Auto merge of #6980 - Jarcho:len_without_is_empty_sig, r=llogiq

`len_without_is_empty` improvements

fixes: #6958
fixes: #6972

changelog: Check the return type of `len`. Only integral types, or an `Option` or `Result` wrapping one.
changelog: Ensure the return type of `is_empty` matches. e.g. `Option<usize>` -> `Option<bool>`

tests/ui/len_without_is_empty.rs

@@ -1,3 +1,5 @@
+// edition:2018
+
 #![warn(clippy::len_without_is_empty)]
 #![allow(dead_code, unused)]
 
@@ -172,9 +174,9 @@ pub trait DependsOnFoo: Foo {
     fn len(&mut self) -> usize;
 }
 
+// issue #1562
 pub struct MultipleImpls;
 
-// issue #1562
 impl MultipleImpls {
     pub fn len(&self) -> usize {
         1
@@ -187,4 +189,99 @@ impl MultipleImpls {
     }
 }
 
+// issue #6958
+pub struct OptionalLen;
+
+impl OptionalLen {
+    pub fn len(&self) -> Option<usize> {
+        Some(0)
+    }
+
+    pub fn is_empty(&self) -> Option<bool> {
+        Some(true)
+    }
+}
+
+pub struct OptionalLen2;
+impl OptionalLen2 {
+    pub fn len(&self) -> Option<usize> {
+        Some(0)
+    }
+
+    pub fn is_empty(&self) -> bool {
+        true
+    }
+}
+
+pub struct OptionalLen3;
+impl OptionalLen3 {
+    pub fn len(&self) -> usize {
+        0
+    }
+
+    // should lint, len is not an option
+    pub fn is_empty(&self) -> Option<bool> {
+        None
+    }
+}
+
+pub struct ResultLen;
+impl ResultLen {
+    pub fn len(&self) -> Result<usize, ()> {
+        Ok(0)
+    }
+
+    // Differing result types
+    pub fn is_empty(&self) -> Option<bool> {
+        Some(true)
+    }
+}
+
+pub struct ResultLen2;
+impl ResultLen2 {
+    pub fn len(&self) -> Result<usize, ()> {
+        Ok(0)
+    }
+
+    pub fn is_empty(&self) -> Result<bool, ()> {
+        Ok(true)
+    }
+}
+
+pub struct ResultLen3;
+impl ResultLen3 {
+    pub fn len(&self) -> Result<usize, ()> {
+        Ok(0)
+    }
+
+    // Non-fallible result is ok.
+    pub fn is_empty(&self) -> bool {
+        true
+    }
+}
+
+pub struct OddLenSig;
+impl OddLenSig {
+    // don't lint
+    pub fn len(&self) -> bool {
+        true
+    }
+}
+
+// issue #6958
+pub struct AsyncLen;
+impl AsyncLen {
+    async fn async_task(&self) -> bool {
+        true
+    }
+
+    pub async fn len(&self) -> usize {
+        if self.async_task().await { 0 } else { 1 }
+    }
+
+    pub async fn is_empty(&self) -> bool {
+        self.len().await == 0
+    }
+}
+
 fn main() {}

tests/ui/len_without_is_empty.stderr

@@ -1,5 +1,5 @@
 error: struct `PubOne` has a public `len` method, but no `is_empty` method
-  --> $DIR/len_without_is_empty.rs:7:5
+  --> $DIR/len_without_is_empty.rs:9:5
    |
 LL |     pub fn len(&self) -> isize {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -7,7 +7,7 @@ LL |     pub fn len(&self) -> isize {
    = 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:55:1
+  --> $DIR/len_without_is_empty.rs:57:1
    |
 LL | / pub trait PubTraitsToo {
 LL | |     fn len(&self) -> isize;
@@ -15,50 +15,109 @@ LL | | }
    | |_^
 
 error: struct `HasIsEmpty` has a public `len` method, but a private `is_empty` method
-  --> $DIR/len_without_is_empty.rs:68:5
+  --> $DIR/len_without_is_empty.rs:70:5
    |
 LL |     pub fn len(&self) -> isize {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: `is_empty` defined here
-  --> $DIR/len_without_is_empty.rs:72:5
+  --> $DIR/len_without_is_empty.rs:74:5
    |
 LL |     fn is_empty(&self) -> bool {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: struct `HasWrongIsEmpty` has a public `len` method, but the `is_empty` method has an unexpected signature
-  --> $DIR/len_without_is_empty.rs:80:5
+  --> $DIR/len_without_is_empty.rs:82:5
    |
 LL |     pub fn len(&self) -> isize {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: `is_empty` defined here
-  --> $DIR/len_without_is_empty.rs:84:5
+  --> $DIR/len_without_is_empty.rs:86:5
    |
 LL |     pub fn is_empty(&self, x: u32) -> bool {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: expected signature: `(&self) -> bool`
 
 error: struct `MismatchedSelf` has a public `len` method, but the `is_empty` method has an unexpected signature
-  --> $DIR/len_without_is_empty.rs:92:5
+  --> $DIR/len_without_is_empty.rs:94:5
    |
 LL |     pub fn len(self) -> isize {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: `is_empty` defined here
-  --> $DIR/len_without_is_empty.rs:96:5
+  --> $DIR/len_without_is_empty.rs:98:5
    |
 LL |     pub fn is_empty(&self) -> bool {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: expected signature: `(self) -> bool`
 
 error: trait `DependsOnFoo` has a `len` method but no (possibly inherited) `is_empty` method
-  --> $DIR/len_without_is_empty.rs:171:1
+  --> $DIR/len_without_is_empty.rs:173:1
    |
 LL | / pub trait DependsOnFoo: Foo {
 LL | |     fn len(&mut self) -> usize;
 LL | | }
    | |_^
 
-error: aborting due to 6 previous errors
+error: struct `OptionalLen3` has a public `len` method, but the `is_empty` method has an unexpected signature
+  --> $DIR/len_without_is_empty.rs:218:5
+   |
+LL |     pub fn len(&self) -> usize {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: `is_empty` defined here
+  --> $DIR/len_without_is_empty.rs:223:5
+   |
+LL |     pub fn is_empty(&self) -> Option<bool> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: expected signature: `(&self) -> bool`
+
+error: struct `ResultLen` has a public `len` method, but the `is_empty` method has an unexpected signature
+  --> $DIR/len_without_is_empty.rs:230:5
+   |
+LL |     pub fn len(&self) -> Result<usize, ()> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: `is_empty` defined here
+  --> $DIR/len_without_is_empty.rs:235:5
+   |
+LL |     pub fn is_empty(&self) -> Option<bool> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: expected signature: `(&self) -> bool` or `(&self) -> Result<bool>
+
+error: this returns a `Result<_, ()>
+  --> $DIR/len_without_is_empty.rs:230:5
+   |
+LL |     pub fn len(&self) -> Result<usize, ()> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::result-unit-err` implied by `-D warnings`
+   = help: use a custom Error type instead
+
+error: this returns a `Result<_, ()>
+  --> $DIR/len_without_is_empty.rs:242:5
+   |
+LL |     pub fn len(&self) -> Result<usize, ()> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use a custom Error type instead
+
+error: this returns a `Result<_, ()>
+  --> $DIR/len_without_is_empty.rs:246:5
+   |
+LL |     pub fn is_empty(&self) -> Result<bool, ()> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use a custom Error type instead
+
+error: this returns a `Result<_, ()>
+  --> $DIR/len_without_is_empty.rs:253:5
+   |
+LL |     pub fn len(&self) -> Result<usize, ()> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use a custom Error type instead
+
+error: aborting due to 12 previous errors