Add invariant to Vec::pop that len < cap if pop successful

Fixes: #114334

Author: krtab

library/alloc/src/vec/mod.rs

@@ -1903,6 +1903,7 @@ impl<T, A: Allocator> Vec<T, A> {
         } else {
             unsafe {
                 self.len -= 1;
+                core::intrinsics::assume(self.len < self.capacity());
                 Some(ptr::read(self.as_ptr().add(self.len())))
             }
         }

tests/codegen/vec_pop_push_noop.rs

@@ -0,0 +1,25 @@
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+#[no_mangle]
+pub fn noop(v: &mut Vec<u8>) {
+    // CHECK: start:
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: load
+    // CHECK-NEXT: icmp eq
+    // CHECK-NEXT: br
+
+    // CHECK: add
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: load
+    // CHECK-NEXT: icmp
+    // CHECK-NEXT: tail call
+    // CHECK-NEXT: br label %bb3
+
+    // CHECK: bb3:
+    // CHECK-NEXT:     ret void
+    if let Some(x) = v.pop() {
+        v.push(x)
+    }
+}