remove some dead code

and add a comment in the AVX test

src/tools/miri/src/helpers.rs

@@ -1,5 +1,3 @@
-pub mod convert;
-
 use std::cmp;
 use std::iter;
 use std::num::NonZeroUsize;

src/tools/miri/src/helpers/convert.rs

@@ -1,49 +0,0 @@
-use implementations::NarrowerThan;
-
-/// Replacement for `as` casts going from wide integer to narrower integer.
-///
-/// # Example
-///
-/// ```ignore
-/// let x = 99_u64;
-/// let lo = x.truncate::<u16>();
-/// // lo is of type u16, equivalent to `x as u16`.
-/// ```
-pub(crate) trait Truncate: Sized {
-    fn truncate<To>(self) -> To
-    where
-        To: NarrowerThan<Self>,
-    {
-        NarrowerThan::truncate_from(self)
-    }
-}
-
-impl Truncate for u16 {}
-impl Truncate for u32 {}
-impl Truncate for u64 {}
-impl Truncate for u128 {}
-
-mod implementations {
-    pub(crate) trait NarrowerThan<T> {
-        fn truncate_from(wide: T) -> Self;
-    }
-
-    macro_rules! impl_narrower_than {
-        ($(NarrowerThan<{$($ty:ty),*}> for $self:ty)*) => {
-            $($(
-                impl NarrowerThan<$ty> for $self {
-                    fn truncate_from(wide: $ty) -> Self {
-                        wide as Self
-                    }
-                }
-            )*)*
-        };
-    }
-
-    impl_narrower_than! {
-        NarrowerThan<{u128, u64, u32, u16}> for u8
-        NarrowerThan<{u128, u64, u32}> for u16
-        NarrowerThan<{u128, u64}> for u32
-        NarrowerThan<{u128}> for u64
-    }
-}

src/tools/miri/tests/pass/intrinsics-x86-avx512.rs

@@ -1,5 +1,6 @@
 // Ignore everything except x86 and x86_64
 // Any additional target are added to CI should be ignored here
+// (We cannot use `cfg`-based tricks here since the `target-feature` flags below only work on x86.)
 //@ignore-target-aarch64
 //@ignore-target-arm
 //@ignore-target-avr