Baffling suggestion labelled "help: try introducing a local generic parameter here"

Code

https://github.com/BLAKE3-team/BLAKE3 @ 60ed9295f58ba975df58a67b2bfe435f478d05ea +

diff --git a/benches/bench.rs b/benches/bench.rs
index e89f04c..ea21f3c 100644
--- a/benches/bench.rs
+++ b/benches/bench.rs
@@ -85,24 +85,24 @@ fn bench_single_compression_avx512(b: &mut Bencher) {
     }
 }
 
-fn bench_many_chunks_fn(b: &mut Bencher, platform: Platform) {
+fn bench_many_fn<const LEN: usize>(increment: blake3::IncrementCounter, b: &mut Bencher, platform: Platform) {
     let degree = platform.simd_degree();
     let mut inputs = Vec::new();
     for _ in 0..degree {
-        inputs.push(RandomInput::new(b, CHUNK_LEN));
+        inputs.push(RandomInput::new(b, LEN));
     }
     b.iter(|| {
-        let input_arrays: ArrayVec<&[u8; CHUNK_LEN], MAX_SIMD_DEGREE> = inputs
+        let input_arrays: ArrayVec<&[u8; LEN], MAX_SIMD_DEGREE> = inputs
             .iter_mut()
             .take(degree)
-            .map(|i| array_ref!(i.get(), 0, CHUNK_LEN))
+            .map(|i| array_ref!(i.get(), 0, LEN))
             .collect();
         let mut out = [0; MAX_SIMD_DEGREE * OUT_LEN];
         platform.hash_many(
             &input_arrays[..],
             &[0; 8],
             0,
-            blake3::IncrementCounter::Yes,
+            increment,
             0,
             0,
             0,
@@ -111,6 +111,10 @@ fn bench_many_chunks_fn(b: &mut Bencher, platform: Platform) {
     });
 }
 
+fn bench_many_chunks_fn(b: &mut Bencher, platform: Platform) {
+    bench_many_fn::<CHUNK_LEN>(blake3::IncrementCounter::Yes, b, platform)
+}
+
 #[bench]
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 fn bench_many_chunks_sse2(b: &mut Bencher) {
@@ -155,31 +159,8 @@ fn bench_many_chunks_wasm(b: &mut Bencher) {
     bench_many_chunks_fn(b, Platform::wasm32_simd().unwrap());
 }
 
-// TODO: When we get const generics we can unify this with the chunks code.
 fn bench_many_parents_fn(b: &mut Bencher, platform: Platform) {
-    let degree = platform.simd_degree();
-    let mut inputs = Vec::new();
-    for _ in 0..degree {
-        inputs.push(RandomInput::new(b, BLOCK_LEN));
-    }
-    b.iter(|| {
-        let input_arrays: ArrayVec<&[u8; BLOCK_LEN], MAX_SIMD_DEGREE> = inputs
-            .iter_mut()
-            .take(degree)
-            .map(|i| array_ref!(i.get(), 0, BLOCK_LEN))
-            .collect();
-        let mut out = [0; MAX_SIMD_DEGREE * OUT_LEN];
-        platform.hash_many(
-            &input_arrays[..],
-            &[0; 8],
-            0,
-            blake3::IncrementCounter::No,
-            0,
-            0,
-            0,
-            &mut out,
-        );
-    });
+    bench_many_fn::<BLOCK_LEN>(blake3::IncrementCounter::No, b, platform)
 }
 
 #[bench]

+ `cargo bench`

Current output

error[E0401]: can't use generic parameters from outer item
  --> benches/bench.rs:98:45
   |
88 | fn bench_many_fn<const LEN: usize>(increment: blake3::IncrementCounter, b: &mut Bencher, platform: Platform) {
   |                        --- const parameter from outer item
...
98 |             .map(|i| array_ref!(i.get(), 0, LEN))
   |                      -----------------------^^^-
   |                      |                      |
   |                      |                      use of generic parameter from outer item
   |                      help: try introducing a local generic parameter here: `LEN,`

Desired output

idk

Rationale and extra context

This is baffling. Where does it want me to do this. What is this about.

Other cases

Rust Version

rustc 1.87.0 (17067e9ac 2025-05-09)
binary: rustc
commit-hash: 17067e9ac6d7ecb70e50f92c1944e545188d2359
commit-date: 2025-05-09
host: x86_64-unknown-linux-gnu
release: 1.87.0
LLVM version: 20.1.1

Anything else?

No response

Reproduce the diagnostic with cargo bench using the code in benches/bench.rs, focusing on the array_ref!(i.get(), 0, LEN) call and its suggested local generic parameter. Trace the compiler diagnostic path that emits this message; done means the report is no longer baffling and points to an actionable location or explanation.