Auto merge of #6179 - flip1995:rewrite_use_self, r=phansch

Rework use_self impl based on ty::Ty comparison #3410 | Take 2

This builds on top of #5531

I already reviewed and approved the commits by `@montrivo.` So only the review of my commits should be necessary.

I would also appreciate your review `@montrivo,` since you are familiar with the challenges here.

Fixes #3410 and Fixes #4143 (same problem)
Fixes #2843
Fixes #3859
Fixes #4734 and fixes #6221
Fixes #4305
Fixes #5078 (even at expression level now 🎉)
Fixes #3881 and Fixes #4887 (same problem)
Fixes #3909

Not yet: #4140 (test added)

All the credit for the fixes goes to `@montrivo.` I only refactored and copy and pasted his code.

changelog: rewrite [`use_self`] lint and fix multiple (8) FPs. One to go.

tests/ui/auxiliary/proc_macro_derive.rs

@@ -41,3 +41,15 @@ pub fn derive_foo(_input: TokenStream) -> TokenStream {
         }
     }
 }
+
+#[proc_macro_derive(StructAUseSelf)]
+pub fn derive_use_self(_input: TokenStream) -> proc_macro::TokenStream {
+    quote! {
+        struct A;
+        impl A {
+            fn new() -> A {
+                A
+            }
+        }
+    }
+}

tests/ui/use_self.fixed

@@ -1,9 +1,13 @@
 // run-rustfix
 // edition:2018
+// aux-build:proc_macro_derive.rs
 
 #![warn(clippy::use_self)]
 #![allow(dead_code)]
-#![allow(clippy::should_implement_trait, clippy::upper_case_acronyms)]
+#![allow(clippy::should_implement_trait, clippy::upper_case_acronyms, clippy::from_over_into)]
+
+#[macro_use]
+extern crate proc_macro_derive;
 
 fn main() {}
 
@@ -71,13 +75,12 @@ mod lifetimes {
 
 mod issue2894 {
     trait IntoBytes {
-        #[allow(clippy::wrong_self_convention)]
-        fn into_bytes(&self) -> Vec<u8>;
+        fn to_bytes(&self) -> Vec<u8>;
     }
 
     // This should not be linted
     impl IntoBytes for u8 {
-        fn into_bytes(&self) -> Vec<u8> {
+        fn to_bytes(&self) -> Vec<u8> {
             vec![*self]
         }
     }
@@ -110,17 +113,20 @@ mod tuple_structs {
 mod macros {
     macro_rules! use_self_expand {
         () => {
-            fn new() -> Self {
-                Self {}
+            fn new() -> Foo {
+                Foo {}
             }
         };
     }
 
     struct Foo {}
 
     impl Foo {
-        use_self_expand!(); // Should lint in local macros
+        use_self_expand!(); // Should not lint in local macros
     }
+
+    #[derive(StructAUseSelf)] // Should not lint in derives
+    struct A;
 }
 
 mod nesting {
@@ -177,11 +183,22 @@ mod issue3410 {
     struct B;
 
     trait Trait<T> {
-        fn a(v: T);
+        fn a(v: T) -> Self;
     }
 
     impl Trait<Vec<A>> for Vec<B> {
-        fn a(_: Vec<A>) {}
+        fn a(_: Vec<A>) -> Self {
+            unimplemented!()
+        }
+    }
+
+    impl<T> Trait<Vec<A>> for Vec<T>
+    where
+        T: Trait<B>,
+    {
+        fn a(v: Vec<A>) -> Self {
+            <Vec<B>>::a(v).into_iter().map(Trait::a).collect()
+        }
     }
 }
 
@@ -252,3 +269,192 @@ mod paths_created_by_lowering {
         }
     }
 }
+
+// reused from #1997
+mod generics {
+    struct Foo<T> {
+        value: T,
+    }
+
+    impl<T> Foo<T> {
+        // `Self` is applicable here
+        fn foo(value: T) -> Self {
+            Self { value }
+        }
+
+        // `Cannot` use `Self` as a return type as the generic types are different
+        fn bar(value: i32) -> Foo<i32> {
+            Foo { value }
+        }
+    }
+}
+
+mod issue4140 {
+    pub struct Error<From, To> {
+        _from: From,
+        _too: To,
+    }
+
+    pub trait From<T> {
+        type From;
+        type To;
+
+        fn from(value: T) -> Self;
+    }
+
+    pub trait TryFrom<T>
+    where
+        Self: Sized,
+    {
+        type From;
+        type To;
+
+        fn try_from(value: T) -> Result<Self, Error<Self::From, Self::To>>;
+    }
+
+    impl<F, T> TryFrom<F> for T
+    where
+        T: From<F>,
+    {
+        type From = Self;
+        type To = Self;
+
+        fn try_from(value: F) -> Result<Self, Error<Self::From, Self::To>> {
+            Ok(From::from(value))
+        }
+    }
+
+    impl From<bool> for i64 {
+        type From = bool;
+        type To = Self;
+
+        fn from(value: bool) -> Self {
+            if value {
+                100
+            } else {
+                0
+            }
+        }
+    }
+}
+
+mod issue2843 {
+    trait Foo {
+        type Bar;
+    }
+
+    impl Foo for usize {
+        type Bar = u8;
+    }
+
+    impl<T: Foo> Foo for Option<T> {
+        type Bar = Option<T::Bar>;
+    }
+}
+
+mod issue3859 {
+    pub struct Foo;
+    pub struct Bar([usize; 3]);
+
+    impl Foo {
+        pub const BAR: usize = 3;
+
+        pub fn foo() {
+            const _X: usize = Foo::BAR;
+            // const _Y: usize = Self::BAR;
+        }
+    }
+}
+
+mod issue4305 {
+    trait Foo: 'static {}
+
+    struct Bar;
+
+    impl Foo for Bar {}
+
+    impl<T: Foo> From<T> for Box<dyn Foo> {
+        fn from(t: T) -> Self {
+            Box::new(t)
+        }
+    }
+}
+
+mod lint_at_item_level {
+    struct Foo {}
+
+    #[allow(clippy::use_self)]
+    impl Foo {
+        fn new() -> Foo {
+            Foo {}
+        }
+    }
+
+    #[allow(clippy::use_self)]
+    impl Default for Foo {
+        fn default() -> Foo {
+            Foo::new()
+        }
+    }
+}
+
+mod lint_at_impl_item_level {
+    struct Foo {}
+
+    impl Foo {
+        #[allow(clippy::use_self)]
+        fn new() -> Foo {
+            Foo {}
+        }
+    }
+
+    impl Default for Foo {
+        #[allow(clippy::use_self)]
+        fn default() -> Foo {
+            Foo::new()
+        }
+    }
+}
+
+mod issue4734 {
+    #[repr(C, packed)]
+    pub struct X {
+        pub x: u32,
+    }
+
+    impl From<X> for u32 {
+        fn from(c: X) -> Self {
+            unsafe { core::mem::transmute(c) }
+        }
+    }
+}
+
+mod nested_paths {
+    use std::convert::Into;
+    mod submod {
+        pub struct B {}
+        pub struct C {}
+
+        impl Into<C> for B {
+            fn into(self) -> C {
+                C {}
+            }
+        }
+    }
+
+    struct A<T> {
+        t: T,
+    }
+
+    impl<T> A<T> {
+        fn new<V: Into<T>>(v: V) -> Self {
+            Self { t: Into::into(v) }
+        }
+    }
+
+    impl A<submod::C> {
+        fn test() -> Self {
+            Self::new::<submod::B>(submod::B {})
+        }
+    }
+}