Auto merge of #7265 - Jarcho:manual_str_repeat, r=giraffate

Add lint `manual_str_repeat`

fixes: #7260

There's a similar function for slices. Should this be renamed to include it, or should that be a separate lint? If we are going to have them as one lint a better name will be needed. `manual_repeat` isn't exactly clear as it's replacing a call to `iter::repeat`.

changelog: Add new lint `manual_str_repeat`

3 files changed, 196 insertions, 0 deletions

diff --git a/tests/ui/manual_str_repeat.fixed b/tests/ui/manual_str_repeat.fixed
new file mode 100644
index 00000000000..dc140257f32
--- /dev/null
+++ b/tests/ui/manual_str_repeat.fixed
@@ -0,0 +1,66 @@
+// run-rustfix
+
+#![feature(custom_inner_attributes)]
+#![warn(clippy::manual_str_repeat)]
+
+use std::borrow::Cow;
+use std::iter::{repeat, FromIterator};
+
+fn main() {
+    let _: String = "test".repeat(10);
+    let _: String = "x".repeat(10);
+    let _: String = "'".repeat(10);
+    let _: String = "\"".repeat(10);
+
+    let x = "test";
+    let count = 10;
+    let _ = x.repeat(count + 2);
+
+    macro_rules! m {
+        ($e:expr) => {{ $e }};
+    }
+    // FIXME: macro args are fine
+    let _: String = repeat(m!("test")).take(m!(count)).collect();
+
+    let x = &x;
+    let _: String = (*x).repeat(count);
+
+    macro_rules! repeat_m {
+        ($e:expr) => {{ repeat($e) }};
+    }
+    // Don't lint, repeat is from a macro.
+    let _: String = repeat_m!("test").take(count).collect();
+
+    let x: Box<str> = Box::from("test");
+    let _: String = x.repeat(count);
+
+    #[derive(Clone)]
+    struct S;
+    impl FromIterator<Box<S>> for String {
+        fn from_iter<T: IntoIterator<Item = Box<S>>>(_: T) -> Self {
+            Self::new()
+        }
+    }
+    // Don't lint, wrong box type
+    let _: String = repeat(Box::new(S)).take(count).collect();
+
+    let _: String = Cow::Borrowed("test").repeat(count);
+
+    let x = "x".to_owned();
+    let _: String = x.repeat(count);
+
+    let x = 'x';
+    // Don't lint, not char literal
+    let _: String = repeat(x).take(count).collect();
+}
+
+fn _msrv_1_15() {
+    #![clippy::msrv = "1.15"]
+    // `str::repeat` was stabilized in 1.16. Do not lint this
+    let _: String = std::iter::repeat("test").take(10).collect();
+}
+
+fn _msrv_1_16() {
+    #![clippy::msrv = "1.16"]
+    let _: String = "test".repeat(10);
+}
diff --git a/tests/ui/manual_str_repeat.rs b/tests/ui/manual_str_repeat.rs
new file mode 100644
index 00000000000..0d69c989b2e
--- /dev/null
+++ b/tests/ui/manual_str_repeat.rs
@@ -0,0 +1,66 @@
+// run-rustfix
+
+#![feature(custom_inner_attributes)]
+#![warn(clippy::manual_str_repeat)]
+
+use std::borrow::Cow;
+use std::iter::{repeat, FromIterator};
+
+fn main() {
+    let _: String = std::iter::repeat("test").take(10).collect();
+    let _: String = std::iter::repeat('x').take(10).collect();
+    let _: String = std::iter::repeat('\'').take(10).collect();
+    let _: String = std::iter::repeat('"').take(10).collect();
+
+    let x = "test";
+    let count = 10;
+    let _ = repeat(x).take(count + 2).collect::<String>();
+
+    macro_rules! m {
+        ($e:expr) => {{ $e }};
+    }
+    // FIXME: macro args are fine
+    let _: String = repeat(m!("test")).take(m!(count)).collect();
+
+    let x = &x;
+    let _: String = repeat(*x).take(count).collect();
+
+    macro_rules! repeat_m {
+        ($e:expr) => {{ repeat($e) }};
+    }
+    // Don't lint, repeat is from a macro.
+    let _: String = repeat_m!("test").take(count).collect();
+
+    let x: Box<str> = Box::from("test");
+    let _: String = repeat(x).take(count).collect();
+
+    #[derive(Clone)]
+    struct S;
+    impl FromIterator<Box<S>> for String {
+        fn from_iter<T: IntoIterator<Item = Box<S>>>(_: T) -> Self {
+            Self::new()
+        }
+    }
+    // Don't lint, wrong box type
+    let _: String = repeat(Box::new(S)).take(count).collect();
+
+    let _: String = repeat(Cow::Borrowed("test")).take(count).collect();
+
+    let x = "x".to_owned();
+    let _: String = repeat(x).take(count).collect();
+
+    let x = 'x';
+    // Don't lint, not char literal
+    let _: String = repeat(x).take(count).collect();
+}
+
+fn _msrv_1_15() {
+    #![clippy::msrv = "1.15"]
+    // `str::repeat` was stabilized in 1.16. Do not lint this
+    let _: String = std::iter::repeat("test").take(10).collect();
+}
+
+fn _msrv_1_16() {
+    #![clippy::msrv = "1.16"]
+    let _: String = std::iter::repeat("test").take(10).collect();
+}
diff --git a/tests/ui/manual_str_repeat.stderr b/tests/ui/manual_str_repeat.stderr
new file mode 100644
index 00000000000..c6511689716
--- /dev/null
+++ b/tests/ui/manual_str_repeat.stderr
@@ -0,0 +1,64 @@
+error: manual implementation of `str::repeat` using iterators
+  --> $DIR/manual_str_repeat.rs:10:21
+   |
+LL |     let _: String = std::iter::repeat("test").take(10).collect();
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `"test".repeat(10)`
+   |
+   = note: `-D clippy::manual-str-repeat` implied by `-D warnings`
+
+error: manual implementation of `str::repeat` using iterators
+  --> $DIR/manual_str_repeat.rs:11:21
+   |
+LL |     let _: String = std::iter::repeat('x').take(10).collect();
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `"x".repeat(10)`
+
+error: manual implementation of `str::repeat` using iterators
+  --> $DIR/manual_str_repeat.rs:12:21
+   |
+LL |     let _: String = std::iter::repeat('/'').take(10).collect();
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `"'".repeat(10)`
+
+error: manual implementation of `str::repeat` using iterators
+  --> $DIR/manual_str_repeat.rs:13:21
+   |
+LL |     let _: String = std::iter::repeat('"').take(10).collect();
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `"/"".repeat(10)`
+
+error: manual implementation of `str::repeat` using iterators
+  --> $DIR/manual_str_repeat.rs:17:13
+   |
+LL |     let _ = repeat(x).take(count + 2).collect::<String>();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `x.repeat(count + 2)`
+
+error: manual implementation of `str::repeat` using iterators
+  --> $DIR/manual_str_repeat.rs:26:21
+   |
+LL |     let _: String = repeat(*x).take(count).collect();
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `(*x).repeat(count)`
+
+error: manual implementation of `str::repeat` using iterators
+  --> $DIR/manual_str_repeat.rs:35:21
+   |
+LL |     let _: String = repeat(x).take(count).collect();
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `x.repeat(count)`
+
+error: manual implementation of `str::repeat` using iterators
+  --> $DIR/manual_str_repeat.rs:47:21
+   |
+LL |     let _: String = repeat(Cow::Borrowed("test")).take(count).collect();
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `Cow::Borrowed("test").repeat(count)`
+
+error: manual implementation of `str::repeat` using iterators
+  --> $DIR/manual_str_repeat.rs:50:21
+   |
+LL |     let _: String = repeat(x).take(count).collect();
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `x.repeat(count)`
+
+error: manual implementation of `str::repeat` using iterators
+  --> $DIR/manual_str_repeat.rs:65:21
+   |
+LL |     let _: String = std::iter::repeat("test").take(10).collect();
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `"test".repeat(10)`
+
+error: aborting due to 10 previous errors
+