gopls/internal/cache: analysis: repair start/end and refine bug report

Poor parser error recovery may cause Node.End to be zero, or
a small positive displacement from zero due to recursive Node.End
computation (#66683).

This change further refines the bug.Reports for such problems,
and additionally repairs the values heuristically to avoid
downstream bug.Reports after toGobDiagnostics (#64547).

Updates golang/go#66683
Updates golang/go#64547

Change-Id: I7c795622ec6b63574978d2953c82036fcc4425af
Reviewed-on: https://go-review.googlesource.com/c/tools/+/576655
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

gopls/internal/cache/analysis.go

@@ -1289,11 +1289,24 @@ func (act *action) exec() (interface{}, *actionSummary, error) {
 				}
 
 				// debugging #64547
-				if start < token.Pos(tokFile.Base()) {
+				fileStart := token.Pos(tokFile.Base())
+				fileEnd := fileStart + token.Pos(tokFile.Size())
+				if start < fileStart {
 					bug.Reportf("start < start of file")
+					start = fileStart
 				}
-				if end > token.Pos(tokFile.Base()+tokFile.Size()+1) {
+				if end < start {
+					// This can happen if End is zero (#66683)
+					// or a small positive displacement from zero
+					// due to recursively Node.End() computation.
+					// This usually arises from poor parser recovery
+					// of an incomplete term at EOF.
+					bug.Reportf("end < start of file")
+					end = fileEnd
+				}
+				if end > fileEnd+1 {
 					bug.Reportf("end > end of file + 1")
+					end = fileEnd
 				}
 
 				return p.PosLocation(start, end)