Add the slim_slice data-structures (#528)

* add the slim_slice data-structures

* slim_slice: addres some review comments

* slim_slice: add basic SlimSmallSliceBox type + comment on SlimNonEmptyBox

* slim_slice: prefix conversions with 'from_'

* slim_slice: use transmute_copy

* add empty LICENSE file

* pacify clippy

crates/data-structures/Cargo.toml

@@ -5,4 +5,10 @@ edition = "2021"
 license-file = "LICENSE"
 description = "Assorted data structures used in spacetimedb"
 
+[features]
+serde = ["dep:serde"]
+
 [dependencies]
+nonempty.workspace = true
+serde = { workspace = true, optional = true }
+thiserror.workspace = true

crates/data-structures/LICENSE

@@ -0,0 +1 @@
+

crates/data-structures/src/lib.rs

@@ -2,3 +2,4 @@
 //! These structures are general and could be used outside spacetimedb.
 
 pub mod nstr;
+pub mod slim_slice;

crates/data-structures/src/nstr.rs

@@ -5,6 +5,7 @@
 //! An example would be `nstr!("spacetime"): NStr<9>`.
 
 use core::{fmt, ops::Deref, str};
+use std::ops::DerefMut;
 
 /// A UTF-8 string of known length `N`.
 ///
@@ -68,6 +69,15 @@ impl<const N: usize> Deref for NStr<N> {
     }
 }
 
+impl<const N: usize> DerefMut for NStr<N> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        // SAFETY: An `NStr<N>` can only be made through `__nstr(..)`
+        // and which receives an `&str` which is valid UTF-8.
+        unsafe { str::from_utf8_unchecked_mut(&mut self.0) }
+    }
+}
+
 impl<const N: usize> fmt::Debug for NStr<N> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str(self.deref())