Identify more cases of useless `into_iter()` calls

If the type of the result of a call to `IntoIterator::into_iter()`
and the type of the receiver are the same, then the receiver
implements `Iterator` and `into_iter()` is the identity function.

The call to `into_iter()` may be removed in all but two cases:

- If the receiver implements `Copy`, `into_iter()` will produce
  a copy of the receiver and cannot be removed. For example,
  `x.into_iter().next()` will not advance `x` while `x.next()` will.
- If the receiver is an immutable local variable and the call to
  `into_iter()` appears in a larger expression, removing the call to
  `into_iter()` might cause mutability issues. For example, if `x`
  is an immutable local variable, `x.into_iter().next()` will
  compile while `x.next()` will not as `next()` receives
  `&mut self`.

3 files changed, 182 insertions, 18 deletions

diff --git a/tests/ui/useless_conversion.fixed b/tests/ui/useless_conversion.fixed
index 70ff08f3655..94b206d8e58 100644
--- a/tests/ui/useless_conversion.fixed
+++ b/tests/ui/useless_conversion.fixed
@@ -33,12 +33,71 @@ fn test_issue_3913() -> Result<(), std::io::Error> {
     Ok(())
 }
 
-fn test_issue_5833() -> Result<(), ()> {
+fn dont_lint_into_iter_on_immutable_local_implementing_iterator_in_expr() {
     let text = "foo\r\nbar\n\nbaz\n";
     let lines = text.lines();
     if Some("ok") == lines.into_iter().next() {}
+}
 
-    Ok(())
+fn lint_into_iter_on_mutable_local_implementing_iterator_in_expr() {
+    let text = "foo\r\nbar\n\nbaz\n";
+    let mut lines = text.lines();
+    if Some("ok") == lines.next() {}
+}
+
+fn lint_into_iter_on_expr_implementing_iterator() {
+    let text = "foo\r\nbar\n\nbaz\n";
+    let mut lines = text.lines();
+    if Some("ok") == lines.next() {}
+}
+
+fn lint_into_iter_on_expr_implementing_iterator_2() {
+    let text = "foo\r\nbar\n\nbaz\n";
+    if Some("ok") == text.lines().next() {}
+}
+
+#[allow(const_item_mutation)]
+fn lint_into_iter_on_const_implementing_iterator() {
+    const NUMBERS: std::ops::Range<i32> = 0..10;
+    let _ = NUMBERS.next();
+}
+
+fn lint_into_iter_on_const_implementing_iterator_2() {
+    const NUMBERS: std::ops::Range<i32> = 0..10;
+    let mut n = NUMBERS;
+    n.next();
+}
+
+#[derive(Clone, Copy)]
+struct CopiableCounter {
+    counter: u32,
+}
+
+impl Iterator for CopiableCounter {
+    type Item = u32;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.counter = self.counter.wrapping_add(1);
+        Some(self.counter)
+    }
+}
+
+fn dont_lint_into_iter_on_copy_iter() {
+    let mut c = CopiableCounter { counter: 0 };
+    assert_eq!(c.into_iter().next(), Some(1));
+    assert_eq!(c.into_iter().next(), Some(1));
+    assert_eq!(c.next(), Some(1));
+    assert_eq!(c.next(), Some(2));
+}
+
+fn dont_lint_into_iter_on_static_copy_iter() {
+    static mut C: CopiableCounter = CopiableCounter { counter: 0 };
+    unsafe {
+        assert_eq!(C.into_iter().next(), Some(1));
+        assert_eq!(C.into_iter().next(), Some(1));
+        assert_eq!(C.next(), Some(1));
+        assert_eq!(C.next(), Some(2));
+    }
 }
 
 fn main() {
@@ -46,7 +105,15 @@ fn main() {
     test_generic2::<i32, i32>(10i32);
     test_questionmark().unwrap();
     test_issue_3913().unwrap();
-    test_issue_5833().unwrap();
+
+    dont_lint_into_iter_on_immutable_local_implementing_iterator_in_expr();
+    lint_into_iter_on_mutable_local_implementing_iterator_in_expr();
+    lint_into_iter_on_expr_implementing_iterator();
+    lint_into_iter_on_expr_implementing_iterator_2();
+    lint_into_iter_on_const_implementing_iterator();
+    lint_into_iter_on_const_implementing_iterator_2();
+    dont_lint_into_iter_on_copy_iter();
+    dont_lint_into_iter_on_static_copy_iter();
 
     let _: String = "foo".into();
     let _: String = From::from("foo");
diff --git a/tests/ui/useless_conversion.rs b/tests/ui/useless_conversion.rs
index f2444a8f436..c7ae927941b 100644
--- a/tests/ui/useless_conversion.rs
+++ b/tests/ui/useless_conversion.rs
@@ -33,12 +33,71 @@ fn test_issue_3913() -> Result<(), std::io::Error> {
     Ok(())
 }
 
-fn test_issue_5833() -> Result<(), ()> {
+fn dont_lint_into_iter_on_immutable_local_implementing_iterator_in_expr() {
     let text = "foo\r\nbar\n\nbaz\n";
     let lines = text.lines();
     if Some("ok") == lines.into_iter().next() {}
+}
 
-    Ok(())
+fn lint_into_iter_on_mutable_local_implementing_iterator_in_expr() {
+    let text = "foo\r\nbar\n\nbaz\n";
+    let mut lines = text.lines();
+    if Some("ok") == lines.into_iter().next() {}
+}
+
+fn lint_into_iter_on_expr_implementing_iterator() {
+    let text = "foo\r\nbar\n\nbaz\n";
+    let mut lines = text.lines().into_iter();
+    if Some("ok") == lines.next() {}
+}
+
+fn lint_into_iter_on_expr_implementing_iterator_2() {
+    let text = "foo\r\nbar\n\nbaz\n";
+    if Some("ok") == text.lines().into_iter().next() {}
+}
+
+#[allow(const_item_mutation)]
+fn lint_into_iter_on_const_implementing_iterator() {
+    const NUMBERS: std::ops::Range<i32> = 0..10;
+    let _ = NUMBERS.into_iter().next();
+}
+
+fn lint_into_iter_on_const_implementing_iterator_2() {
+    const NUMBERS: std::ops::Range<i32> = 0..10;
+    let mut n = NUMBERS.into_iter();
+    n.next();
+}
+
+#[derive(Clone, Copy)]
+struct CopiableCounter {
+    counter: u32,
+}
+
+impl Iterator for CopiableCounter {
+    type Item = u32;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.counter = self.counter.wrapping_add(1);
+        Some(self.counter)
+    }
+}
+
+fn dont_lint_into_iter_on_copy_iter() {
+    let mut c = CopiableCounter { counter: 0 };
+    assert_eq!(c.into_iter().next(), Some(1));
+    assert_eq!(c.into_iter().next(), Some(1));
+    assert_eq!(c.next(), Some(1));
+    assert_eq!(c.next(), Some(2));
+}
+
+fn dont_lint_into_iter_on_static_copy_iter() {
+    static mut C: CopiableCounter = CopiableCounter { counter: 0 };
+    unsafe {
+        assert_eq!(C.into_iter().next(), Some(1));
+        assert_eq!(C.into_iter().next(), Some(1));
+        assert_eq!(C.next(), Some(1));
+        assert_eq!(C.next(), Some(2));
+    }
 }
 
 fn main() {
@@ -46,7 +105,15 @@ fn main() {
     test_generic2::<i32, i32>(10i32);
     test_questionmark().unwrap();
     test_issue_3913().unwrap();
-    test_issue_5833().unwrap();
+
+    dont_lint_into_iter_on_immutable_local_implementing_iterator_in_expr();
+    lint_into_iter_on_mutable_local_implementing_iterator_in_expr();
+    lint_into_iter_on_expr_implementing_iterator();
+    lint_into_iter_on_expr_implementing_iterator_2();
+    lint_into_iter_on_const_implementing_iterator();
+    lint_into_iter_on_const_implementing_iterator_2();
+    dont_lint_into_iter_on_copy_iter();
+    dont_lint_into_iter_on_static_copy_iter();
 
     let _: String = "foo".into();
     let _: String = From::from("foo");
diff --git a/tests/ui/useless_conversion.stderr b/tests/ui/useless_conversion.stderr
index 65ee3807fa9..be067c6843a 100644
--- a/tests/ui/useless_conversion.stderr
+++ b/tests/ui/useless_conversion.stderr
@@ -22,71 +22,101 @@ error: useless conversion to the same type: `i32`
 LL |         let _: i32 = 0i32.into();
    |                      ^^^^^^^^^^^ help: consider removing `.into()`: `0i32`
 
+error: useless conversion to the same type: `std::str::Lines<'_>`
+  --> $DIR/useless_conversion.rs:45:22
+   |
+LL |     if Some("ok") == lines.into_iter().next() {}
+   |                      ^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `lines`
+
+error: useless conversion to the same type: `std::str::Lines<'_>`
+  --> $DIR/useless_conversion.rs:50:21
+   |
+LL |     let mut lines = text.lines().into_iter();
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `text.lines()`
+
+error: useless conversion to the same type: `std::str::Lines<'_>`
+  --> $DIR/useless_conversion.rs:56:22
+   |
+LL |     if Some("ok") == text.lines().into_iter().next() {}
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `text.lines()`
+
+error: useless conversion to the same type: `std::ops::Range<i32>`
+  --> $DIR/useless_conversion.rs:62:13
+   |
+LL |     let _ = NUMBERS.into_iter().next();
+   |             ^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `NUMBERS`
+
+error: useless conversion to the same type: `std::ops::Range<i32>`
+  --> $DIR/useless_conversion.rs:67:17
+   |
+LL |     let mut n = NUMBERS.into_iter();
+   |                 ^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `NUMBERS`
+
 error: useless conversion to the same type: `std::string::String`
-  --> $DIR/useless_conversion.rs:61:21
+  --> $DIR/useless_conversion.rs:128:21
    |
 LL |     let _: String = "foo".to_string().into();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into()`: `"foo".to_string()`
 
 error: useless conversion to the same type: `std::string::String`
-  --> $DIR/useless_conversion.rs:62:21
+  --> $DIR/useless_conversion.rs:129:21
    |
 LL |     let _: String = From::from("foo".to_string());
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `From::from()`: `"foo".to_string()`
 
 error: useless conversion to the same type: `std::string::String`
-  --> $DIR/useless_conversion.rs:63:13
+  --> $DIR/useless_conversion.rs:130:13
    |
 LL |     let _ = String::from("foo".to_string());
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `String::from()`: `"foo".to_string()`
 
 error: useless conversion to the same type: `std::string::String`
-  --> $DIR/useless_conversion.rs:64:13
+  --> $DIR/useless_conversion.rs:131:13
    |
 LL |     let _ = String::from(format!("A: {:04}", 123));
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `String::from()`: `format!("A: {:04}", 123)`
 
 error: useless conversion to the same type: `std::str::Lines<'_>`
-  --> $DIR/useless_conversion.rs:65:13
+  --> $DIR/useless_conversion.rs:132:13
    |
 LL |     let _ = "".lines().into_iter();
    |             ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `"".lines()`
 
 error: useless conversion to the same type: `std::vec::IntoIter<i32>`
-  --> $DIR/useless_conversion.rs:66:13
+  --> $DIR/useless_conversion.rs:133:13
    |
 LL |     let _ = vec![1, 2, 3].into_iter().into_iter();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `vec![1, 2, 3].into_iter()`
 
 error: useless conversion to the same type: `std::string::String`
-  --> $DIR/useless_conversion.rs:67:21
+  --> $DIR/useless_conversion.rs:134:21
    |
 LL |     let _: String = format!("Hello {}", "world").into();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into()`: `format!("Hello {}", "world")`
 
 error: useless conversion to the same type: `i32`
-  --> $DIR/useless_conversion.rs:72:13
+  --> $DIR/useless_conversion.rs:139:13
    |
 LL |     let _ = i32::from(a + b) * 3;
    |             ^^^^^^^^^^^^^^^^ help: consider removing `i32::from()`: `(a + b)`
 
 error: useless conversion to the same type: `Foo<'a'>`
-  --> $DIR/useless_conversion.rs:78:23
+  --> $DIR/useless_conversion.rs:145:23
    |
 LL |     let _: Foo<'a'> = s2.into();
    |                       ^^^^^^^^^ help: consider removing `.into()`: `s2`
 
 error: useless conversion to the same type: `Foo<'a'>`
-  --> $DIR/useless_conversion.rs:80:13
+  --> $DIR/useless_conversion.rs:147:13
    |
 LL |     let _ = Foo::<'a'>::from(s3);
    |             ^^^^^^^^^^^^^^^^^^^^ help: consider removing `Foo::<'a'>::from()`: `s3`
 
 error: useless conversion to the same type: `std::vec::IntoIter<Foo<'a'>>`
-  --> $DIR/useless_conversion.rs:82:13
+  --> $DIR/useless_conversion.rs:149:13
    |
 LL |     let _ = vec![s4, s4, s4].into_iter().into_iter();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `vec![s4, s4, s4].into_iter()`
 
-error: aborting due to 14 previous errors
+error: aborting due to 19 previous errors