Introduce the TreeLevel struct

milli/src/tree_level.rs

@@ -0,0 +1,47 @@
+use std::convert::TryFrom;
+use std::fmt;
+
+/// This is just before the lowest printable character (space, sp, 32)
+const MAX_VALUE: u8 = 31;
+
+#[derive(Debug, Copy, Clone)]
+pub enum Error {
+    LevelTooHigh(u8),
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[repr(transparent)]
+pub struct TreeLevel(u8);
+
+impl TreeLevel {
+    pub const fn max_value() -> TreeLevel {
+        TreeLevel(MAX_VALUE)
+    }
+
+    pub const fn min_value() -> TreeLevel {
+        TreeLevel(0)
+    }
+}
+
+impl Into<u8> for TreeLevel {
+    fn into(self) -> u8 {
+        self.0
+    }
+}
+
+impl TryFrom<u8> for TreeLevel {
+    type Error = Error;
+
+    fn try_from(value: u8) -> Result<TreeLevel, Error> {
+        match value {
+            0..=MAX_VALUE => Ok(TreeLevel(value)),
+            _ => Err(Error::LevelTooHigh(value)),
+        }
+    }
+}
+
+impl fmt::Display for TreeLevel {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}