use Vec's try_reserve_exact method (#3656)

Cargo.toml

@@ -12,7 +12,7 @@ description = """
 Easy support for interacting between JS and Rust.
 """
 edition = "2018"
-rust-version = "1.56"
+rust-version = "1.57"
 
 [package.metadata.docs.rs]
 features = ["serde-serialize"]

src/externref.rs

@@ -1,8 +1,5 @@
 use crate::JsValue;
-use std::alloc::{self, Layout};
 use std::cell::Cell;
-use std::mem;
-use std::ptr;
 use std::slice;
 use std::vec::Vec;
 use std::cmp::max;
@@ -46,23 +43,8 @@ impl Slab {
                     internal_error("someone else allocated table entries?")
                 }
 
-                // poor man's `try_reserve_exact` until that's stable
-                unsafe {
-                    let new_cap = self.data.capacity() + extra;
-                    let size = mem::size_of::<usize>() * new_cap;
-                    let align = mem::align_of::<usize>();
-                    let layout = match Layout::from_size_align(size, align) {
-                        Ok(l) => l,
-                        Err(_) => internal_error("size/align layout failure"),
-                    };
-                    let ptr = alloc::alloc(layout) as *mut usize;
-                    if ptr.is_null() {
-                        internal_error("allocation failure");
-                    }
-                    ptr::copy_nonoverlapping(self.data.as_ptr(), ptr, self.data.len());
-                    let new_vec = Vec::from_raw_parts(ptr, self.data.len(), new_cap);
-                    let mut old = mem::replace(&mut self.data, new_vec);
-                    old.set_len(0);
+                if self.data.try_reserve_exact(extra).is_err() {
+                    internal_error("allocation failure");
                 }
             }