Round generator sizes to multiple of their alignment

Fixes rust-lang#62658.

Author: tmandry

src/librustc/ty/layout.rs

@@ -1540,6 +1540,8 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
             Ok(variant)
         }).collect::<Result<IndexVec<VariantIdx, _>, _>>()?;
 
+        size = size.align_to(align.abi);
+
         let abi = if prefix.abi.is_uninhabited() ||
                      variants.iter().all(|v| v.abi.is_uninhabited()) {
             Abi::Uninhabited

src/test/ui/async-await/issue-62658.rs

@@ -0,0 +1,29 @@
+// This test created a generator whose size was not rounded to a multiple of its
+// alignment. This caused an assertion error in codegen.
+
+// build-pass
+// edition:2018
+
+#![feature(async_await)]
+
+async fn noop() {}
+
+async fn foo() {
+    // This suspend should be the largest variant.
+    {
+        let x = [0u8; 17];
+        noop().await;
+        println!("{:?}", x);
+    }
+
+    // Add one variant that's aligned to 8 bytes.
+    {
+        let x = 0u64;
+        noop().await;
+        println!("{:?}", x);
+    }
+}
+
+fn main() {
+    let _ = foo();
+}