Rollup merge of rust-lang#80834 - bugadani:vecdeque, r=oli-obk

Remove unreachable panics from VecDeque::{front/back}[_mut]

`VecDeque`'s `front`, `front_mut`, `back` and `back_mut` methods are implemented in terms of the index operator, which causes these functions to contain [unreachable panic calls](https://rust.godbolt.org/z/MTnq1o).

This PR reimplements these methods in terms of `get[_mut]` instead.

library/alloc/src/collections/vec_deque/mod.rs

@@ -1292,7 +1292,7 @@ impl<T> VecDeque<T> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn front(&self) -> Option<&T> {
-        if !self.is_empty() { Some(&self[0]) } else { None }
+        self.get(0)
     }
 
     /// Provides a mutable reference to the front element, or `None` if the
@@ -1316,7 +1316,7 @@ impl<T> VecDeque<T> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn front_mut(&mut self) -> Option<&mut T> {
-        if !self.is_empty() { Some(&mut self[0]) } else { None }
+        self.get_mut(0)
     }
 
     /// Provides a reference to the back element, or `None` if the `VecDeque` is
@@ -1336,7 +1336,7 @@ impl<T> VecDeque<T> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn back(&self) -> Option<&T> {
-        if !self.is_empty() { Some(&self[self.len() - 1]) } else { None }
+        self.get(self.len().wrapping_sub(1))
     }
 
     /// Provides a mutable reference to the back element, or `None` if the
@@ -1360,8 +1360,7 @@ impl<T> VecDeque<T> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn back_mut(&mut self) -> Option<&mut T> {
-        let len = self.len();
-        if !self.is_empty() { Some(&mut self[len - 1]) } else { None }
+        self.get_mut(self.len().wrapping_sub(1))
     }
 
     /// Removes the first element and returns it, or `None` if the `VecDeque` is

src/test/codegen/vecdeque_no_panic.rs

@@ -0,0 +1,19 @@
+// This test checks that `VecDeque::front[_mut]()` and `VecDeque::back[_mut]()` can't panic.
+
+// compile-flags: -O
+// ignore-debug: the debug assertions get in the way
+
+#![crate_type = "lib"]
+
+use std::collections::VecDeque;
+
+// CHECK-LABEL: @dont_panic
+#[no_mangle]
+pub fn dont_panic(v: &mut VecDeque<usize>) {
+    // CHECK-NOT: expect
+    // CHECK-NOT: panic
+    v.front();
+    v.front_mut();
+    v.back();
+    v.back_mut();
+}