Auto merge of #3978 - phansch:rustfix_len_zero, r=flip1995

Add run-rustfix for len_zero lint

* Extracts len_without_is_empty into separate file
* Adds `// run-rustfix` to `tests/ui/len_zero.rs`

cc #3630

tests/ui/len_without_is_empty.rs

@@ -0,0 +1,145 @@
+#![warn(clippy::len_without_is_empty)]
+#![allow(dead_code, unused)]
+
+pub struct PubOne;
+
+impl PubOne {
+    pub fn len(self: &Self) -> isize {
+        1
+    }
+}
+
+impl PubOne {
+    // A second impl for this struct -- the error span shouldn't mention this.
+    pub fn irrelevant(self: &Self) -> bool {
+        false
+    }
+}
+
+// Identical to `PubOne`, but with an `allow` attribute on the impl complaining `len`.
+pub struct PubAllowed;
+
+#[allow(clippy::len_without_is_empty)]
+impl PubAllowed {
+    pub fn len(self: &Self) -> isize {
+        1
+    }
+}
+
+// No `allow` attribute on this impl block, but that doesn't matter -- we only require one on the
+// impl containing `len`.
+impl PubAllowed {
+    pub fn irrelevant(self: &Self) -> bool {
+        false
+    }
+}
+
+pub trait PubTraitsToo {
+    fn len(self: &Self) -> isize;
+}
+
+impl PubTraitsToo for One {
+    fn len(self: &Self) -> isize {
+        0
+    }
+}
+
+pub struct HasIsEmpty;
+
+impl HasIsEmpty {
+    pub fn len(self: &Self) -> isize {
+        1
+    }
+
+    fn is_empty(self: &Self) -> bool {
+        false
+    }
+}
+
+pub struct HasWrongIsEmpty;
+
+impl HasWrongIsEmpty {
+    pub fn len(self: &Self) -> isize {
+        1
+    }
+
+    pub fn is_empty(self: &Self, x: u32) -> bool {
+        false
+    }
+}
+
+struct NotPubOne;
+
+impl NotPubOne {
+    pub fn len(self: &Self) -> isize {
+        // No error; `len` is pub but `NotPubOne` is not exported anyway.
+        1
+    }
+}
+
+struct One;
+
+impl One {
+    fn len(self: &Self) -> isize {
+        // No error; `len` is private; see issue #1085.
+        1
+    }
+}
+
+trait TraitsToo {
+    fn len(self: &Self) -> isize;
+    // No error; `len` is private; see issue #1085.
+}
+
+impl TraitsToo for One {
+    fn len(self: &Self) -> isize {
+        0
+    }
+}
+
+struct HasPrivateIsEmpty;
+
+impl HasPrivateIsEmpty {
+    pub fn len(self: &Self) -> isize {
+        1
+    }
+
+    fn is_empty(self: &Self) -> bool {
+        false
+    }
+}
+
+struct Wither;
+
+pub trait WithIsEmpty {
+    fn len(self: &Self) -> isize;
+    fn is_empty(self: &Self) -> bool;
+}
+
+impl WithIsEmpty for Wither {
+    fn len(self: &Self) -> isize {
+        1
+    }
+
+    fn is_empty(self: &Self) -> bool {
+        false
+    }
+}
+
+pub trait Empty {
+    fn is_empty(&self) -> bool;
+}
+
+pub trait InheritingEmpty: Empty {
+    // Must not trigger `LEN_WITHOUT_IS_EMPTY`.
+    fn len(&self) -> isize;
+}
+
+// This used to ICE.
+pub trait Foo: Sized {}
+
+pub trait DependsOnFoo: Foo {
+    fn len(&mut self) -> usize;
+}
+
+fn main() {}

tests/ui/len_without_is_empty.stderr

@@ -0,0 +1,54 @@
+error: item `PubOne` has a public `len` method but no corresponding `is_empty` method
+  --> $DIR/len_without_is_empty.rs:6:1
+   |
+LL | / impl PubOne {
+LL | |     pub fn len(self: &Self) -> isize {
+LL | |         1
+LL | |     }
+LL | | }
+   | |_^
+   |
+   = note: `-D clippy::len-without-is-empty` implied by `-D warnings`
+
+error: trait `PubTraitsToo` has a `len` method but no (possibly inherited) `is_empty` method
+  --> $DIR/len_without_is_empty.rs:37:1
+   |
+LL | / pub trait PubTraitsToo {
+LL | |     fn len(self: &Self) -> isize;
+LL | | }
+   | |_^
+
+error: item `HasIsEmpty` has a public `len` method but a private `is_empty` method
+  --> $DIR/len_without_is_empty.rs:49:1
+   |
+LL | / impl HasIsEmpty {
+LL | |     pub fn len(self: &Self) -> isize {
+LL | |         1
+LL | |     }
+...  |
+LL | |     }
+LL | | }
+   | |_^
+
+error: item `HasWrongIsEmpty` has a public `len` method but no corresponding `is_empty` method
+  --> $DIR/len_without_is_empty.rs:61:1
+   |
+LL | / impl HasWrongIsEmpty {
+LL | |     pub fn len(self: &Self) -> isize {
+LL | |         1
+LL | |     }
+...  |
+LL | |     }
+LL | | }
+   | |_^
+
+error: trait `DependsOnFoo` has a `len` method but no (possibly inherited) `is_empty` method
+  --> $DIR/len_without_is_empty.rs:141:1
+   |
+LL | / pub trait DependsOnFoo: Foo {
+LL | |     fn len(&mut self) -> usize;
+LL | | }
+   | |_^
+
+error: aborting due to 5 previous errors
+

tests/ui/len_zero.fixed

@@ -0,0 +1,143 @@
+// run-rustfix
+
+#![warn(clippy::len_zero)]
+#![allow(dead_code, unused, clippy::len_without_is_empty)]
+
+pub struct One;
+struct Wither;
+
+trait TraitsToo {
+    fn len(self: &Self) -> isize;
+    // No error; `len` is private; see issue #1085.
+}
+
+impl TraitsToo for One {
+    fn len(self: &Self) -> isize {
+        0
+    }
+}
+
+pub struct HasIsEmpty;
+
+impl HasIsEmpty {
+    pub fn len(self: &Self) -> isize {
+        1
+    }
+
+    fn is_empty(self: &Self) -> bool {
+        false
+    }
+}
+
+pub struct HasWrongIsEmpty;
+
+impl HasWrongIsEmpty {
+    pub fn len(self: &Self) -> isize {
+        1
+    }
+
+    pub fn is_empty(self: &Self, x: u32) -> bool {
+        false
+    }
+}
+
+pub trait WithIsEmpty {
+    fn len(self: &Self) -> isize;
+    fn is_empty(self: &Self) -> bool;
+}
+
+impl WithIsEmpty for Wither {
+    fn len(self: &Self) -> isize {
+        1
+    }
+
+    fn is_empty(self: &Self) -> bool {
+        false
+    }
+}
+
+fn main() {
+    let x = [1, 2];
+    if x.is_empty() {
+        println!("This should not happen!");
+    }
+
+    if "".is_empty() {}
+
+    let y = One;
+    if y.len() == 0 {
+        // No error; `One` does not have `.is_empty()`.
+        println!("This should not happen either!");
+    }
+
+    let z: &TraitsToo = &y;
+    if z.len() > 0 {
+        // No error; `TraitsToo` has no `.is_empty()` method.
+        println!("Nor should this!");
+    }
+
+    let has_is_empty = HasIsEmpty;
+    if has_is_empty.is_empty() {
+        println!("Or this!");
+    }
+    if !has_is_empty.is_empty() {
+        println!("Or this!");
+    }
+    if !has_is_empty.is_empty() {
+        println!("Or this!");
+    }
+    if has_is_empty.is_empty() {
+        println!("Or this!");
+    }
+    if !has_is_empty.is_empty() {
+        println!("Or this!");
+    }
+    if has_is_empty.len() > 1 {
+        // No error.
+        println!("This can happen.");
+    }
+    if has_is_empty.len() <= 1 {
+        // No error.
+        println!("This can happen.");
+    }
+    if has_is_empty.is_empty() {
+        println!("Or this!");
+    }
+    if !has_is_empty.is_empty() {
+        println!("Or this!");
+    }
+    if !has_is_empty.is_empty() {
+        println!("Or this!");
+    }
+    if !has_is_empty.is_empty() {
+        println!("Or this!");
+    }
+    if has_is_empty.is_empty() {
+        println!("Or this!");
+    }
+    if 1 < has_is_empty.len() {
+        // No error.
+        println!("This can happen.");
+    }
+    if 1 >= has_is_empty.len() {
+        // No error.
+        println!("This can happen.");
+    }
+    assert!(!has_is_empty.is_empty());
+
+    let with_is_empty: &WithIsEmpty = &Wither;
+    if with_is_empty.is_empty() {
+        println!("Or this!");
+    }
+    assert!(!with_is_empty.is_empty());
+
+    let has_wrong_is_empty = HasWrongIsEmpty;
+    if has_wrong_is_empty.len() == 0 {
+        // No error; `HasWrongIsEmpty` does not have `.is_empty()`.
+        println!("Or this!");
+    }
+}
+
+fn test_slice(b: &[u8]) {
+    if !b.is_empty() {}
+}