From d36d351afcb439144621159d8642892cde26eff6 Mon Sep 17 00:00:00 2001
From: Nathan Corbyn <me@nathancorbyn.com>
Date: Tue, 23 Jun 2020 16:46:24 +0100
Subject: Implement intrinsic

---
 src/libcore/intrinsics.rs | 15 +++++++++++++++
 src/libcore/lib.rs        |  1 +
 src/libcore/mem/mod.rs    | 27 +++++++++++++++++++++++++++
 3 files changed, 43 insertions(+)

(limited to 'src/libcore')

diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index 50e321f9c71..4074087e1e6 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -1917,6 +1917,15 @@ extern "rust-intrinsic" {
     #[rustc_const_unstable(feature = "const_discriminant", issue = "69821")]
     pub fn discriminant_value<T>(v: &T) -> <T as DiscriminantKind>::Discriminant;
 
+    /// Returns the number of variants of the type `T` cast to a `usize`;
+    /// if `T` has no variants, returns 0. Uninhabited variants will be counted.
+    ///
+    /// The to-be-stabilized version of this intrinsic is
+    /// [`std::mem::variant_count`](../../std/mem/fn.variant_count.html)
+    #[rustc_const_unstable(feature = "variant_count", issue = "73662")]
+    #[cfg(not(bootstrap))]
+    pub fn variant_count<T>() -> usize;
+
     /// Rust's "try catch" construct which invokes the function pointer `try_fn`
     /// with the data pointer `data`.
     ///
@@ -1960,6 +1969,12 @@ extern "rust-intrinsic" {
     pub fn ptr_guaranteed_ne<T>(ptr: *const T, other: *const T) -> bool;
 }
 
+#[rustc_const_unstable(feature = "variant_count", issue = "73662")]
+#[cfg(bootstrap)]
+pub const fn variant_count<T>() -> usize {
+    0
+}
+
 // Some functions are defined here because they accidentally got made
 // available in this module on stable. See <https://github.com/rust-lang/rust/issues/15702>.
 // (`transmute` also falls into this category, but it cannot be wrapped due to the
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index 4eb2fdbd078..2b26e5303a8 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -124,6 +124,7 @@
 #![feature(unsized_locals)]
 #![feature(untagged_unions)]
 #![feature(unwind_attributes)]
+#![feature(variant_count)]
 #![feature(doc_alias)]
 #![feature(mmx_target_feature)]
 #![feature(tbm_target_feature)]
diff --git a/src/libcore/mem/mod.rs b/src/libcore/mem/mod.rs
index 066bb8b3dc7..d041571f260 100644
--- a/src/libcore/mem/mod.rs
+++ b/src/libcore/mem/mod.rs
@@ -999,3 +999,30 @@ impl<T> fmt::Debug for Discriminant<T> {
 pub const fn discriminant<T>(v: &T) -> Discriminant<T> {
     Discriminant(intrinsics::discriminant_value(v))
 }
+
+/// Returns the number of variants in the enum type `T`.
+///
+/// If `T` is not an enum, calling this function will not result in undefined behavior, but the
+/// return value is unspecified. Equally, if `T` is an enum with more variants than `usize::MAX`
+/// the return value is unspecified. Uninhabited variants will be counted.
+///
+/// # Examples
+///
+/// ```
+/// use std::mem;
+///
+/// enum Void {}
+/// enum Foo { A(&'static str), B(i32), C(i32) }
+///
+/// assert_eq!(mem::variant_count::<Void>(), 0);
+/// assert_eq!(mem::variant_count::<Foo>(), 3);
+///
+/// assert_eq!(mem::variant_count::<Option<!>>(), 2);
+/// assert_eq!(mem::variant_count::<Result<!, !>>(), 2);
+/// ```
+#[inline(always)]
+#[unstable(feature = "variant_count", issue = "73662")]
+#[rustc_const_unstable(feature = "variant_count", issue = "73662")]
+pub const fn variant_count<T>() -> usize {
+    intrinsics::variant_count::<T>()
+}
-- 
cgit 1.4.1-3-g733a5


From c2dfc25c0eeb74d6c723312d612cd6899058971c Mon Sep 17 00:00:00 2001
From: Nathan Corbyn <me@nathancorbyn.com>
Date: Wed, 24 Jun 2020 15:10:10 +0100
Subject: Fix tests

---
 src/libcore/mem/mod.rs                    | 2 ++
 src/test/ui/consts/const-variant-count.rs | 1 +
 2 files changed, 3 insertions(+)

(limited to 'src/libcore')

diff --git a/src/libcore/mem/mod.rs b/src/libcore/mem/mod.rs
index d041571f260..2be105e68eb 100644
--- a/src/libcore/mem/mod.rs
+++ b/src/libcore/mem/mod.rs
@@ -1009,6 +1009,8 @@ pub const fn discriminant<T>(v: &T) -> Discriminant<T> {
 /// # Examples
 ///
 /// ```
+/// # #![feature(never_type)]
+///
 /// use std::mem;
 ///
 /// enum Void {}
diff --git a/src/test/ui/consts/const-variant-count.rs b/src/test/ui/consts/const-variant-count.rs
index f21f6e9c291..455419d2c7f 100644
--- a/src/test/ui/consts/const-variant-count.rs
+++ b/src/test/ui/consts/const-variant-count.rs
@@ -1,6 +1,7 @@
 // run-pass
 #![allow(dead_code)]
 #![feature(variant_count)]
+#![feature(never_type)]
 
 use std::mem::variant_count;
 
-- 
cgit 1.4.1-3-g733a5


From 493199626baf8a0a8f6d3a19a089165d18b3f1fb Mon Sep 17 00:00:00 2001
From: Nathan Corbyn <me@nathancorbyn.com>
Date: Wed, 24 Jun 2020 15:36:04 +0100
Subject: Fix tests

---
 src/libcore/mem/mod.rs | 1 +
 1 file changed, 1 insertion(+)

(limited to 'src/libcore')

diff --git a/src/libcore/mem/mod.rs b/src/libcore/mem/mod.rs
index 2be105e68eb..1bd7ae3a34e 100644
--- a/src/libcore/mem/mod.rs
+++ b/src/libcore/mem/mod.rs
@@ -1010,6 +1010,7 @@ pub const fn discriminant<T>(v: &T) -> Discriminant<T> {
 ///
 /// ```
 /// # #![feature(never_type)]
+/// # #![feature(variant_count)]
 ///
 /// use std::mem;
 ///
-- 
cgit 1.4.1-3-g733a5