1 files changed, 137 insertions, 0 deletions
diff --git a/tests/ui/generator/discriminant.rs b/tests/ui/generator/discriminant.rs
new file mode 100644
index 00000000000..195e7702299
--- /dev/null
+++ b/tests/ui/generator/discriminant.rs
@@ -0,0 +1,137 @@
+//! Tests that generator discriminant sizes and ranges are chosen optimally and that they are
+//! reflected in the output of `mem::discriminant`.
+
+// run-pass
+
+#![feature(generators, generator_trait, core_intrinsics, discriminant_kind)]
+
+use std::intrinsics::discriminant_value;
+use std::marker::{Unpin, DiscriminantKind};
+use std::mem::size_of_val;
+use std::{cmp, ops::*};
+
+macro_rules! yield25 {
+    ($e:expr) => {
+        yield $e;
+        yield $e;
+        yield $e;
+        yield $e;
+        yield $e;
+
+        yield $e;
+        yield $e;
+        yield $e;
+        yield $e;
+        yield $e;
+
+        yield $e;
+        yield $e;
+        yield $e;
+        yield $e;
+        yield $e;
+
+        yield $e;
+        yield $e;
+        yield $e;
+        yield $e;
+        yield $e;
+
+        yield $e;
+        yield $e;
+        yield $e;
+        yield $e;
+        yield $e;
+    };
+}
+
+/// Yields 250 times.
+macro_rules! yield250 {
+    () => {
+        yield250!(())
+    };
+
+    ($e:expr) => {
+        yield25!($e);
+        yield25!($e);
+        yield25!($e);
+        yield25!($e);
+        yield25!($e);
+
+        yield25!($e);
+        yield25!($e);
+        yield25!($e);
+        yield25!($e);
+        yield25!($e);
+    };
+}
+
+fn cycle(
+    gen: impl Generator<()> + Unpin + DiscriminantKind<Discriminant = u32>,
+    expected_max_discr: u32
+) {
+    let mut gen = Box::pin(gen);
+    let mut max_discr = 0;
+    loop {
+        max_discr = cmp::max(max_discr, discriminant_value(gen.as_mut().get_mut()));
+        match gen.as_mut().resume(()) {
+            GeneratorState::Yielded(_) => {}
+            GeneratorState::Complete(_) => {
+                assert_eq!(max_discr, expected_max_discr);
+                return;
+            }
+        }
+    }
+}
+
+fn main() {
+    // Has only one invalid discr. value.
+    let gen_u8_tiny_niche = || {
+        || {
+            // 3 reserved variants
+
+            yield250!(); // 253 variants
+
+            yield; // 254
+            yield; // 255
+        }
+    };
+
+    // Uses all values in the u8 discriminant.
+    let gen_u8_full = || {
+        || {
+            // 3 reserved variants
+
+            yield250!(); // 253 variants
+
+            yield; // 254
+            yield; // 255
+            yield; // 256
+        }
+    };
+
+    // Barely needs a u16 discriminant.
+    let gen_u16 = || {
+        || {
+            // 3 reserved variants
+
+            yield250!(); // 253 variants
+
+            yield; // 254
+            yield; // 255
+            yield; // 256
+            yield; // 257
+        }
+    };
+
+    assert_eq!(size_of_val(&gen_u8_tiny_niche()), 1);
+    assert_eq!(size_of_val(&Some(gen_u8_tiny_niche())), 1); // uses niche
+    assert_eq!(size_of_val(&Some(Some(gen_u8_tiny_niche()))), 2); // cannot use niche anymore
+    assert_eq!(size_of_val(&gen_u8_full()), 1);
+    assert_eq!(size_of_val(&Some(gen_u8_full())), 2); // cannot use niche
+    assert_eq!(size_of_val(&gen_u16()), 2);
+    assert_eq!(size_of_val(&Some(gen_u16())), 2); // uses niche
+
+    cycle(gen_u8_tiny_niche(), 254);
+    cycle(gen_u8_full(), 255);
+    cycle(gen_u16(), 256);
+}