Auto merge of #11597 - y21:repeat_vec_with_capacity, r=dswij

new lint: `repeat_vec_with_capacity`

Closes #11537

[Lint description](https://github.com/y21/rust-clippy/blob/repeat_vec_with_capacity/clippy_lints/src/repeat_vec_with_capacity.rs#L14) should explain this PR :)

changelog: new lint: `repeat_vec_with_capacity`

tests/ui/repeat_vec_with_capacity.fixed

@@ -0,0 +1,38 @@
+#![warn(clippy::repeat_vec_with_capacity)]
+
+fn main() {
+    {
+        (0..123).map(|_| Vec::<()>::with_capacity(42)).collect::<Vec<_>>();
+        //~^ ERROR: repeating `Vec::with_capacity` using `vec![x; n]`, which does not retain capacity
+    }
+
+    {
+        let n = 123;
+        (0..n).map(|_| Vec::<()>::with_capacity(42)).collect::<Vec<_>>();
+        //~^ ERROR: repeating `Vec::with_capacity` using `vec![x; n]`, which does not retain capacity
+    }
+
+    {
+        macro_rules! from_macro {
+            ($x:expr) => {
+                vec![$x; 123];
+            };
+        }
+        // vec expansion is from another macro, don't lint
+        from_macro!(Vec::<()>::with_capacity(42));
+    }
+
+    {
+        std::iter::repeat_with(|| Vec::<()>::with_capacity(42));
+        //~^ ERROR: repeating `Vec::with_capacity` using `iter::repeat`, which does not retain capacity
+    }
+
+    {
+        macro_rules! from_macro {
+            ($x:expr) => {
+                std::iter::repeat($x)
+            };
+        }
+        from_macro!(Vec::<()>::with_capacity(42));
+    }
+}

tests/ui/repeat_vec_with_capacity.rs

@@ -0,0 +1,38 @@
+#![warn(clippy::repeat_vec_with_capacity)]
+
+fn main() {
+    {
+        vec![Vec::<()>::with_capacity(42); 123];
+        //~^ ERROR: repeating `Vec::with_capacity` using `vec![x; n]`, which does not retain capacity
+    }
+
+    {
+        let n = 123;
+        vec![Vec::<()>::with_capacity(42); n];
+        //~^ ERROR: repeating `Vec::with_capacity` using `vec![x; n]`, which does not retain capacity
+    }
+
+    {
+        macro_rules! from_macro {
+            ($x:expr) => {
+                vec![$x; 123];
+            };
+        }
+        // vec expansion is from another macro, don't lint
+        from_macro!(Vec::<()>::with_capacity(42));
+    }
+
+    {
+        std::iter::repeat(Vec::<()>::with_capacity(42));
+        //~^ ERROR: repeating `Vec::with_capacity` using `iter::repeat`, which does not retain capacity
+    }
+
+    {
+        macro_rules! from_macro {
+            ($x:expr) => {
+                std::iter::repeat($x)
+            };
+        }
+        from_macro!(Vec::<()>::with_capacity(42));
+    }
+}

tests/ui/repeat_vec_with_capacity.stderr

@@ -0,0 +1,40 @@
+error: repeating `Vec::with_capacity` using `vec![x; n]`, which does not retain capacity
+  --> $DIR/repeat_vec_with_capacity.rs:5:9
+   |
+LL |         vec![Vec::<()>::with_capacity(42); 123];
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only the last `Vec` will have the capacity
+   = note: `-D clippy::repeat-vec-with-capacity` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::repeat_vec_with_capacity)]`
+help: if you intended to initialize multiple `Vec`s with an initial capacity, try
+   |
+LL |         (0..123).map(|_| Vec::<()>::with_capacity(42)).collect::<Vec<_>>();
+   |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: repeating `Vec::with_capacity` using `vec![x; n]`, which does not retain capacity
+  --> $DIR/repeat_vec_with_capacity.rs:11:9
+   |
+LL |         vec![Vec::<()>::with_capacity(42); n];
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only the last `Vec` will have the capacity
+help: if you intended to initialize multiple `Vec`s with an initial capacity, try
+   |
+LL |         (0..n).map(|_| Vec::<()>::with_capacity(42)).collect::<Vec<_>>();
+   |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: repeating `Vec::with_capacity` using `iter::repeat`, which does not retain capacity
+  --> $DIR/repeat_vec_with_capacity.rs:26:9
+   |
+LL |         std::iter::repeat(Vec::<()>::with_capacity(42));
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: none of the yielded `Vec`s will have the requested capacity
+help: if you intended to create an iterator that yields `Vec`s with an initial capacity, try
+   |
+LL |         std::iter::repeat_with(|| Vec::<()>::with_capacity(42));
+   |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to 3 previous errors
+