Rollup merge of #62421 - JohnTitor:U007D-master, r=alexcrichton

Introduce `as_deref` to Option

This is re-submission for #59628.
Renames `deref()` to `as_deref()` and adds `deref_mut()` impls and tests.

CC #50264

r? @Kimundi
(I picked you as you're the previous reviewer.)

4 files changed, 241 insertions, 56 deletions

diff --git a/src/libcore/option.rs b/src/libcore/option.rs
index 33d6afdc975..abc8883d398 100644
--- a/src/libcore/option.rs
+++ b/src/libcore/option.rs
@@ -136,7 +136,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
-use crate::{convert, fmt, hint, mem, ops::{self, Deref}};
+use crate::{convert, fmt, hint, mem, ops::{self, Deref, DerefMut}};
 use crate::pin::Pin;
 
 // Note that this is not a lang item per se, but it has a hidden dependency on
@@ -1104,17 +1104,28 @@ impl<T: Default> Option<T> {
 
 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
 impl<T: Deref> Option<T> {
-    /// Converts from `&Option<T>` to `Option<&T::Target>`.
+    /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
     ///
     /// Leaves the original Option in-place, creating a new one with a reference
     /// to the original one, additionally coercing the contents via [`Deref`].
     ///
     /// [`Deref`]: ../../std/ops/trait.Deref.html
-    pub fn deref(&self) -> Option<&T::Target> {
+    pub fn as_deref(&self) -> Option<&T::Target> {
         self.as_ref().map(|t| t.deref())
     }
 }
 
+#[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
+impl<T: DerefMut> Option<T> {
+    /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
+    ///
+    /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
+    /// the inner type's `Deref::Target` type.
+    pub fn as_deref_mut(&mut self) -> Option<&mut T::Target> {
+        self.as_mut().map(|t| t.deref_mut())
+    }
+}
+
 impl<T, E> Option<Result<T, E>> {
     /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
     ///
diff --git a/src/libcore/result.rs b/src/libcore/result.rs
index 3a38b66ad01..cb6bc058730 100644
--- a/src/libcore/result.rs
+++ b/src/libcore/result.rs
@@ -232,7 +232,7 @@
 
 use crate::fmt;
 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
-use crate::ops::{self, Deref};
+use crate::ops::{self, Deref, DerefMut};
 
 /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
 ///
@@ -981,24 +981,22 @@ impl<T: Default, E> Result<T, E> {
 
 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
 impl<T: Deref, E> Result<T, E> {
-    /// Converts from `&Result<T, E>` to `Result<&T::Target, &E>`.
+    /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&T::Target, &E>`.
     ///
-    /// Leaves the original Result in-place, creating a new one with a reference
-    /// to the original one, additionally coercing the `Ok` arm of the Result via
-    /// `Deref`.
-    pub fn deref_ok(&self) -> Result<&T::Target, &E> {
+    /// Leaves the original `Result` in-place, creating a new one containing a reference to the
+    /// `Ok` type's `Deref::Target` type.
+    pub fn as_deref_ok(&self) -> Result<&T::Target, &E> {
         self.as_ref().map(|t| t.deref())
     }
 }
 
 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
 impl<T, E: Deref> Result<T, E> {
-    /// Converts from `&Result<T, E>` to `Result<&T, &E::Target>`.
+    /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&T, &E::Target>`.
     ///
-    /// Leaves the original Result in-place, creating a new one with a reference
-    /// to the original one, additionally coercing the `Err` arm of the Result via
-    /// `Deref`.
-    pub fn deref_err(&self) -> Result<&T, &E::Target>
+    /// Leaves the original `Result` in-place, creating a new one containing a reference to the
+    /// `Err` type's `Deref::Target` type.
+    pub fn as_deref_err(&self) -> Result<&T, &E::Target>
     {
         self.as_ref().map_err(|e| e.deref())
     }
@@ -1006,17 +1004,52 @@ impl<T, E: Deref> Result<T, E> {
 
 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
 impl<T: Deref, E: Deref> Result<T, E> {
-    /// Converts from `&Result<T, E>` to `Result<&T::Target, &E::Target>`.
+    /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&T::Target, &E::Target>`.
     ///
-    /// Leaves the original Result in-place, creating a new one with a reference
-    /// to the original one, additionally coercing both the `Ok` and `Err` arms
-    /// of the Result via `Deref`.
-    pub fn deref(&self) -> Result<&T::Target, &E::Target>
+    /// Leaves the original `Result` in-place, creating a new one containing a reference to both
+    /// the `Ok` and `Err` types' `Deref::Target` types.
+    pub fn as_deref(&self) -> Result<&T::Target, &E::Target>
     {
         self.as_ref().map(|t| t.deref()).map_err(|e| e.deref())
     }
 }
 
+#[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
+impl<T: DerefMut, E> Result<T, E> {
+    /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut T::Target, &mut E>`.
+    ///
+    /// Leaves the original `Result` in-place, creating a new one containing a mutable reference to
+    /// the `Ok` type's `Deref::Target` type.
+    pub fn as_deref_mut_ok(&mut self) -> Result<&mut T::Target, &mut E> {
+        self.as_mut().map(|t| t.deref_mut())
+    }
+}
+
+#[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
+impl<T, E: DerefMut> Result<T, E> {
+    /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut T, &mut E::Target>`.
+    ///
+    /// Leaves the original `Result` in-place, creating a new one containing a mutable reference to
+    /// the `Err` type's `Deref::Target` type.
+    pub fn as_deref_mut_err(&mut self) -> Result<&mut T, &mut E::Target>
+    {
+        self.as_mut().map_err(|e| e.deref_mut())
+    }
+}
+
+#[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
+impl<T: DerefMut, E: DerefMut> Result<T, E> {
+    /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to
+    /// `Result<&mut T::Target, &mut E::Target>`.
+    ///
+    /// Leaves the original `Result` in-place, creating a new one containing a mutable reference to
+    /// both the `Ok` and `Err` types' `Deref::Target` types.
+    pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E::Target>
+    {
+        self.as_mut().map(|t| t.deref_mut()).map_err(|e| e.deref_mut())
+    }
+}
+
 impl<T, E> Result<Option<T>, E> {
     /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
     ///
diff --git a/src/libcore/tests/option.rs b/src/libcore/tests/option.rs
index b059b134868..ff43fc49f71 100644
--- a/src/libcore/tests/option.rs
+++ b/src/libcore/tests/option.rs
@@ -1,6 +1,8 @@
 use core::option::*;
 use core::mem;
 use core::clone::Clone;
+use core::array::FixedSizeArray;
+use core::ops::DerefMut;
 
 #[test]
 fn test_get_ptr() {
@@ -310,20 +312,38 @@ fn test_try() {
 }
 
 #[test]
-fn test_option_deref() {
+fn test_option_as_deref() {
     // Some: &Option<T: Deref>::Some(T) -> Option<&T::Deref::Target>::Some(&*T)
     let ref_option = &Some(&42);
-    assert_eq!(ref_option.deref(), Some(&42));
+    assert_eq!(ref_option.as_deref(), Some(&42));
 
     let ref_option = &Some(String::from("a result"));
-    assert_eq!(ref_option.deref(), Some("a result"));
+    assert_eq!(ref_option.as_deref(), Some("a result"));
 
     let ref_option = &Some(vec![1, 2, 3, 4, 5]);
-    assert_eq!(ref_option.deref(), Some(&[1, 2, 3, 4, 5][..]));
+    assert_eq!(ref_option.as_deref(), Some([1, 2, 3, 4, 5].as_slice()));
 
     // None: &Option<T: Deref>>::None -> None
     let ref_option: &Option<&i32> = &None;
-    assert_eq!(ref_option.deref(), None);
+    assert_eq!(ref_option.as_deref(), None);
+}
+
+#[test]
+fn test_option_as_deref_mut() {
+    // Some: &mut Option<T: Deref>::Some(T) -> Option<&mut T::Deref::Target>::Some(&mut *T)
+    let mut val = 42;
+    let ref_option = &mut Some(&mut val);
+    assert_eq!(ref_option.as_deref_mut(), Some(&mut 42));
+
+    let ref_option = &mut Some(String::from("a result"));
+    assert_eq!(ref_option.as_deref_mut(), Some(String::from("a result").deref_mut()));
+
+    let ref_option = &mut Some(vec![1, 2, 3, 4, 5]);
+    assert_eq!(ref_option.as_deref_mut(), Some([1, 2, 3, 4, 5].as_mut_slice()));
+
+    // None: &mut Option<T: Deref>>::None -> None
+    let ref_option: &mut Option<&mut i32> = &mut None;
+    assert_eq!(ref_option.as_deref_mut(), None);
 }
 
 #[test]
diff --git a/src/libcore/tests/result.rs b/src/libcore/tests/result.rs
index 1fab07526a0..163f8d0ab37 100644
--- a/src/libcore/tests/result.rs
+++ b/src/libcore/tests/result.rs
@@ -1,4 +1,6 @@
 use core::option::*;
+use core::array::FixedSizeArray;
+use core::ops::DerefMut;
 
 fn op1() -> Result<isize, &'static str> { Ok(666) }
 fn op2() -> Result<isize, &'static str> { Err("sadface") }
@@ -225,94 +227,213 @@ fn test_try() {
 }
 
 #[test]
-fn test_result_deref() {
-    // &Result<T: Deref, E>::Ok(T).deref_ok() ->
+fn test_result_as_deref() {
+    // &Result<T: Deref, E>::Ok(T).as_deref_ok() ->
     //      Result<&T::Deref::Target, &E>::Ok(&*T)
     let ref_ok = &Result::Ok::<&i32, u8>(&42);
     let expected_result = Result::Ok::<&i32, &u8>(&42);
-    assert_eq!(ref_ok.deref_ok(), expected_result);
+    assert_eq!(ref_ok.as_deref_ok(), expected_result);
 
     let ref_ok = &Result::Ok::<String, u32>(String::from("a result"));
     let expected_result = Result::Ok::<&str, &u32>("a result");
-    assert_eq!(ref_ok.deref_ok(), expected_result);
+    assert_eq!(ref_ok.as_deref_ok(), expected_result);
 
     let ref_ok = &Result::Ok::<Vec<i32>, u32>(vec![1, 2, 3, 4, 5]);
-    let expected_result = Result::Ok::<&[i32], &u32>(&[1, 2, 3, 4, 5][..]);
-    assert_eq!(ref_ok.deref_ok(), expected_result);
+    let expected_result = Result::Ok::<&[i32], &u32>([1, 2, 3, 4, 5].as_slice());
+    assert_eq!(ref_ok.as_deref_ok(), expected_result);
 
-    // &Result<T: Deref, E: Deref>::Ok(T).deref() ->
+    // &Result<T: Deref, E: Deref>::Ok(T).as_deref() ->
     //      Result<&T::Deref::Target, &E::Deref::Target>::Ok(&*T)
     let ref_ok = &Result::Ok::<&i32, &u8>(&42);
     let expected_result = Result::Ok::<&i32, &u8>(&42);
-    assert_eq!(ref_ok.deref(), expected_result);
+    assert_eq!(ref_ok.as_deref(), expected_result);
 
     let ref_ok = &Result::Ok::<String, &u32>(String::from("a result"));
     let expected_result = Result::Ok::<&str, &u32>("a result");
-    assert_eq!(ref_ok.deref(), expected_result);
+    assert_eq!(ref_ok.as_deref(), expected_result);
 
     let ref_ok = &Result::Ok::<Vec<i32>, &u32>(vec![1, 2, 3, 4, 5]);
-    let expected_result = Result::Ok::<&[i32], &u32>(&[1, 2, 3, 4, 5][..]);
-    assert_eq!(ref_ok.deref(), expected_result);
+    let expected_result = Result::Ok::<&[i32], &u32>([1, 2, 3, 4, 5].as_slice());
+    assert_eq!(ref_ok.as_deref(), expected_result);
 
-    // &Result<T, E: Deref>::Err(T).deref_err() ->
+    // &Result<T, E: Deref>::Err(T).as_deref_err() ->
     //      Result<&T, &E::Deref::Target>::Err(&*E)
     let ref_err = &Result::Err::<u8, &i32>(&41);
     let expected_result = Result::Err::<&u8, &i32>(&41);
-    assert_eq!(ref_err.deref_err(), expected_result);
+    assert_eq!(ref_err.as_deref_err(), expected_result);
 
     let ref_err = &Result::Err::<u32, String>(String::from("an error"));
     let expected_result = Result::Err::<&u32, &str>("an error");
-    assert_eq!(ref_err.deref_err(), expected_result);
+    assert_eq!(ref_err.as_deref_err(), expected_result);
 
     let ref_err = &Result::Err::<u32, Vec<i32>>(vec![5, 4, 3, 2, 1]);
-    let expected_result = Result::Err::<&u32, &[i32]>(&[5, 4, 3, 2, 1][..]);
-    assert_eq!(ref_err.deref_err(), expected_result);
+    let expected_result = Result::Err::<&u32, &[i32]>([5, 4, 3, 2, 1].as_slice());
+    assert_eq!(ref_err.as_deref_err(), expected_result);
 
-    // &Result<T: Deref, E: Deref>::Err(T).deref_err() ->
+    // &Result<T: Deref, E: Deref>::Err(T).as_deref_err() ->
     //      Result<&T, &E::Deref::Target>::Err(&*E)
     let ref_err = &Result::Err::<&u8, &i32>(&41);
     let expected_result = Result::Err::<&u8, &i32>(&41);
-    assert_eq!(ref_err.deref(), expected_result);
+    assert_eq!(ref_err.as_deref(), expected_result);
 
     let ref_err = &Result::Err::<&u32, String>(String::from("an error"));
     let expected_result = Result::Err::<&u32, &str>("an error");
-    assert_eq!(ref_err.deref(), expected_result);
+    assert_eq!(ref_err.as_deref(), expected_result);
 
     let ref_err = &Result::Err::<&u32, Vec<i32>>(vec![5, 4, 3, 2, 1]);
-    let expected_result = Result::Err::<&u32, &[i32]>(&[5, 4, 3, 2, 1][..]);
-    assert_eq!(ref_err.deref(), expected_result);
+    let expected_result = Result::Err::<&u32, &[i32]>([5, 4, 3, 2, 1].as_slice());
+    assert_eq!(ref_err.as_deref(), expected_result);
 
-    // The following cases test calling deref_* with the wrong variant (i.e.
-    // `deref_ok()` with a `Result::Err()`, or `deref_err()` with a `Result::Ok()`.
-    // While unusual, these cases are supported to ensure that an `inner_deref`
+    // The following cases test calling `as_deref_*` with the wrong variant (i.e.
+    // `as_deref_ok()` with a `Result::Err()`, or `as_deref_err()` with a `Result::Ok()`.
+    // While uncommon, these cases are supported to ensure that an `as_deref_*`
     // call can still be made even when one of the Result types does not implement
     // `Deref` (for example, std::io::Error).
 
-    // &Result<T, E: Deref>::Ok(T).deref_err() ->
+    // &Result<T, E: Deref>::Ok(T).as_deref_err() ->
     //      Result<&T, &E::Deref::Target>::Ok(&T)
     let ref_ok = &Result::Ok::<i32, &u8>(42);
     let expected_result = Result::Ok::<&i32, &u8>(&42);
-    assert_eq!(ref_ok.deref_err(), expected_result);
+    assert_eq!(ref_ok.as_deref_err(), expected_result);
 
     let ref_ok = &Result::Ok::<&str, &u32>("a result");
     let expected_result = Result::Ok::<&&str, &u32>(&"a result");
-    assert_eq!(ref_ok.deref_err(), expected_result);
+    assert_eq!(ref_ok.as_deref_err(), expected_result);
 
     let ref_ok = &Result::Ok::<[i32; 5], &u32>([1, 2, 3, 4, 5]);
     let expected_result = Result::Ok::<&[i32; 5], &u32>(&[1, 2, 3, 4, 5]);
-    assert_eq!(ref_ok.deref_err(), expected_result);
+    assert_eq!(ref_ok.as_deref_err(), expected_result);
 
-    // &Result<T: Deref, E>::Err(E).deref_ok() ->
+    // &Result<T: Deref, E>::Err(E).as_deref_ok() ->
     //      Result<&T::Deref::Target, &E>::Err(&E)
     let ref_err = &Result::Err::<&u8, i32>(41);
     let expected_result = Result::Err::<&u8, &i32>(&41);
-    assert_eq!(ref_err.deref_ok(), expected_result);
+    assert_eq!(ref_err.as_deref_ok(), expected_result);
 
     let ref_err = &Result::Err::<&u32, &str>("an error");
     let expected_result = Result::Err::<&u32, &&str>(&"an error");
-    assert_eq!(ref_err.deref_ok(), expected_result);
+    assert_eq!(ref_err.as_deref_ok(), expected_result);
 
     let ref_err = &Result::Err::<&u32, [i32; 5]>([5, 4, 3, 2, 1]);
     let expected_result = Result::Err::<&u32, &[i32; 5]>(&[5, 4, 3, 2, 1]);
-    assert_eq!(ref_err.deref_ok(), expected_result);
+    assert_eq!(ref_err.as_deref_ok(), expected_result);
+}
+
+#[test]
+fn test_result_as_deref_mut() {
+    // &mut Result<T: Deref, E>::Ok(T).as_deref_mut_ok() ->
+    //      Result<&mut T::Deref::Target, &mut E>::Ok(&mut *T)
+    let mut val = 42;
+    let mut expected_val = 42;
+    let mut_ok = &mut Result::Ok::<&mut i32, u8>(&mut val);
+    let expected_result = Result::Ok::<&mut i32, &mut u8>(&mut expected_val);
+    assert_eq!(mut_ok.as_deref_mut_ok(), expected_result);
+
+    let mut expected_string = String::from("a result");
+    let mut_ok = &mut Result::Ok::<String, u32>(expected_string.clone());
+    let expected_result = Result::Ok::<&mut str, &mut u32>(expected_string.deref_mut());
+    assert_eq!(mut_ok.as_deref_mut_ok(), expected_result);
+
+    let mut expected_vec = vec![1, 2, 3, 4, 5];
+    let mut_ok = &mut Result::Ok::<Vec<i32>, u32>(expected_vec.clone());
+    let expected_result = Result::Ok::<&mut [i32], &mut u32>(expected_vec.as_mut_slice());
+    assert_eq!(mut_ok.as_deref_mut_ok(), expected_result);
+
+    // &mut Result<T: Deref, E: Deref>::Ok(T).as_deref_mut() ->
+    //      Result<&mut T::Deref::Target, &mut E::Deref::Target>::Ok(&mut *T)
+    let mut val = 42;
+    let mut expected_val = 42;
+    let mut_ok = &mut Result::Ok::<&mut i32, &mut u8>(&mut val);
+    let expected_result = Result::Ok::<&mut i32, &mut u8>(&mut expected_val);
+    assert_eq!(mut_ok.as_deref_mut(), expected_result);
+
+    let mut expected_string = String::from("a result");
+    let mut_ok = &mut Result::Ok::<String, &mut u32>(expected_string.clone());
+    let expected_result = Result::Ok::<&mut str, &mut u32>(expected_string.deref_mut());
+    assert_eq!(mut_ok.as_deref_mut(), expected_result);
+
+    let mut expected_vec = vec![1, 2, 3, 4, 5];
+    let mut_ok = &mut Result::Ok::<Vec<i32>, &mut u32>(expected_vec.clone());
+    let expected_result = Result::Ok::<&mut [i32], &mut u32>(expected_vec.as_mut_slice());
+    assert_eq!(mut_ok.as_deref_mut(), expected_result);
+
+    // &mut Result<T, E: Deref>::Err(T).as_deref_mut_err() ->
+    //      Result<&mut T, &mut E::Deref::Target>::Err(&mut *E)
+    let mut val = 41;
+    let mut expected_val = 41;
+    let mut_err = &mut Result::Err::<u8, &mut i32>(&mut val);
+    let expected_result = Result::Err::<&mut u8, &mut i32>(&mut expected_val);
+    assert_eq!(mut_err.as_deref_mut_err(), expected_result);
+
+    let mut expected_string = String::from("an error");
+    let mut_err = &mut Result::Err::<u32, String>(expected_string.clone());
+    let expected_result = Result::Err::<&mut u32, &mut str>(expected_string.deref_mut());
+    assert_eq!(mut_err.as_deref_mut_err(), expected_result);
+
+    let mut expected_vec = vec![5, 4, 3, 2, 1];
+    let mut_err = &mut Result::Err::<u32, Vec<i32>>(expected_vec.clone());
+    let expected_result = Result::Err::<&mut u32, &mut [i32]>(expected_vec.as_mut_slice());
+    assert_eq!(mut_err.as_deref_mut_err(), expected_result);
+
+    // &mut Result<T: Deref, E: Deref>::Err(T).as_deref_mut_err() ->
+    //      Result<&mut T, &mut E::Deref::Target>::Err(&mut *E)
+    let mut val = 41;
+    let mut expected_val = 41;
+    let mut_err = &mut Result::Err::<&mut u8, &mut i32>(&mut val);
+    let expected_result = Result::Err::<&mut u8, &mut i32>(&mut expected_val);
+    assert_eq!(mut_err.as_deref_mut(), expected_result);
+
+    let mut expected_string = String::from("an error");
+    let mut_err = &mut Result::Err::<&mut u32, String>(expected_string.clone());
+    let expected_result = Result::Err::<&mut u32, &mut str>(expected_string.as_mut_str());
+    assert_eq!(mut_err.as_deref_mut(), expected_result);
+
+    let mut expected_vec = vec![5, 4, 3, 2, 1];
+    let mut_err = &mut Result::Err::<&mut u32, Vec<i32>>(expected_vec.clone());
+    let expected_result = Result::Err::<&mut u32, &mut [i32]>(expected_vec.as_mut_slice());
+    assert_eq!(mut_err.as_deref_mut(), expected_result);
+
+    // The following cases test calling `as_deref_mut_*` with the wrong variant (i.e.
+    // `as_deref_mut_ok()` with a `Result::Err()`, or `as_deref_mut_err()` with a `Result::Ok()`.
+    // While uncommon, these cases are supported to ensure that an `as_deref_mut_*`
+    // call can still be made even when one of the Result types does not implement
+    // `Deref` (for example, std::io::Error).
+
+    // &mut Result<T, E: Deref>::Ok(T).as_deref_mut_err() ->
+    //      Result<&mut T, &mut E::Deref::Target>::Ok(&mut T)
+    let mut expected_val = 42;
+    let mut_ok = &mut Result::Ok::<i32, &mut u8>(expected_val.clone());
+    let expected_result = Result::Ok::<&mut i32, &mut u8>(&mut expected_val);
+    assert_eq!(mut_ok.as_deref_mut_err(), expected_result);
+
+    let string = String::from("a result");
+    let expected_string = string.clone();
+    let mut ref_str = expected_string.as_ref();
+    let mut_ok = &mut Result::Ok::<&str, &mut u32>(string.as_str());
+    let expected_result = Result::Ok::<&mut &str, &mut u32>(&mut ref_str);
+    assert_eq!(mut_ok.as_deref_mut_err(), expected_result);
+
+    let mut expected_arr = [1, 2, 3, 4, 5];
+    let mut_ok = &mut Result::Ok::<[i32; 5], &mut u32>(expected_arr.clone());
+    let expected_result = Result::Ok::<&mut [i32; 5], &mut u32>(&mut expected_arr);
+    assert_eq!(mut_ok.as_deref_mut_err(), expected_result);
+
+    // &mut Result<T: Deref, E>::Err(E).as_deref_mut_ok() ->
+    //      Result<&mut T::Deref::Target, &mut E>::Err(&mut E)
+    let mut expected_val = 41;
+    let mut_err = &mut Result::Err::<&mut u8, i32>(expected_val.clone());
+    let expected_result = Result::Err::<&mut u8, &mut i32>(&mut expected_val);
+    assert_eq!(mut_err.as_deref_mut_ok(), expected_result);
+
+    let string = String::from("an error");
+    let expected_string = string.clone();
+    let mut ref_str = expected_string.as_ref();
+    let mut_err = &mut Result::Err::<&mut u32, &str>(string.as_str());
+    let expected_result = Result::Err::<&mut u32, &mut &str>(&mut ref_str);
+    assert_eq!(mut_err.as_deref_mut_ok(), expected_result);
+
+    let mut expected_arr = [5, 4, 3, 2, 1];
+    let mut_err = &mut Result::Err::<&mut u32, [i32; 5]>(expected_arr.clone());
+    let expected_result = Result::Err::<&mut u32, &mut [i32; 5]>(&mut expected_arr);
+    assert_eq!(mut_err.as_deref_mut_ok(), expected_result);
 }