Update and add ui tests for core/std suggestions

tests/ui/default_instead_of_iter_empty_no_std.fixed

@@ -0,0 +1,28 @@
+#![warn(clippy::default_instead_of_iter_empty)]
+#![allow(dead_code)]
+#![feature(lang_items)]
+#![no_std]
+
+use core::panic::PanicInfo;
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+#[panic_handler]
+fn panic(info: &PanicInfo) -> ! {
+    loop {}
+}
+
+#[derive(Default)]
+struct Iter {
+    iter: core::iter::Empty<usize>,
+}
+
+fn main() {
+    // Do lint.
+    let _ = core::iter::empty::<usize>();
+    let _foo: core::iter::Empty<usize> = core::iter::empty();
+
+    // Do not lint.
+    let _ = Iter::default();
+}

tests/ui/default_instead_of_iter_empty_no_std.rs

@@ -0,0 +1,28 @@
+#![warn(clippy::default_instead_of_iter_empty)]
+#![allow(dead_code)]
+#![feature(lang_items)]
+#![no_std]
+
+use core::panic::PanicInfo;
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+#[panic_handler]
+fn panic(info: &PanicInfo) -> ! {
+    loop {}
+}
+
+#[derive(Default)]
+struct Iter {
+    iter: core::iter::Empty<usize>,
+}
+
+fn main() {
+    // Do lint.
+    let _ = core::iter::Empty::<usize>::default();
+    let _foo: core::iter::Empty<usize> = core::iter::Empty::default();
+
+    // Do not lint.
+    let _ = Iter::default();
+}

tests/ui/default_instead_of_iter_empty_no_std.stderr

@@ -0,0 +1,17 @@
+error: `core::iter::empty()` is the more idiomatic way
+  --> $DIR/default_instead_of_iter_empty_no_std.rs:23:13
+   |
+LL |     let _ = core::iter::Empty::<usize>::default();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `core::iter::empty::<usize>()`
+   |
+   = note: `-D clippy::default-instead-of-iter-empty` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::default_instead_of_iter_empty)]`
+
+error: `core::iter::empty()` is the more idiomatic way
+  --> $DIR/default_instead_of_iter_empty_no_std.rs:24:42
+   |
+LL |     let _foo: core::iter::Empty<usize> = core::iter::Empty::default();
+   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `core::iter::empty()`
+
+error: aborting due to 2 previous errors
+

tests/ui/mem_replace_no_std.fixed

@@ -0,0 +1,82 @@
+#![allow(unused)]
+#![warn(
+    clippy::all,
+    clippy::style,
+    clippy::mem_replace_option_with_none,
+    clippy::mem_replace_with_default
+)]
+#![feature(lang_items)]
+#![no_std]
+
+use core::mem;
+use core::panic::PanicInfo;
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+#[panic_handler]
+fn panic(info: &PanicInfo) -> ! {
+    loop {}
+}
+
+fn replace_option_with_none() {
+    let mut an_option = Some(1);
+    let _ = an_option.take();
+    let an_option = &mut Some(1);
+    let _ = an_option.take();
+}
+
+fn replace_with_default() {
+    let mut refstr = "hello";
+    let _ = core::mem::take(&mut refstr);
+
+    let mut slice: &[i32] = &[1, 2, 3];
+    let _ = core::mem::take(&mut slice);
+}
+
+// lint is disabled for primitives because in this case `take`
+// has no clear benefit over `replace` and sometimes is harder to read
+fn dont_lint_primitive() {
+    let mut pbool = true;
+    let _ = mem::replace(&mut pbool, false);
+
+    let mut pint = 5;
+    let _ = mem::replace(&mut pint, 0);
+}
+
+fn main() {
+    replace_option_with_none();
+    replace_with_default();
+    dont_lint_primitive();
+}
+
+fn issue9824() {
+    struct Foo<'a>(Option<&'a str>);
+    impl<'a> core::ops::Deref for Foo<'a> {
+        type Target = Option<&'a str>;
+
+        fn deref(&self) -> &Self::Target {
+            &self.0
+        }
+    }
+    impl<'a> core::ops::DerefMut for Foo<'a> {
+        fn deref_mut(&mut self) -> &mut Self::Target {
+            &mut self.0
+        }
+    }
+
+    struct Bar {
+        opt: Option<u8>,
+        val: u8,
+    }
+
+    let mut f = Foo(Some("foo"));
+    let mut b = Bar { opt: Some(1), val: 12 };
+
+    // replace option with none
+    let _ = f.0.take();
+    let _ = (*f).take();
+    let _ = b.opt.take();
+    // replace with default
+    let _ = mem::replace(&mut b.val, u8::default());
+}

tests/ui/mem_replace_no_std.rs

@@ -0,0 +1,82 @@
+#![allow(unused)]
+#![warn(
+    clippy::all,
+    clippy::style,
+    clippy::mem_replace_option_with_none,
+    clippy::mem_replace_with_default
+)]
+#![feature(lang_items)]
+#![no_std]
+
+use core::mem;
+use core::panic::PanicInfo;
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+#[panic_handler]
+fn panic(info: &PanicInfo) -> ! {
+    loop {}
+}
+
+fn replace_option_with_none() {
+    let mut an_option = Some(1);
+    let _ = mem::replace(&mut an_option, None);
+    let an_option = &mut Some(1);
+    let _ = mem::replace(an_option, None);
+}
+
+fn replace_with_default() {
+    let mut refstr = "hello";
+    let _ = mem::replace(&mut refstr, "");
+
+    let mut slice: &[i32] = &[1, 2, 3];
+    let _ = mem::replace(&mut slice, &[]);
+}
+
+// lint is disabled for primitives because in this case `take`
+// has no clear benefit over `replace` and sometimes is harder to read
+fn dont_lint_primitive() {
+    let mut pbool = true;
+    let _ = mem::replace(&mut pbool, false);
+
+    let mut pint = 5;
+    let _ = mem::replace(&mut pint, 0);
+}
+
+fn main() {
+    replace_option_with_none();
+    replace_with_default();
+    dont_lint_primitive();
+}
+
+fn issue9824() {
+    struct Foo<'a>(Option<&'a str>);
+    impl<'a> core::ops::Deref for Foo<'a> {
+        type Target = Option<&'a str>;
+
+        fn deref(&self) -> &Self::Target {
+            &self.0
+        }
+    }
+    impl<'a> core::ops::DerefMut for Foo<'a> {
+        fn deref_mut(&mut self) -> &mut Self::Target {
+            &mut self.0
+        }
+    }
+
+    struct Bar {
+        opt: Option<u8>,
+        val: u8,
+    }
+
+    let mut f = Foo(Some("foo"));
+    let mut b = Bar { opt: Some(1), val: 12 };
+
+    // replace option with none
+    let _ = mem::replace(&mut f.0, None);
+    let _ = mem::replace(&mut *f, None);
+    let _ = mem::replace(&mut b.opt, None);
+    // replace with default
+    let _ = mem::replace(&mut b.val, u8::default());
+}

tests/ui/mem_replace_no_std.stderr

@@ -0,0 +1,50 @@
+error: replacing an `Option` with `None`
+  --> $DIR/mem_replace_no_std.rs:24:13
+   |
+LL |     let _ = mem::replace(&mut an_option, None);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider `Option::take()` instead: `an_option.take()`
+   |
+   = note: `-D clippy::mem-replace-option-with-none` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::mem_replace_option_with_none)]`
+
+error: replacing an `Option` with `None`
+  --> $DIR/mem_replace_no_std.rs:26:13
+   |
+LL |     let _ = mem::replace(an_option, None);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider `Option::take()` instead: `an_option.take()`
+
+error: replacing a value of type `T` with `T::default()` is better expressed using `core::mem::take`
+  --> $DIR/mem_replace_no_std.rs:31:13
+   |
+LL |     let _ = mem::replace(&mut refstr, "");
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `core::mem::take(&mut refstr)`
+   |
+   = note: `-D clippy::mem-replace-with-default` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::mem_replace_with_default)]`
+
+error: replacing a value of type `T` with `T::default()` is better expressed using `core::mem::take`
+  --> $DIR/mem_replace_no_std.rs:34:13
+   |
+LL |     let _ = mem::replace(&mut slice, &[]);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `core::mem::take(&mut slice)`
+
+error: replacing an `Option` with `None`
+  --> $DIR/mem_replace_no_std.rs:77:13
+   |
+LL |     let _ = mem::replace(&mut f.0, None);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider `Option::take()` instead: `f.0.take()`
+
+error: replacing an `Option` with `None`
+  --> $DIR/mem_replace_no_std.rs:78:13
+   |
+LL |     let _ = mem::replace(&mut *f, None);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider `Option::take()` instead: `(*f).take()`
+
+error: replacing an `Option` with `None`
+  --> $DIR/mem_replace_no_std.rs:79:13
+   |
+LL |     let _ = mem::replace(&mut b.opt, None);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider `Option::take()` instead: `b.opt.take()`
+
+error: aborting due to 7 previous errors
+

tests/ui/ptr_eq_no_std.fixed

@@ -0,0 +1,49 @@
+#![warn(clippy::ptr_eq)]
+#![no_std]
+#![feature(lang_items)]
+
+use core::panic::PanicInfo;
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+#[panic_handler]
+fn panic(info: &PanicInfo) -> ! {
+    loop {}
+}
+
+macro_rules! mac {
+    ($a:expr, $b:expr) => {
+        $a as *const _ as usize == $b as *const _ as usize
+    };
+}
+
+macro_rules! another_mac {
+    ($a:expr, $b:expr) => {
+        $a as *const _ == $b as *const _
+    };
+}
+
+fn main() {
+    let a = &[1, 2, 3];
+    let b = &[1, 2, 3];
+
+    let _ = core::ptr::eq(a, b);
+    let _ = core::ptr::eq(a, b);
+    let _ = a.as_ptr() == b as *const _;
+    let _ = a.as_ptr() == b.as_ptr();
+
+    // Do not lint
+
+    let _ = mac!(a, b);
+    let _ = another_mac!(a, b);
+
+    let a = &mut [1, 2, 3];
+    let b = &mut [1, 2, 3];
+
+    let _ = a.as_mut_ptr() == b as *mut [i32] as *mut _;
+    let _ = a.as_mut_ptr() == b.as_mut_ptr();
+
+    let _ = a == b;
+    let _ = core::ptr::eq(a, b);
+}

tests/ui/ptr_eq_no_std.rs

@@ -0,0 +1,49 @@
+#![warn(clippy::ptr_eq)]
+#![no_std]
+#![feature(lang_items)]
+
+use core::panic::PanicInfo;
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+#[panic_handler]
+fn panic(info: &PanicInfo) -> ! {
+    loop {}
+}
+
+macro_rules! mac {
+    ($a:expr, $b:expr) => {
+        $a as *const _ as usize == $b as *const _ as usize
+    };
+}
+
+macro_rules! another_mac {
+    ($a:expr, $b:expr) => {
+        $a as *const _ == $b as *const _
+    };
+}
+
+fn main() {
+    let a = &[1, 2, 3];
+    let b = &[1, 2, 3];
+
+    let _ = a as *const _ as usize == b as *const _ as usize;
+    let _ = a as *const _ == b as *const _;
+    let _ = a.as_ptr() == b as *const _;
+    let _ = a.as_ptr() == b.as_ptr();
+
+    // Do not lint
+
+    let _ = mac!(a, b);
+    let _ = another_mac!(a, b);
+
+    let a = &mut [1, 2, 3];
+    let b = &mut [1, 2, 3];
+
+    let _ = a.as_mut_ptr() == b as *mut [i32] as *mut _;
+    let _ = a.as_mut_ptr() == b.as_mut_ptr();
+
+    let _ = a == b;
+    let _ = core::ptr::eq(a, b);
+}