rust: use `kernel::{fmt,prelude::fmt!}`

Reduce coupling to implementation details of the formatting machinery by
avoiding direct use for `core`'s formatting traits and macros.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250704-core-cstr-prepare-v1-3-a91524037783@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/error.rs

@@ -6,10 +6,10 @@
 
 use crate::{
     alloc::{layout::LayoutError, AllocError},
+    fmt,
     str::CStr,
 };
 
-use core::fmt;
 use core::num::NonZeroI32;
 use core::num::TryFromIntError;
 use core::str::Utf8Error;
@@ -219,8 +219,8 @@ fn from(_: LayoutError) -> Error {
     }
 }
 
-impl From<core::fmt::Error> for Error {
+impl From<fmt::Error> for Error {
-    fn from(_: core::fmt::Error) -> Error {
+    fn from(_: fmt::Error) -> Error {
         code::EINVAL
     }
 }

rust/kernel/print.rs

@@ -8,10 +8,10 @@
 
 use crate::{
     ffi::{c_char, c_void},
+    fmt,
     prelude::*,
     str::RawFormatter,
 };
-use core::fmt;
 
 // Called from `vsprintf` with format specifier `%pA`.
 #[expect(clippy::missing_safety_doc)]
@@ -149,7 +149,7 @@ macro_rules! print_macro (
         // takes borrows on the arguments, but does not extend the scope of temporaries.
         // Therefore, a `match` expression is used to keep them around, since
         // the scrutinee is kept until the end of the `match`.
-        match format_args!($($arg)+) {
+        match $crate::prelude::fmt!($($arg)+) {
             // SAFETY: This hidden macro should only be called by the documented
             // printing macros which ensure the format string is one of the fixed
             // ones. All `__LOG_PREFIX`s are null-terminated as they are generated
@@ -168,7 +168,7 @@ macro_rules! print_macro (
     // The `CONT` case.
     ($format_string:path, true, $($arg:tt)+) => (
         $crate::print::call_printk_cont(
-            format_args!($($arg)+),
+            $crate::prelude::fmt!($($arg)+),
         );
     );
 );

rust/kernel/str.rs

@@ -3,7 +3,7 @@
 //! String representations.
 
 use crate::alloc::{flags::*, AllocError, KVec};
-use core::fmt::{self, Write};
+use crate::fmt::{self, Write};
 use core::ops::{self, Deref, DerefMut, Index};
 
 use crate::prelude::*;

samples/rust/rust_print_main.rs

@@ -40,7 +40,7 @@ fn arc_print() -> Result {
         // behaviour, contract or protocol on both `i32` and `&str` into a single `Arc` of
         // type `Arc<dyn Display>`.
 
-        use core::fmt::Display;
+        use kernel::fmt::Display;
         fn arc_dyn_print(arc: &Arc<dyn Display>) {
             pr_info!("Arc<dyn Display> says {arc}");
         }