internal/core/adt: drop overlayContext.generation

The field hasn't been used for a while, and the global nextGeneration
counter was causing data races in parallel tests which used
multiple cue.Contexts concurrently.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I7f83556323d6d228312699fad33c6d57ffbdb1bf
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1212032
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mvdan

internal/core/adt/overlay.go

@@ -38,11 +38,8 @@ import "slices"
 // TODO(perf): implement copy on write: instead of copying the entire tree, we
 // could get by with only copying arcs to that are modified in the copy.
 
-var nextGeneration int
-
 func newOverlayContext(ctx *OpContext) *overlayContext {
-	nextGeneration++
-	return &overlayContext{ctx: ctx, generation: nextGeneration}
+	return &overlayContext{ctx: ctx}
 }
 
 // An overlayContext keeps track of copied vertices, closeContexts, and tasks.
@@ -51,12 +48,6 @@ func newOverlayContext(ctx *OpContext) *overlayContext {
 type overlayContext struct {
 	ctx *OpContext
 
-	// generation is used to identify the current overlayContext. All
-	// closeContexts created by this overlayContext will have this generation.
-	// Whenever a counter of a closedContext is changed, this may only cause
-	// a cascade of changes if the generation is the same.
-	generation int
-
 	// vertices holds the original, non-overlay vertices. The overlay for a
 	// vertex v can be obtained by looking up v.cc.overlay.src.
 	vertices []*Vertex