If the user selects "diesel generator" (i.e., misses the 's'), the resultant HTML would be

<a href=foo>diesel generator</a>s

The correct wikitext version of this is indeed

[[diesel generator]]<nowiki />s

… which is probably not what the user wanted to do, but it appears to be what they asked VisualEditor to do.

Consequently, I'm going to mark this as INVALID [If this is not the case - that the user did indeed select the entirety of the two words and there's a bug, I'd really love to know what browser they were using so we can hunt down the problem in our selection code!]

Had they instead selected "diesel generators", VisualEditor/Parsoid would have created this as:

[[diesel generator|diesel generators]]

… which is not ideal (I'd prefer the shorter [[diesel generator]]s) but does the job. (I've just triple-checked this in Firefox, Chrome and Safari - it doesn't appear to have broken.)

We could always expand users' selections to what we think they should have selected, but this would make it impossible for users to do some things that they may want to do, albeit less frequently. I'm not sure that preventing users from doing things that they can do in wikitext would be a good solution, in general.