs::remove_file, io::Result, path::Path};
 
 use version_check as rustc;
 
-fn main() {
-    if !Path::new("src/bindings.rs").exists() {
-        panic!(
-            "No generated C bindings found! Either build them manually with bindgen or with meson \
-             (`ninja bindings.rs`) and copy them to src/bindings.rs, or build through meson."
-        );
+fn main() -> Result<()> {
+    // Placing bindings.inc.rs in the source directory is supported
+    // but not documented or encouraged.
+    let path = env::var("MESON_BUILD_ROOT")
+        .unwrap_or_else(|_| format!("{}/src", env!("CARGO_MANIFEST_DIR")));
+
+    let file = format!("{}/bindings.inc.rs", path);
+    let file = Path::new(&file);
+    if !Path::new(&file).exists() {
+        panic!(concat!(
+            "No generated C bindings found! If you want to run `cargo`, start a subshell\n",
+            "with `meson devenv`, or point MESON_BUILD_ROOT to the top of the build tree."
+        ));
+    }
+
+    let out_dir = env::var("OUT_DIR").unwrap();
+    let dest_path = format!("{}/bindings.inc.rs", out_dir);
+    let dest_path = Path::new(&dest_path);
+    if dest_path.symlink_metadata().is_ok() {
+        remove_file(dest_path)?;
     }
+    symlink_file(file, dest_path)?;
 
     // Check for available rustc features
     if rustc::is_min_version("1.77.0").unwrap_or(false) {
@@ -20,4 +39,5 @@ fn main() {
     }
 
     println!("cargo:rerun-if-changed=build.rs");
+    Ok(())
 }
diff --git a/rust/qemu-api/meson.build b/rust/qemu-api/meson.build
index cad9ac4..3be7b7e 100644
--- a/rust/qemu-api/meson.build
+++ b/rust/qemu-api/meson.build
@@ -9,6 +9,7 @@ _qemu_api_rs = static_library(
   structured_sources(
     [
       'src/lib.rs',
+      'src/bindings.rs',
       'src/c_str.rs',
       'src/definitions.rs',
       'src/device_class.rs',
diff --git a/rust/qemu-api/src/bindings.rs b/rust/qemu-api/src/bindings.rs
new file mode 100644
index 0000000..0b76ec5
--- /dev/null
+++ b/rust/qemu-api/src/bindings.rs
@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#![allow(
+    dead_code,
+    improper_ctypes_definitions,
+    improper_ctypes,
+    non_camel_case_types,
+    non_snake_case,
+    non_upper_case_globals,
+    unsafe_op_in_unsafe_fn,
+    clippy::missing_const_for_fn,
+    clippy::too_many_arguments,
+    clippy::approx_constant,
+    clippy::use_self,
+    clippy::useless_transmute,
+    clippy::missing_safety_doc
+)]
+
+#[cfg(MESON)]
+include!("bindings.inc.rs");
+
+#[cfg(not(MESON))]
+include!(concat!(env!("OUT_DIR"), "/bindings.inc.rs"));
+
+unsafe impl Send for Property {}
+unsafe impl Sync for Property {}
+unsafe impl Sync for TypeInfo {}
+unsafe impl Sync for VMStateDescription {}
+unsafe impl Sync for VMStateField {}
+unsafe impl Sync for VMStateInfo {}
diff --git a/rust/qemu-api/src/lib.rs b/rust/qemu-api/src/lib.rs
index aa8d16e..440aff3 100644
--- a/rust/qemu-api/src/lib.rs
+++ b/rust/qemu-api/src/lib.rs
@@ -4,31 +4,9 @@
 
 #![cfg_attr(not(MESON), doc = include_str!("../README.md"))]
 
-#[allow(
-    dead_code,
-    improper_ctypes_definitions,
-    improper_ctypes,
-    non_camel_case_types,
-    non_snake_case,
-    non_upper_case_globals,
-    unsafe_op_in_unsafe_fn,
-    clippy::missing_const_for_fn,
-    clippy::too_many_arguments,
-    clippy::approx_constant,
-    clippy::use_self,
-    clippy::useless_transmute,
-    clippy::missing_safety_doc,
-)]
 #[rustfmt::skip]
 pub mod bindings;
 
-unsafe impl Send for bindings::Property {}
-unsafe impl Sync for bindings::Property {}
-unsafe impl Sync for bindings::TypeInfo {}
-unsafe impl Sync for bindings::VMStateDescription {}
-unsafe impl Sync for bindings::VMStateField {}
-unsafe impl Sync for bindings::VMStateInfo {}
-
 pub mod c_str;
 pub mod definitions;
 pub mod device_class;