Rollup merge of rust-lang#78249 - lcnr:ct-infer-origin, r=varkor

improve const infer error

For type inference we probably have to be careful about subtyping and stuff but considering that subtyping shouldn't be relevant for constants I don't really see a reason why we may not want to reuse the const origin here.

r? `@varkor`

Author: jonas-schievink

compiler/rustc_middle/src/infer/unify_key.rs

@@ -175,19 +175,15 @@ impl<'tcx> UnifyKey for ty::ConstVid<'tcx> {
 impl<'tcx> UnifyValue for ConstVarValue<'tcx> {
     type Error = (&'tcx ty::Const<'tcx>, &'tcx ty::Const<'tcx>);
 
-    fn unify_values(value1: &Self, value2: &Self) -> Result<Self, Self::Error> {
-        let (val, span) = match (value1.val, value2.val) {
+    fn unify_values(&value1: &Self, &value2: &Self) -> Result<Self, Self::Error> {
+        Ok(match (value1.val, value2.val) {
             (ConstVariableValue::Known { .. }, ConstVariableValue::Known { .. }) => {
                 bug!("equating two const variables, both of which have known values")
             }
 
             // If one side is known, prefer that one.
-            (ConstVariableValue::Known { .. }, ConstVariableValue::Unknown { .. }) => {
-                (value1.val, value1.origin.span)
-            }
-            (ConstVariableValue::Unknown { .. }, ConstVariableValue::Known { .. }) => {
-                (value2.val, value2.origin.span)
-            }
+            (ConstVariableValue::Known { .. }, ConstVariableValue::Unknown { .. }) => value1,
+            (ConstVariableValue::Unknown { .. }, ConstVariableValue::Known { .. }) => value2,
 
             // If both sides are *unknown*, it hardly matters, does it?
             (
@@ -200,16 +196,11 @@ impl<'tcx> UnifyValue for ConstVarValue<'tcx> {
                 // universe is the minimum of the two universes, because that is
                 // the one which contains the fewest names in scope.
                 let universe = cmp::min(universe1, universe2);
-                (ConstVariableValue::Unknown { universe }, value1.origin.span)
+                ConstVarValue {
+                    val: ConstVariableValue::Unknown { universe },
+                    origin: value1.origin,
+                }
             }
-        };
-
-        Ok(ConstVarValue {
-            origin: ConstVariableOrigin {
-                kind: ConstVariableOriginKind::ConstInference,
-                span: span,
-            },
-            val,
         })
     }
 }

src/test/ui/const-generics/infer/issue-77092.stderr

@@ -2,7 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/issue-77092.rs:13:26
    |
 LL |         println!("{:?}", take_array_from_mut(&mut arr, i));
-   |                          ^^^^^^^^^^^^^^^^^^^ cannot infer the value of the constant `{_: usize}`
+   |                          ^^^^^^^^^^^^^^^^^^^ cannot infer the value of const parameter `N` declared on the function `take_array_from_mut`
 
 error: aborting due to previous error