Add Borrow and BorrowMut derives

Cargo.toml

@@ -45,6 +45,8 @@ add_assign = []
 add = []
 as_mut = []
 as_ref = []
+borrow = []
+borrow_mut = []
 constructor = []
 deref = []
 deref_mut = []
@@ -71,6 +73,8 @@ default = [
     "add",
     "as_mut",
     "as_ref",
+    "borrow",
+    "borrow_mut",
     "constructor",
     "deref",
     "deref_mut",

README.md

@@ -82,6 +82,8 @@ These are traits that are used to convert automatically between types.
 5. [`IntoIterator`]
 6. [`AsRef`]
 7. [`AsMut`]
+8. [`Borrow`]
+9. [`BorrowMut`]
 
 ### Formatting traits
 
@@ -162,6 +164,8 @@ extern crate derive_more;
 [`IntoIterator`]: https://jeltef.github.io/derive_more/derive_more/into_iterator.html
 [`AsRef`]: https://jeltef.github.io/derive_more/derive_more/as_ref.html
 [`AsMut`]: https://jeltef.github.io/derive_more/derive_more/as_mut.html
+[`Borrow`]: https://jeltef.github.io/derive_more/derive_more/borrow.html
+[`BorrowMut`]: https://jeltef.github.io/derive_more/derive_more/borrow_mut.html
 
 [`Display`-like]: https://jeltef.github.io/derive_more/derive_more/display.html
 

doc/borrow.md

@@ -0,0 +1,130 @@
+% What #[derive(Borrow)] generates
+
+Deriving `Borrow` generates one or more implementations of `Borrow`, each
+corresponding to one of the fields of the decorated type.
+This allows types which contain some `T` to be passed anywhere that an
+`Borrow<T>` is accepted.
+
+# Newtypes and Structs with One Field
+
+When `Borrow` is derived for a newtype or struct with one field, a single
+implementation is generated to expose the underlying field.
+
+```rust
+# use core::borrow::Borrow;
+# #[macro_use] extern crate derive_more;
+# fn main(){}
+#[derive(Borrow)]
+struct MyWrapper(String);
+```
+
+Generates:
+
+```rust
+# use core::borrow::Borrow;
+# struct MyWrapper(String);
+impl Borrow<String> for MyWrapper {
+    fn borrow(&self) -> &String {
+        &self.0
+    }
+}
+```
+
+It's also possible to use the `#[borrow(forward)]` attribute to forward
+to the `borrow` implementation of the field. So here `SigleFieldForward`
+implements all `Borrow` for all types that `Vec<i32>` implements `Borrow` for.
+
+```rust
+# use core::borrow::Borrow;
+# #[macro_use] extern crate derive_more;
+#[derive(Borrow)]
+#[borrow(forward)]
+struct SingleFieldForward(Vec<i32>);
+
+fn main() {
+    let item = SingleFieldForward(vec![]);
+    let _: &[i32] = (&item).borrow();
+}
+
+```
+
+This generates:
+
+```rust
+# struct SingleFieldForward(Vec<i32>);
+impl<__BorrowT: ?::core::marker::Sized> ::core::borrow::Borrow<__BorrowT> for SingleFieldForward
+where
+    Vec<i32>: ::core::borrow::Borrow<__BorrowT>,
+{
+    #[inline]
+    fn borrow(&self) -> &__BorrowT {
+        <Vec<i32> as ::core::borrow::Borrow<__BorrowT>>::borrow(&self.0)
+    }
+}
+```
+
+# Structs with Multiple Fields
+
+When `Borrow` is derived for a struct with more than one field (including tuple
+structs), you must also mark one or more fields with the `#[borrow]` attribute.
+An implementation will be generated for each indicated field.
+You can also exclude a specific field by using `#[borrow(ignore)]`.
+
+```rust
+# use core::borrow::Borrow;
+# #[macro_use] extern crate derive_more;
+# fn main(){}
+#[derive(Borrow)]
+struct MyWrapper {
+    #[borrow]
+    name: String,
+    #[borrow]
+    num: i32,
+    valid: bool,
+}
+
+```
+
+Generates:
+
+```rust
+# use core::borrow::Borrow;
+# struct MyWrapper {
+#     name: String,
+#     num: i32,
+#     valid: bool,
+# }
+impl Borrow<String> for MyWrapper {
+    fn borrow(&self) -> &String {
+        &self.name
+    }
+}
+
+impl Borrow<i32> for MyWrapper {
+    fn borrow(&self) -> &i32 {
+        &self.num
+    }
+}
+```
+
+Note that `Borrow<T>` may only be implemented once for any given type `T`.
+This means any attempt to mark more than one field of the same type with
+`#[borrow]` will result in a compilation error.
+
+```compile_fail
+# use core::borrow::Borrow;
+# #[macro_use] extern crate derive_more;
+# fn main(){}
+// Error! Conflicting implementations of Borrow<String>
+#[derive(Borrow)]
+struct MyWrapper {
+    #[borrow]
+    str1: String,
+    #[borrow]
+    str2: String,
+}
+```
+
+# Enums
+
+Deriving `Borrow` for enums is not supported.

doc/borrow_mut.md

@@ -0,0 +1,149 @@
+% What #[derive(BorrowMut)] generates
+
+Deriving `BorrowMut` generates one or more implementations of `BorrowMut`, each
+corresponding to one of the fields of the decorated type.
+This allows types which contain some `T` to be passed anywhere that an
+`BorrowMut<T>` is accepted.
+
+Note that `BorrowMut<T>` expects the type to also implement `Borrow<T>`.
+
+# Newtypes and Structs with One Field
+
+When `BorrowMut` is derived for a newtype or struct with one field, a single
+implementation is generated to expose the underlying field.
+
+```rust
+# #[macro_use] extern crate derive_more;
+# use core::borrow::Borrow;
+# use core::borrow::BorrowMut;
+# fn main(){}
+#[derive(Borrow, BorrowMut)]
+struct MyWrapper(String);
+```
+
+Generates:
+
+```rust
+# #[macro_use] extern crate derive_more;
+# use core::borrow::Borrow;
+# use core::borrow::BorrowMut;
+# #[derive(Borrow)]
+# struct MyWrapper(String);
+impl BorrowMut<String> for MyWrapper {
+    fn borrow_mut(&mut self) -> &mut String {
+        &mut self.0
+    }
+}
+```
+
+It's also possible to use the `#[borrow_mut(forward)]` attribute to forward
+to the `borrow_mut` implementation of the field. So here `SigleFieldForward`
+implements all `BorrowMut` for all types that `Vec<i32>` implements `BorrowMut` for.
+
+```rust
+# #[macro_use] extern crate derive_more;
+# use core::borrow::Borrow;
+# use core::borrow::BorrowMut;
+#[derive(Borrow, BorrowMut)]
+#[borrow(forward)]
+#[borrow_mut(forward)]
+struct SingleFieldForward(Vec<i32>);
+
+fn main() {
+    let mut item = SingleFieldForward(vec![]);
+    let _: &mut [i32] = (&mut item).borrow_mut();
+}
+
+```
+
+This generates:
+
+```rust
+# #[macro_use] extern crate derive_more;
+# #[derive(Borrow)]
+# #[borrow(forward)]
+# struct SingleFieldForward(Vec<i32>);
+impl<__BorrowMutT: ?::core::marker::Sized> ::core::borrow::BorrowMut<__BorrowMutT> for SingleFieldForward
+where
+    Vec<i32>: ::core::borrow::BorrowMut<__BorrowMutT>,
+{
+    #[inline]
+    fn borrow_mut(&mut self) -> &mut __BorrowMutT {
+        <Vec<i32> as ::core::borrow::BorrowMut<__BorrowMutT>>::borrow_mut(&mut self.0)
+    }
+}
+```
+
+
+# Structs with Multiple Fields
+
+When `BorrowMut` is derived for a struct with more than one field (including tuple
+structs), you must also mark one or more fields with the `#[borrow_mut]` attribute.
+An implementation will be generated for each indicated field.
+You can also exclude a specific field by using `#[borrow_mut(ignore)]`.
+
+```rust
+# #[macro_use] extern crate derive_more;
+# use core::borrow::Borrow;
+# use core::borrow::BorrowMut;
+# fn main(){}
+#[derive(Borrow, BorrowMut)]
+struct MyWrapper {
+    #[borrow]
+    #[borrow_mut]
+    name: String,
+    #[borrow]
+    #[borrow_mut]
+    num: i32,
+    valid: bool,
+}
+
+
+```
+
+Generates:
+
+```rust
+# #[macro_use] extern crate derive_more;
+# use core::borrow::Borrow;
+# use core::borrow::BorrowMut;
+# #[derive(Borrow)]
+# struct MyWrapper {
+#     name: String,
+#     num: i32,
+#     valid: bool,
+# }
+impl BorrowMut<String> for MyWrapper {
+    fn borrow_mut(&mut self) -> &mut String {
+        &mut self.name
+    }
+}
+
+impl BorrowMut<i32> for MyWrapper {
+    fn borrow_mut(&mut self) -> &mut i32 {
+        &mut self.num
+    }
+}
+```
+
+Note that `BorrowMut<T>` may only be implemented once for any given type `T`. This means any attempt to
+mark more than one field of the same type with `#[borrow_mut]` will result in a compilation error.
+
+```compile_fail
+# #[macro_use] extern crate derive_more;
+# use core::borrow::Borrow;
+# use core::borrow::BorrowMut;
+# fn main(){}
+// Error! Conflicting implementations of BorrowMut<String>
+#[derive(BorrowMut)]
+struct MyWrapper {
+    #[borrow_mut]
+    str1: String,
+    #[borrow_mut]
+    str2: String,
+}
+```
+
+# Enums
+
+Deriving `BorrowMut` for enums is not supported.