Rollup merge of #79291 - JulianKnodt:ce_priv, r=petrochenkov

Add error message for private fn

Attempts to add a more detailed error when a `const_evaluatable` fn from another scope is used inside of a scope which cannot access it.

r? ````@lcnr````

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -6,7 +6,7 @@ pub mod auto_trait;
 mod chalk_fulfill;
 pub mod codegen;
 mod coherence;
-mod const_evaluatable;
+pub mod const_evaluatable;
 mod engine;
 pub mod error_reporting;
 mod fulfill;