Each `bindgen` release may upgrade the list of Rust targets. For instance,
currently, in their master branch [1], the latest ones are:

    Nightly => {
        vectorcall_abi: #124485,
        ptr_metadata: #81513,
        layout_for_ptr: #69835,
    },
    Stable_1_77(77) => { offset_of: #106655 },
    Stable_1_73(73) => { thiscall_abi: #42202 },
    Stable_1_71(71) => { c_unwind_abi: #106075 },
    Stable_1_68(68) => { abi_efiapi: #105795 },

By default, the highest stable release in their list is used, and users
are expected to set one if they need to support older Rust versions
(e.g. see [2]).

Thus, over time, new Rust features are used by default, and at some
point, it is likely that `bindgen` will emit Rust code that requires a
Rust version higher than our minimum (or perhaps enabling an unstable
feature). Currently, there is no problem because the maximum they have,
as seen above, is Rust 1.77.0, and our current minimum is Rust 1.78.0.

Therefore, set a Rust target explicitly now to prevent going forward in
time too much and thus getting potential build failures at some point.

Since we also support a minimum `bindgen` version, and since `bindgen`
does not support passing unknown Rust target versions, we need to use
the list of our minimum `bindgen` version, rather than the latest. So,
since `bindgen` 0.65.1 had this list [3], we need to use Rust 1.68.0:

    /// Rust stable 1.64
    ///  * `core_ffi_c` ([Tracking issue](rust-lang/rust#94501))
    => Stable_1_64 => 1.64;
    /// Rust stable 1.68
    ///  * `abi_efiapi` calling convention ([Tracking issue](rust-lang/rust#65815))
    => Stable_1_68 => 1.68;
    /// Nightly rust
    ///  * `thiscall` calling convention ([Tracking issue](rust-lang/rust#42202))
    ///  * `vectorcall` calling convention (no tracking issue)
    ///  * `c_unwind` calling convention ([Tracking issue](rust-lang/rust#74990))
    => Nightly => nightly;

    ...

    /// Latest stable release of Rust
    pub const LATEST_STABLE_RUST: RustTarget = RustTarget::Stable_1_68;

Thus add the `--rust-target 1.68` parameter. Add a comment as well
explaining this.

An alternative would be to use the currently running (i.e. actual) `rustc`
and `bindgen` versions to pick a "better" Rust target version. However,
that would introduce more moving parts depending on the user setup and
is also more complex to implement.

Starting with `bindgen` 0.71.0 [4], we will be able to set any future
Rust version instead, i.e. we will be able to set here our minimum
supported Rust version. Christian implemented it [5] after seeing this
patch. Thanks!

Cc: [email protected] # 6.12.y only (since older LTSs only support a single `bindgen` version)
Cc: Christian Poveda <[email protected]>
Cc: Emilio Cobos Álvarez <[email protected]>
Link: https://github.com/rust-lang/rust-bindgen/blob/21c60f473f4e824d4aa9b2b508056320d474b110/bindgen/features.rs#L97-L105 [1]
Link: rust-lang/rust-bindgen#2960 [2]
Link: https://github.com/rust-lang/rust-bindgen/blob/7d243056d335fdc4537f7bca73c06d01aae24ddc/bindgen/features.rs#L131-L150 [3]
Link: https://github.com/rust-lang/rust-bindgen/blob/main/CHANGELOG.md#0710-2024-12-06 [4]
Link: rust-lang/rust-bindgen#2993 [5]
Reviewed-by: Alice Ryhl <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

Each `bindgen` release may upgrade the list of Rust targets. For instance,
currently, in their master branch [1], the latest ones are:

    Nightly => {
        vectorcall_abi: #124485,
        ptr_metadata: #81513,
        layout_for_ptr: #69835,
    },
    Stable_1_77(77) => { offset_of: #106655 },
    Stable_1_73(73) => { thiscall_abi: #42202 },
    Stable_1_71(71) => { c_unwind_abi: #106075 },
    Stable_1_68(68) => { abi_efiapi: #105795 },

By default, the highest stable release in their list is used, and users
are expected to set one if they need to support older Rust versions
(e.g. see [2]).

Thus, over time, new Rust features are used by default, and at some
point, it is likely that `bindgen` will emit Rust code that requires a
Rust version higher than our minimum (or perhaps enabling an unstable
feature). Currently, there is no problem because the maximum they have,
as seen above, is Rust 1.77.0, and our current minimum is Rust 1.78.0.

Therefore, set a Rust target explicitly now to prevent going forward in
time too much and thus getting potential build failures at some point.

Since we also support a minimum `bindgen` version, and since `bindgen`
does not support passing unknown Rust target versions, we need to use
the list of our minimum `bindgen` version, rather than the latest. So,
since `bindgen` 0.65.1 had this list [3], we need to use Rust 1.68.0:

    /// Rust stable 1.64
    ///  * `core_ffi_c` ([Tracking issue](rust-lang/rust#94501))
    => Stable_1_64 => 1.64;
    /// Rust stable 1.68
    ///  * `abi_efiapi` calling convention ([Tracking issue](rust-lang/rust#65815))
    => Stable_1_68 => 1.68;
    /// Nightly rust
    ///  * `thiscall` calling convention ([Tracking issue](rust-lang/rust#42202))
    ///  * `vectorcall` calling convention (no tracking issue)
    ///  * `c_unwind` calling convention ([Tracking issue](rust-lang/rust#74990))
    => Nightly => nightly;

    ...

    /// Latest stable release of Rust
    pub const LATEST_STABLE_RUST: RustTarget = RustTarget::Stable_1_68;

Thus add the `--rust-target 1.68` parameter. Add a comment as well
explaining this.

An alternative would be to use the currently running (i.e. actual) `rustc`
and `bindgen` versions to pick a "better" Rust target version. However,
that would introduce more moving parts depending on the user setup and
is also more complex to implement.

Starting with `bindgen` 0.71.0 [4], we will be able to set any future
Rust version instead, i.e. we will be able to set here our minimum
supported Rust version. Christian implemented it [5] after seeing this
patch. Thanks!

Cc: Christian Poveda <[email protected]>
Cc: Emilio Cobos Álvarez <[email protected]>
Cc: [email protected] # needed for 6.12.y; unneeded for 6.6.y; do not apply to 6.1.y
Fixes: c844fa6 ("rust: start supporting several `bindgen` versions")
Link: https://github.com/rust-lang/rust-bindgen/blob/21c60f473f4e824d4aa9b2b508056320d474b110/bindgen/features.rs#L97-L105 [1]
Link: rust-lang/rust-bindgen#2960 [2]
Link: https://github.com/rust-lang/rust-bindgen/blob/7d243056d335fdc4537f7bca73c06d01aae24ddc/bindgen/features.rs#L131-L150 [3]
Link: https://github.com/rust-lang/rust-bindgen/blob/main/CHANGELOG.md#0710-2024-12-06 [4]
Link: rust-lang/rust-bindgen#2993 [5]
Reviewed-by: Alice Ryhl <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>