From ad47e0ca6dfdc030e0645f0364a34e629a6d67a7 Mon Sep 17 00:00:00 2001
From: Taiki Endo <te316e89@gmail.com>
Date: Sun, 10 Jul 2022 02:09:55 +0900
Subject: [PATCH] Make Slab::new const on Rust 1.39+

---
 Cargo.toml    |  4 ++++
 build.rs      | 21 +++++++++++++++++++++
 src/lib.rs    | 25 +++++++++++++++++++++++--
 tests/slab.rs |  6 ++++++
 4 files changed, 54 insertions(+), 2 deletions(-)
 create mode 100644 build.rs

diff --git a/Cargo.toml b/Cargo.toml
index 540de1a..d120413 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -21,9 +21,13 @@ exclude = ["/.*"]
 std = []
 default = ["std"]
 
+[build-dependencies]
+autocfg = "1"
+
 [dependencies]
 serde = { version = "1.0.95", optional = true, default-features = false, features = ["alloc"] }
 
 [dev-dependencies]
+rustversion = "1"
 serde = { version = "1", features = ["derive"] }
 serde_test = "1"
diff --git a/build.rs b/build.rs
new file mode 100644
index 0000000..28840e4
--- /dev/null
+++ b/build.rs
@@ -0,0 +1,21 @@
+fn main() {
+    let cfg = match autocfg::AutoCfg::new() {
+        Ok(cfg) => cfg,
+        Err(e) => {
+            // If we couldn't detect the compiler version and features, just
+            // print a warning. This isn't a fatal error: we can still build
+            // Tokio, we just can't enable cfgs automatically.
+            println!(
+                "cargo:warning=slab: failed to detect compiler features: {}",
+                e
+            );
+            return;
+        }
+    };
+    // Note that this is `no_`*, not `has_*`. This allows treating as the latest
+    // stable rustc is used when the build script doesn't run. This is useful
+    // for non-cargo build systems that don't run the build script.
+    if !cfg.probe_rustc_version(1, 39) {
+        println!("cargo:rustc-cfg=slab_no_const_vec_new");
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index f207455..e369f46 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -219,14 +219,35 @@ impl<T> Slab<T> {
     /// The function does not allocate and the returned slab will have no
     /// capacity until `insert` is called or capacity is explicitly reserved.
     ///
+    /// This is `const fn` on Rust 1.39+.
+    ///
     /// # Examples
     ///
     /// ```
     /// # use slab::*;
     /// let slab: Slab<i32> = Slab::new();
     /// ```
-    pub fn new() -> Slab<T> {
-        Slab::with_capacity(0)
+    #[cfg(not(slab_no_const_vec_new))]
+    pub const fn new() -> Self {
+        Self {
+            entries: Vec::new(),
+            next: 0,
+            len: 0,
+        }
+    }
+    /// Construct a new, empty `Slab`.
+    ///
+    /// The function does not allocate and the returned slab will have no
+    /// capacity until `insert` is called or capacity is explicitly reserved.
+    ///
+    /// This is `const fn` on Rust 1.39+.
+    #[cfg(slab_no_const_vec_new)]
+    pub fn new() -> Self {
+        Self {
+            entries: Vec::new(),
+            next: 0,
+            len: 0,
+        }
     }
 
     /// Construct a new, empty `Slab` with the specified capacity.
diff --git a/tests/slab.rs b/tests/slab.rs
index 41360fa..fa89ebb 100644
--- a/tests/slab.rs
+++ b/tests/slab.rs
@@ -696,3 +696,9 @@ fn try_remove() {
     assert_eq!(slab.try_remove(key), None);
     assert_eq!(slab.get(key), None);
 }
+
+#[rustversion::since(1.39)]
+#[test]
+fn const_new() {
+    static _SLAB: Slab<()> = Slab::new();
+}