Rollup merge of #42959 - SimonSapin:nonzero-checked, r=sfackler

Make the "main" constructors of NonZero/Shared/Unique return Option

Per discussion in https://github.com/rust-lang/rust/issues/27730#issuecomment-303939441.

This is a breaking change to unstable APIs.

The old behavior is still available under the name `new_unchecked`. Note that only that one can be `const fn`, since `if` is currently not allowed in constant contexts.

In the case of `NonZero` this requires adding a new `is_zero` method to the `Zeroable` trait. I mildly dislike this, but it’s not much worse than having a `Zeroable` trait in the first place. `Zeroable` and `NonZero` are both unstable, this can be reworked later.

src/test/ui/print_type_sizes/nullable.rs

@@ -42,7 +42,7 @@ impl Default for EmbeddedDiscr {
 }
 
 #[derive(Default)]
-pub struct IndirectNonZero<T: Zeroable> {
+pub struct IndirectNonZero<T: Zeroable + One> {
     pre: u8,
     nested: NestedNonZero<T>,
     post: u16,
@@ -54,14 +54,20 @@ pub struct NestedNonZero<T: Zeroable> {
     post: u16,
 }
 
-impl<T: Zeroable+Default> Default for NestedNonZero<T> {
+impl<T: Zeroable+One> Default for NestedNonZero<T> {
     fn default() -> Self {
-        unsafe {
-            NestedNonZero { pre: 0, val: NonZero::new(Default::default()), post: 0 }
-        }
+        NestedNonZero { pre: 0, val: NonZero::new(T::one()).unwrap(), post: 0 }
     }
 }
 
+pub trait One {
+    fn one() -> Self;
+}
+
+impl One for u32 {
+    fn one() -> Self { 1 }
+}
+
 pub fn main() {
     let _x: MyOption<NonZero<u32>> = Default::default();
     let _y: EmbeddedDiscr = Default::default();