Rollup merge of #136069 - yotamofek:next-solver-slice, r=compiler-errors

Simplify slice indexing in next trait solver

Unless I'm missing something:
- no need to explicitly specify the end of the slice as the end of the index range
- the `assert` is redundant since the indexing will panic for the same condition

I think this change simplifies it a bit. Also replaced the `for` loop of `push`es with a call to `extend` with an iterator. Might improve performance since it knows how many elements will be added beforehand and can pre-reserve room?

r? `@compiler-errors` , I think

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs

@@ -453,13 +453,11 @@ where
 {
     // In case any fresh inference variables have been created between `state`
     // and the previous instantiation, extend `orig_values` for it.
-    assert!(orig_values.len() <= state.value.var_values.len());
-    for &arg in &state.value.var_values.var_values.as_slice()
-        [orig_values.len()..state.value.var_values.len()]
-    {
-        let unconstrained = delegate.fresh_var_for_kind_with_span(arg, span);
-        orig_values.push(unconstrained);
-    }
+    orig_values.extend(
+        state.value.var_values.var_values.as_slice()[orig_values.len()..]
+            .iter()
+            .map(|&arg| delegate.fresh_var_for_kind_with_span(arg, span)),
+    );
 
     let instantiation =
         EvalCtxt::compute_query_response_instantiation_values(delegate, orig_values, &state, span);