linux-kernel - [PATCH v2 10/28] rust: error: add `From` implementations for `Error`

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Date:   Fri,  2 Dec 2022 17:14:41 +0100
From:   ojeda@...nel.org
To:     Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>
Cc:     rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev, Adam Bratschi-Kaye <ark.email@...il.com>,
        Nándor István Krácser 
        <bonifaido@...il.com>, Finn Behrens <me@...enk.dev>
Subject: [PATCH v2 10/28] rust: error: add `From` implementations for `Error`

From: Wedson Almeida Filho <wedsonaf@...il.com>

Add a set of `From` implementations for the `Error` kernel type.

These implementations allow to easily convert from standard Rust
error types to the usual kernel errors based on one of the `E*`
integer codes.

On top of that, the question mark Rust operator (`?`) implicitly
performs a conversion on the error value using the `From` trait
when propagating. Thus it is extra convenient to use.

For instance, a kernel function that needs to convert a `i64` into
a `i32` and to bubble up the error as a kernel error may write:

    fn f(x: i64) -> Result<...> {
        ...
        let y = i32::try_from(x)?;
        ...
    }

which will transform the `TryFromIntError` into an `Err(EINVAL)`.

Co-developed-by: Adam Bratschi-Kaye <ark.email@...il.com>
Signed-off-by: Adam Bratschi-Kaye <ark.email@...il.com>
Co-developed-by: Nándor István Krácser <bonifaido@...il.com>
Signed-off-by: Nándor István Krácser <bonifaido@...il.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@...il.com>
Reviewed-by: Finn Behrens <me@...enk.dev>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
---
 rust/kernel/error.rs | 45 +++++++++++++++++++++++++++++++++++++++++++-
 rust/kernel/lib.rs   |  1 +
 2 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index 861746f2422d..5b9751d7ff1d 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -4,7 +4,14 @@
 //!
 //! C header: [`include/uapi/asm-generic/errno-base.h`](../../../include/uapi/asm-generic/errno-base.h)
 
-use alloc::collections::TryReserveError;
+use alloc::{
+    alloc::{AllocError, LayoutError},
+    collections::TryReserveError,
+};
+
+use core::convert::From;
+use core::num::TryFromIntError;
+use core::str::Utf8Error;
 
 /// Contains the C-compatible error codes.
 pub mod code {
@@ -71,12 +78,48 @@ impl Error {
     }
 }
 
+impl From<AllocError> for Error {
+    fn from(_: AllocError) -> Error {
+        code::ENOMEM
+    }
+}
+
+impl From<TryFromIntError> for Error {
+    fn from(_: TryFromIntError) -> Error {
+        code::EINVAL
+    }
+}
+
+impl From<Utf8Error> for Error {
+    fn from(_: Utf8Error) -> Error {
+        code::EINVAL
+    }
+}
+
 impl From<TryReserveError> for Error {
     fn from(_: TryReserveError) -> Error {
         code::ENOMEM
     }
 }
 
+impl From<LayoutError> for Error {
+    fn from(_: LayoutError) -> Error {
+        code::ENOMEM
+    }
+}
+
+impl From<core::fmt::Error> for Error {
+    fn from(_: core::fmt::Error) -> Error {
+        code::EINVAL
+    }
+}
+
+impl From<core::convert::Infallible> for Error {
+    fn from(e: core::convert::Infallible) -> Error {
+        match e {}
+    }
+}
+
 /// A [`Result`] with an [`Error`] error type.
 ///
 /// To be used as the return type for functions that may fail.
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index abd46261d385..ffc6626a6d29 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -12,6 +12,7 @@
 //! do so first instead of bypassing this crate.
 
 #![no_std]
+#![feature(allocator_api)]
 #![feature(core_ffi_c)]
 
 // Ensure conditional compilation based on the kernel configuration works;
-- 
2.38.1