repro

.gitignore

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+/target
+rustc-ice-*.txt

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "binary"
+version = "0.1.0"
+dependencies = [
+ "library",
+]
+
+[[package]]
+name = "library"
+version = "0.1.0"

Cargo.toml

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+[workspace]
+members = ["binary", "library"]
+resolver = "2"

binary/Cargo.toml

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+[package]
+name = "binary"
+version = "0.1.0"
+edition = "2021"
+
+[[bin]]
+name = "binary"
+path = "main.rs"
+
+
+[dependencies]
+library = { path = "../library" }

binary/main.rs

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+use library::*;
+
+fn main() {
+    // this doesn't compile
+    let mut inner = ImplementsTraitOverConstGeneric::<4>;
+
+    // but this would
+    // let mut inner = NonTraitWithConstGeneric::<4>;
+
+    // if we don't call this function then it'll compile fine
+    inner.configure(&Config {
+        config: [0, 0, 0, 0],
+    });
+}

library/Cargo.toml

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+[package]
+name = "library"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+path = "lib.rs"

library/lib.rs

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+#![allow(incomplete_features)]
+#![feature(generic_const_exprs)]
+
+pub trait LibTrait {
+    const NUM: usize;
+
+    fn configure(&mut self, cfg: &Config<{ Self::NUM }>);
+}
+
+pub struct Config<const N: usize> {
+    pub config: [u8; N],
+}
+
+pub struct ImplementsTraitOverConstGeneric<const N: usize>;
+
+impl<const N: usize> LibTrait for ImplementsTraitOverConstGeneric<N> {
+    const NUM: usize = N;
+
+    fn configure(&mut self, _: &Config<{ Self::NUM }>) {}
+}
+
+// but this works:
+
+// impl LibTrait for ImplementsTraitOverConstGeneric<4> {
+//     const NUM: usize = 4;
+
+//     fn configure(&mut self, _: &Config<{ Self::NUM }>) {}
+// }
+
+// or this:
+
+pub struct NonTraitWithConstGeneric<const N: usize>;
+
+impl<const N: usize> NonTraitWithConstGeneric<N> {
+    pub const NUM: usize = N;
+
+    pub fn configure(&mut self, _: &Config<{ Self::NUM }>) {}
+}

rust-toolchain.toml

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+[toolchain]
+channel = "nightly"