rust: allow older version of bindgen

Cope with the old version that is provided in Debian 12.

--size_t-is-usize is needed on bindgen <0.61.0, and it was removed in
bindgen 0.65.0, so check for it in meson.build.

--merge-extern-blocks was added in 0.61.0.

--formatter rustfmt was added in 0.65.0 and is the default, so remove it.

Apart from Debian 12 and Ubuntu 22.04, all other supported distros have
version 0.66.x of bindgen or newer (or do not have bindgen at all).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/about/build-platforms.rst

@@ -107,6 +107,18 @@ Python build dependencies
   required, it may be necessary to fetch python modules from the Python
   Package Index (PyPI) via ``pip``, in order to build QEMU.
 
+Rust build dependencies
+  QEMU is generally conservative in adding new Rust dependencies, and all
+  of them are included in the distributed tarballs.  One exception is the
+  bindgen tool, which is too big to package and distribute.  The minimum
+  supported version of bindgen is 0.60.x.  For distributions that do not
+  include bindgen or have an older version, it is recommended to install
+  a newer version using ``cargo install bindgen-cli``.
+
+  Developers may want to use Cargo-based tools in the QEMU source tree;
+  this requires Cargo 1.74.0.  Note that Cargo is not required in order
+  to build QEMU.
+
 Optional build dependencies
   Build components whose absence does not affect the ability to build
   QEMU may not be available in distros, or may be too old for QEMU's

meson.build

@@ -100,6 +100,23 @@ if have_rust
   endif
 endif
 
+if have_rust
+  bindgen = find_program('bindgen', required: get_option('rust'))
+  if not bindgen.found() or bindgen.version().version_compare('<0.60.0')
+    if get_option('rust').enabled()
+      error('bindgen version ' + bindgen.version() + ' is unsupported. You can install a new version with "cargo install bindgen-cli"')
+    else
+      if bindgen.found()
+        warning('bindgen version ' + bindgen.version() + ' is unsupported, disabling Rust compilation.')
+      else
+        warning('bindgen not found, disabling Rust compilation.')
+      endif
+      message('To use Rust you can install a new version with "cargo install bindgen-cli"')
+      have_rust = false
+    endif
+  endif
+endif
+
 dtrace = not_found
 stap = not_found
 if 'dtrace' in get_option('trace_backends')
@@ -3963,15 +3980,13 @@ common_all = static_library('common',
 if have_rust
   # We would like to use --generate-cstr, but it is only available
   # starting with bindgen 0.66.0.  The oldest supported versions
-  # are in Ubuntu 22.04 (0.59.1) and Debian 12 (0.60.1).
+  # is 0.60.x (Debian 12 has 0.60.1) which introduces --allowlist-file.
   bindgen_args = [
     '--disable-header-comment',
     '--raw-line', '// @generated',
     '--ctypes-prefix', 'std::os::raw',
-    '--formatter', 'rustfmt',
     '--generate-block',
     '--impl-debug',
-    '--merge-extern-blocks',
     '--no-doc-comments',
     '--with-derive-default',
     '--no-layout-tests',
@@ -3980,6 +3995,12 @@ if have_rust
     '--allowlist-file', meson.project_source_root() + '/.*',
     '--allowlist-file', meson.project_build_root() + '/.*'
     ]
+  if bindgen.version().version_compare('<0.61.0')
+    # default in 0.61+
+    bindgen_args += ['--size_t-is-usize']
+  else
+    bindgen_args += ['--merge-extern-blocks']
+  endif
   c_enums = [
     'DeviceCategory',
     'GpioPolarity',
@@ -4015,7 +4036,7 @@ if have_rust
     dependencies: common_ss.all_dependencies(),
     output: 'bindings.rs',
     include_directories: include_directories('.', 'include'),
-    bindgen_version: ['>=0.69.4'],
+    bindgen_version: ['>=0.60.0'],
     args: bindgen_args,
     )
   subdir('rust')
@@ -4481,6 +4502,8 @@ if have_rust
   summary_info += {'Rust target':     config_host['RUST_TARGET_TRIPLE']}
   summary_info += {'rustc':           ' '.join(rustc.cmd_array())}
   summary_info += {'rustc version':   rustc.version()}
+  summary_info += {'bindgen':         bindgen.full_path()}
+  summary_info += {'bindgen version': bindgen.version()}
 endif
 option_cflags = (get_option('debug') ? ['-g'] : [])
 if get_option('optimization') != 'plain'