rust: macros: rewrite module! using syn

Now that `syn` and `quote` are available in the Kernel use them for the
`module!` proc-macro instead of the current string based proc-macro
approach.

Proc-macros written using `syn` are a lot more readable and thus easier
to maintain than proc-macros written using string manipulation. An
additional advantage of `syn` is the improved error reporting, here is
an example:

    module! {
        type: RustMinimal,
        name: "rust_minimal",
        author: "Rust for Linux Contributors",
        description: "Rust minimal sample",
        // we forgot to add a `license` field.
    }

Prior to this patch, the error reads:

    error: proc macro panicked
      --> samples/rust/rust_minimal.rs:7:1
       |
    7  | / module! {
    8  | |     type: RustMinimal,
    9  | |     name: "rust_minimal",
    10 | |     author: "Rust for Linux Contributors",
    11 | |     description: "Rust minimal sample",
    12 | |     // license: "GPL",
    13 | | }
       | |_^
       |
       = help: message: Missing required key "license".

    error[E0425]: cannot find value `__LOG_PREFIX` in the crate root
      --> samples/rust/rust_minimal.rs:21:9
       |
    21 |         pr_info!("Rust minimal sample (init)\n");
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in the crate root
       |
       = note: this error originates in the macro `$crate::print_macro` which comes from the expansion of the macro `pr_info` (in Nightly builds, run with -Z macro-backtrace for more info)

    error[E0425]: cannot find value `__LOG_PREFIX` in the crate root
      --> samples/rust/rust_minimal.rs:22:9
       |
    22 |         pr_info!("Am I built-in? {}\n", !cfg!(MODULE));
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in the crate root
       |
       = note: this error originates in the macro `$crate::print_macro` which comes from the expansion of the macro `pr_info` (in Nightly builds, run with -Z macro-backtrace for more info)

    error[E0425]: cannot find value `__LOG_PREFIX` in the crate root
      --> samples/rust/rust_minimal.rs:35:9
       |
    35 |         pr_info!("My numbers are {:?}\n", self.numbers);
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in the crate root
       |
       = note: this error originates in the macro `$crate::print_macro` which comes from the expansion of the macro `pr_info` (in Nightly builds, run with -Z macro-backtrace for more info)

    error[E0425]: cannot find value `__LOG_PREFIX` in the crate root
      --> samples/rust/rust_minimal.rs:36:9
       |
    36 |         pr_info!("Rust minimal sample (exit)\n");
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in the crate root
       |
       = note: this error originates in the macro `$crate::print_macro` which comes from the expansion of the macro `pr_info` (in Nightly builds, run with -Z macro-backtrace for more info)

    error: aborting due to 5 previous errors

After this patch, the error reads:

    error: Missing required key "license".
      --> samples/rust/rust_minimal.rs:8:5
       |
    8  | /     type: RustMinimal,
    9  | |     name: "rust_minimal",
    10 | |     author: "Rust for Linux Contributors",
    11 | |     description: "Rust minimal sample",
       | |_______________________________________^

    error: aborting due to 1 previous error

Similar improvements in error quality apply to getting the order of the
fields wrong and giving non-ascii strings as values for fields where it
should be ascii-only.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>

Author: BennoLossin

rust/macros/helpers.rs

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 
+#![allow(dead_code)]
 use proc_macro2::{token_stream, Group, TokenStream, TokenTree};
 
 pub(crate) fn try_ident(it: &mut token_stream::IntoIter) -> Option<String> {

rust/macros/lib.rs

@@ -12,6 +12,7 @@ mod vtable;
 mod zeroable;
 
 use proc_macro::TokenStream;
+use quote::quote;
 use syn::parse_macro_input;
 
 /// Declares a kernel module.
@@ -74,8 +75,20 @@ use syn::parse_macro_input;
 ///   - `license`: byte array of the license of the kernel module (required).
 ///   - `alias`: byte array of alias name of the kernel module.
 #[proc_macro]
-pub fn module(ts: TokenStream) -> TokenStream {
-    module::module(ts.into()).into()
+pub fn module(input: TokenStream) -> TokenStream {
+    match syn::parse(input) {
+        Ok(input) => module::module(input),
+        Err(err) => {
+            let err = err.into_compile_error();
+            quote! {
+                // Supresses missing `__LOG_PREFIX` errors from printing macros.
+                const __LOG_PREFIX: &[u8] = b"";
+                // Due to this error, it will not compile, so an empty `__LOG_PREFIX` is fine.
+                #err
+            }
+        }
+    }
+    .into()
 }
 
 /// Declares or implements a vtable trait.