[wip] unit tests for file module

- finalize `Chunk` coverage

src/file/chunk.rs

@@ -1,4 +1,4 @@
-use std::fmt;
+use std::{any, fmt};
 
 use crate::sharry;
 
@@ -23,6 +23,22 @@ impl fmt::Debug for Chunk<'_> {
     }
 }
 
+/// convert usize into other type
+///
+/// # Panics
+///
+/// - if the given value does not fit into the target type
+fn from_usize_or_panic<I>(value: usize) -> I
+where
+    I: TryFrom<usize>,
+    I::Error: std::error::Error,
+{
+    value.try_into().unwrap_or_else(|e| {
+        let target_type = any::type_name::<I>();
+        panic!("usize={value:?} did not fit into {target_type:?}: {e}")
+    })
+}
+
 impl<'t> Chunk<'t> {
     pub(super) fn new_direct(file_id: sharry::FileID, offset: u64, data: &'t [u8]) -> Self {
         Self {
@@ -49,12 +65,10 @@ impl<'t> Chunk<'t> {
 
     /// get the chunk's length
     pub fn get_length(&self) -> u64 {
-        let len = self.data.len();
-
-        // BOOKMARK this might **panic** on platforms where `usize` has more than 64 bit.
+        // BOOKMARK this might **panic** on (hypothetical) platforms where `usize` has more than 64 bit.
         // Also, you've allocated more than 2 EiB ... in ONE chunk.
         // Whoa! Maybe just chill?
-        u64::try_from(len).unwrap_or_else(|e| panic!("usize={len} did not fit into u64: {e}"))
+        from_usize_or_panic(self.data.len())
     }
 }
 
@@ -90,4 +104,17 @@ mod tests {
             assert_eq!(chunk.get_length(), len);
         }
     }
+
+    #[test]
+    #[should_panic(expected = "did not fit into \"u32\"")]
+    fn test_usize_overflow_panics() {
+        // works
+        assert_eq!(from_usize_or_panic::<u64>(usize::MAX), u64::MAX);
+        assert_eq!(from_usize_or_panic::<u32>(u32::MAX as usize), u32::MAX);
+        assert_eq!(from_usize_or_panic::<u16>(u16::MAX as usize), u16::MAX);
+        assert_eq!(from_usize_or_panic::<u8>(u8::MAX as usize), u8::MAX);
+
+        // panics
+        from_usize_or_panic::<u32>(usize::MAX);
+    }
 }