Add diagnostics to unknown buffer types

Cargo.toml

@@ -259,6 +259,7 @@ check-cfg = [
     'cfg(lower_upper_exp_for_non_zero)',
     'cfg(netbsdlike)',
     'cfg(rustc_attrs)',
+    'cfg(rustc_diagnostics)',
     'cfg(solarish)',
     'cfg(staged_api)',
     'cfg(static_assertions)',

build.rs

@@ -84,6 +84,10 @@ fn main() {
         use_feature("lower_upper_exp_for_non_zero");
     }
 
+    if can_compile("#[diagnostic::on_unimplemented()] trait Foo {}") {
+        use_feature("rustc_diagnostics")
+    }
+
     // WASI support can utilize wasi_ext if present.
     if os == "wasi" {
         use_feature_or_nothing("wasi_ext");

src/buffer.rs

@@ -94,6 +94,15 @@ use core::slice;
 /// "captured variable cannot escape `FnMut` closure body",
 /// use an explicit loop instead of `retry_on_intr`, assuming you're using
 /// that. See `error_retry_closure_uninit` in examples/buffer_errors.rs.
+#[cfg_attr(
+    rustc_diagnostics,
+    diagnostic::on_unimplemented(
+        message = "rustix does not accept `{Self}` buffers",
+        label = "Unsupported buffer type",
+        note = "only (potentially uninitialized) byte arrays, slices, and Vecs are supported",
+        note = "please read the docs: https://docs.rs/rustix/latest/rustix/buffer/trait.Buffer.html"
+    )
+)]
 pub trait Buffer<T>: private::Sealed<T> {}
 
 // Implement `Buffer` for all the types that implement `Sealed`.