internal/core/adt: don't allocate when asserting in Environment.up

Assertf takes remaining formatting arguments as ..any,
and since the arguments we pass here are two integers,
they escape when a pointer to them is needed to fit into an interface.
Environment.up is called often, so this resulted in about 4.6%
of the total allocated objects in `cue vet -c=false ./...`.

The small amount of added debug info doesn't seem worth a 2% CPU cost.
We could likely write the assertion in a way that doesn't escape values,
but that also doesn't seem worth the hassle right now.

              │    old     │                new                │
              │   sec/op   │   sec/op    vs base               │
    CueVetAll   1.968 ± 1%   1.929 ± 1%  -1.97% (p=0.000 n=10)

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I9334b91ed41f36d7e755504d37d49a9430c4a174
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194375
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>

internal/core/adt/composite.go

@@ -104,7 +104,7 @@ type cacheKey struct {
 func (e *Environment) up(ctx *OpContext, count int32) *Environment {
 	for i := int32(0); i < count; i++ {
 		e = e.Up
-		ctx.Assertf(ctx.Pos(), e.Vertex != nil, "Environment.up encountered a nil vertex at step %d out of %d", i+1, count)
+		ctx.Assertf(ctx.Pos(), e.Vertex != nil, "Environment.up encountered a nil vertex")
 	}
 	return e
 }