From 7d498a0284af6e0744943867d0e26b858e2b85e6 Mon Sep 17 00:00:00 2001
From: "A. Molzer" <5550310+197g@users.noreply.github.com>
Date: Tue, 6 Jan 2026 15:59:18 +0100
Subject: [PATCH] Add TiffValue proxy implementations

We have a fundamental choice here: We could add more proxy
implementations for `Vec` and `Box` to follow `&'_ T`, for some types
that indirections is clear. Vec and arrays are also both very natural
ways of expressing a repeated value. However, passing an allocated value
to the interface forces us to drop it inside. We might be able to
utilize the allocation for an in-place byteswap in a future increment
but not with the current design.

So, overall, I think that inherently `Drop`-y types should be avoided in
these implementations. Instead, we can use the amazing diagnostic tools
in the compiler to just help the users navigate the type system. We can
still provide the impl for arrays.

For context, see: https://users.rust-lang.org/t/what-type-is-u16/137395

Now they see:

	    |
	 38 |         .write_tag(tiff::tags::Tag::GeoKeyDirectoryTag, &geokey);
	    |                                                          ^^^^^^ the trait `TiffValue` is not implemented for `&Vec<u16>`
	    |
	    = note: the trait is implemented for primitive types (`u8`, `i16`, `f32`, etc.)
	    = note: the trait is implemented for shared references to values
	    = note: the trait is implemented for slices, pass them by reference (e.g. `&[u8]`)
	    = note: the trait is implemented for arrays if it is implemented for a slice
	    = note: values in a `Vec` or `Box` should be dereferenced, e.g. `&vec[..]`
	    = help: the following other types implement trait `TiffValue`:

Previously the compiler would both strip the `&` in its first message
(due to our forwarding impl) and not list any way of getting a
reference-to-slice.
---
 src/encoder/tiff_value.rs | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/src/encoder/tiff_value.rs b/src/encoder/tiff_value.rs
index 57d278d..16ab2c1 100644
--- a/src/encoder/tiff_value.rs
+++ b/src/encoder/tiff_value.rs
@@ -5,6 +5,14 @@ use crate::{bytecast, tags::Type, TiffError, TiffFormatError, TiffResult};
 use super::writer::TiffWriter;
 
 /// Trait for types that can be encoded in a tiff file
+#[diagnostic::on_unimplemented(
+    message = "the type `{Self}` does not implement `TiffValue`",
+    note = "the trait is implemented for primitive types (`u8`, `i16`, `f32`, etc.)",
+    note = "the trait is implemented for shared references to values",
+    note = "the trait is implemented for slices, pass them by reference (e.g. `&[u8]`)",
+    note = "the trait is implemented for arrays if it is implemented for a slice",
+    note = "values in a `Vec` or `Box` should be dereferenced, e.g. `&vec[..]`"
+)]
 pub trait TiffValue {
     const BYTE_LEN: u8;
     const FIELD_TYPE: Type;
@@ -450,6 +458,14 @@ impl TiffValue for str {
     }
 }
 
+// If you pass `&Vec<_>` then you'd get at first sight the  complaint:
+//
+//  `Vec<T>` does not implement `TiffValue`
+//
+// and a list of implementations of `TiffValue` that are quite unrelated. That is not very helpful.
+// We do not *want* `Vec` to implement the trait as an owning type, instead you should pass a
+// reference to a slice. The error message is further customized in the diagnostics of the trait.
+#[diagnostic::do_not_recommend]
 impl<T: TiffValue + ?Sized> TiffValue for &'_ T {
     const BYTE_LEN: u8 = T::BYTE_LEN;
     const FIELD_TYPE: Type = T::FIELD_TYPE;
@@ -467,6 +483,27 @@ impl<T: TiffValue + ?Sized> TiffValue for &'_ T {
     }
 }
 
+/// An array is treated like a slice of the same length.
+impl<T, const N: usize> TiffValue for [T; N]
+where
+    [T]: TiffValue,
+{
+    const BYTE_LEN: u8 = <[T]>::BYTE_LEN;
+    const FIELD_TYPE: Type = <[T]>::FIELD_TYPE;
+
+    fn count(&self) -> usize {
+        self.as_slice().count()
+    }
+
+    fn write<W: Write>(&self, writer: &mut TiffWriter<W>) -> TiffResult<()> {
+        self.as_slice().write(writer)
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        self.as_slice().data()
+    }
+}
+
 macro_rules! impl_tiff_value_for_contiguous_sequence {
     ($inner_type:ty; $bytes:expr; $field_type:expr) => {
         impl $crate::encoder::TiffValue for [$inner_type] {