Fix big endian read/write

Co-authored-by: matthewjasper <mjjasper1@gmail.com>

compiler/rustc_middle/src/mir/interpret/mod.rs

@@ -565,7 +565,7 @@ pub fn write_target_uint(
     // So we do not write all bytes of the u128, just the "payload".
     match endianness {
         Endian::Little => target.write(&data.to_le_bytes())?,
-        Endian::Big => target.write(&data.to_be_bytes())?,
+        Endian::Big => target.write(&data.to_be_bytes()[16 - target.len()..])?,
     };
     debug_assert!(target.len() == 0); // We should have filled the target buffer.
     Ok(())
@@ -576,12 +576,18 @@ pub fn read_target_uint(endianness: Endian, mut source: &[u8]) -> Result<u128, i
     // This u128 holds an "any-size uint" (since smaller uints can fits in it)
     let mut buf = [0u8; std::mem::size_of::<u128>()];
     // So we do not read exactly 16 bytes into the u128, just the "payload".
+    let uint = match endianness {
+        Endian::Little => {
             source.read(&mut buf)?;
-    debug_assert!(source.len() == 0); // We should have consumed the source buffer.
+            Ok(u128::from_le_bytes(buf))
-    match endianness {
-        Endian::Little => Ok(u128::from_le_bytes(buf)),
-        Endian::Big => Ok(u128::from_be_bytes(buf)),
         }
+        Endian::Big => {
+            source.read(&mut buf[16 - source.len()..])?;
+            Ok(u128::from_be_bytes(buf))
+        }
+    };
+    debug_assert!(source.len() == 0); // We should have consumed the source buffer.
+    uint
 }
 
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