Auto merge of #2094 - dtolnay-contrib:truncate, r=RalfJung

Replace `as` casts in llvm.x86.addcarry.64 implementation

Recommended in #2090 (review).

Author: bors

src/helpers.rs

@@ -1,3 +1,5 @@
+pub mod convert;
+
 use std::mem;
 use std::num::NonZeroUsize;
 use std::time::Duration;

src/helpers/convert.rs

@@ -0,0 +1,49 @@
+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/shims/foreign_items.rs

@@ -22,6 +22,7 @@ use rustc_target::{
 };
 
 use super::backtrace::EvalContextExt as _;
+use crate::helpers::convert::Truncate;
 use crate::*;
 
 /// Returned by `emulate_foreign_item_by_name`.
@@ -677,8 +678,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 let a = this.read_scalar(a)?.to_u64()?;
                 let b = this.read_scalar(b)?.to_u64()?;
 
-                let wide_sum = c_in as u128 + a as u128 + b as u128;
-                let (c_out, sum) = ((wide_sum >> 64) as u8, wide_sum as u64);
+                let wide_sum = u128::from(c_in) + u128::from(a) + u128::from(b);
+                let (c_out, sum) = ((wide_sum >> 64).truncate::<u8>(), wide_sum.truncate::<u64>());
 
                 let c_out_field = this.place_field(dest, 0)?;
                 this.write_scalar(Scalar::from_u8(c_out), &c_out_field)?;