add Scalar::from methods for signed integers

Author: RalfJung

src/librustc/mir/interpret/value.rs

@@ -272,11 +272,13 @@ impl<'tcx, Tag> Scalar<Tag> {
 
     #[inline]
     pub fn from_bool(b: bool) -> Self {
+        // Guaranteed to be truncated and does not need sign extension.
         Scalar::Raw { data: b as u128, size: 1 }
     }
 
     #[inline]
     pub fn from_char(c: char) -> Self {
+        // Guaranteed to be truncated and does not need sign extension.
         Scalar::Raw { data: c as u128, size: 4 }
     }
 
@@ -299,21 +301,25 @@ impl<'tcx, Tag> Scalar<Tag> {
 
     #[inline]
     pub fn from_u8(i: u8) -> Self {
+        // Guaranteed to be truncated and does not need sign extension.
         Scalar::Raw { data: i as u128, size: 1 }
     }
 
     #[inline]
     pub fn from_u16(i: u16) -> Self {
+        // Guaranteed to be truncated and does not need sign extension.
         Scalar::Raw { data: i as u128, size: 2 }
     }
 
     #[inline]
     pub fn from_u32(i: u32) -> Self {
+        // Guaranteed to be truncated and does not need sign extension.
         Scalar::Raw { data: i as u128, size: 4 }
     }
 
     #[inline]
     pub fn from_u64(i: u64) -> Self {
+        // Guaranteed to be truncated and does not need sign extension.
         Scalar::Raw { data: i as u128, size: 8 }
     }
 
@@ -341,6 +347,26 @@ impl<'tcx, Tag> Scalar<Tag> {
             .unwrap_or_else(|| bug!("Signed value {:#x} does not fit in {} bits", i, size.bits()))
     }
 
+    #[inline]
+    pub fn from_i8(i: i8) -> Self {
+        Self::from_int(i, Size::from_bits(8))
+    }
+
+    #[inline]
+    pub fn from_i16(i: i16) -> Self {
+        Self::from_int(i, Size::from_bits(16))
+    }
+
+    #[inline]
+    pub fn from_i32(i: i32) -> Self {
+        Self::from_int(i, Size::from_bits(32))
+    }
+
+    #[inline]
+    pub fn from_i64(i: i64) -> Self {
+        Self::from_int(i, Size::from_bits(64))
+    }
+
     #[inline]
     pub fn from_machine_isize(i: i64, cx: &impl HasDataLayout) -> Self {
         Self::from_int(i, cx.data_layout().pointer_size)