Rollup merge of #123016 - compiler-errors:no-type-var-origin, r=lcnr

Remove `TypeVariableOriginKind` and `ConstVariableOriginKind`

It's annoying to have to import `TypeVariableOriginKind` just to fill it with `MiscVariable` for almost every use. Every other usage other than `TypeParameterDefinition` wasn't even used -- I can see how it may have been useful once for debugging, but I do quite a lot of typeck debugging and I've never really needed it.

So let's just remove it, and keep around the only useful thing which is the `DefId` of the param for `var_for_def`.

This is based on #123006, which removed the special use of `TypeVariableOriginKind::OpaqueInference`, which I'm pretty sure I was the one that added.

r? lcnr or re-roll to types

compiler/rustc_middle/src/infer/unify_key.rs

@@ -1,7 +1,6 @@
 use crate::ty::{self, Ty, TyCtxt};
 use rustc_data_structures::unify::{NoError, UnifyKey, UnifyValue};
 use rustc_span::def_id::DefId;
-use rustc_span::symbol::Symbol;
 use rustc_span::Span;
 use std::cmp;
 use std::marker::PhantomData;
@@ -106,16 +105,11 @@ impl ToType for ty::FloatVarValue {
 
 #[derive(Copy, Clone, Debug)]
 pub struct ConstVariableOrigin {
-    pub kind: ConstVariableOriginKind,
     pub span: Span,
-}
-
-/// Reasons to create a const inference variable
-#[derive(Copy, Clone, Debug)]
-pub enum ConstVariableOriginKind {
-    MiscVariable,
-    ConstInference,
-    ConstParameterDefinition(Symbol, DefId),
+    /// `DefId` of the const parameter this was instantiated for, if any.
+    ///
+    /// This should only be used for diagnostics.
+    pub param_def_id: Option<DefId>,
 }
 
 #[derive(Copy, Clone, Debug)]