Add a storage for the origin of const inference variables.

[lcnr]

We currently don't store the origin of const inference variables. This is needed to emit more helpful error messages.

For type inference variables, this is done using https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/struct.InferCtxtInner.html#method.type_variables and then
https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/type_variable/struct.TypeVariableTable.html#method.var_origin.

We use this to provide better diagnostics in case type inference fails:

rust/src/librustc_infer/infer/error_reporting/need_type_info.rs

Lines 198 to 201 in 9912925

	let ty_vars = &inner.type_variables();
	let var_origin = ty_vars.var_origin(ty_vid);
	if let TypeVariableOriginKind::TypeParameterDefinition(name, def_id) = var_origin.kind {
	let parent_def_id = def_id.and_then(|def_id| self.tcx.parent(def_id));

A similar mechanism should probably also be used for consts.

[varkor]

It would be good to have some examples of where diagnostics are subpar because of this missing feature, to decide if it's a prerequisite for stabilising min_const_generics.

[lcnr]

yeah, will look a bit into some interesting test cases here in the near future hopefully

[lcnr]

One issue here is something like this:

#![feature(min_const_generics)]

struct Foo;

impl Foo {
    fn bar(self) -> Foo {
        Foo
    }

    fn baz<const N: usize>(self) -> Foo {
        println!("baz: {}", N);
        Foo
    }
}

fn main() {
    Foo.bar().bar().bar().bar().baz();
}

which results in

error[E0282]: type annotations needed
  --> src/main.rs:17:5
   |
17 |     Foo.bar().bar().bar().bar().baz();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: unable to infer the value of a const parameter

for baz<T> we instead get

error[E0282]: type annotations needed
  --> src/main.rs:17:33
   |
17 |     Foo.bar().bar().bar().bar().baz();
   |                                 ^^^ cannot infer type for type parameter `T` declared on the associated function `baz`

btw I am not actually sure if this issue makes sense, we already have the origin of const infer values stored, so
I don't actually think we need to add a new storage here 😆 we "just" have to use the current one

[pickfire]

Is there any suggestion for user on how to fix this?