Question: The following stanza is from Robert Frost's *Into My Own*:

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> One of my wishes is that those dark trees,  
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>  So old and firm they scarcely show the breeze,  
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>  Were not, as ’twere, the merest mask of gloom,  
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>  But stretched away unto the **edge of doom**.
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(Emphasis mine.)

I am guessing that the "edge of doom" is referring to the end of time -- as in, The trees are stretching into the end of time and space. Is this reading correct? Where does the phrase the "edge of doom" come from? At first when I read it, I thought that the "edge of doom" was a dark reference (similar to "mask of gloom"), but it seems to be closer to a wedding vow, such as "Our marriage will last unto the edge of doom".

Answer: 
Your reading of the poem makes sense. It might help to know that the dark trees are symbolizing the future in this poem. The second line tells us that these trees rarely show a breeze. This means that we cannot predict the future. 

In the fourth line, Frost expresses his wish that those dark trees in the first line were stretched away to the edge of doom. This means that he hoped that the future is stretched far away. So, the edge of doom means here far away (in time). This seems consistent with your view, since it is also used in that way in the sentence you quoted, "Our marriage will last unto the edge of doom". 

Based on: [Analysis of "Into My Own"](https://letterpile.com/writing/Robert-Frosts-Into-My-Own-Analysis).
The author could be alluding to Shakespeare's [*Macbeth* (Act IV, Scene 1, with the witches)](http://www.gutenberg.org/cache/epub/2264/pg2264.html) :

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> A shew of eight Kings, and Banquo last, with a glasse in his hand.
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> Macb. Thou art too like the Spirit of Banquo: Down:
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> Thy Crowne do's seare mine Eye-bals. And thy haire
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> Thou other Gold-bound-brow, is like the first:
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> A third, is like the former. **Filthy Hagges**,
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> Why do you shew me this? - A fourth? Start eyes!
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> **What will the Line stretch out to'th' cracke of Doome?**
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[*Macbeth* (Act IV, Scene 1, emphasis mine)](http://www.gutenberg.org/cache/epub/2264/pg2264.html)

Here, Shakespeare is using the verb *stretch* in a very similar manner, having a character inquire of the witches whether something (in this case, a family dynasty of kings, each succeeding his father, which have appeared in an apparition) will last until doomsday.

The "Filthy Hagges" (i.e. the witches) could be being referenced or alluded to by Frost's gloomy imagery that could be an apt metaphor to describe a stereotypical witch's lair.

Earlier in this scene in Macbeth, we see some tree and forest related imagery, which could map to Frost's use of arboreal language:

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> Thunder 3 Apparation, a **Childe Crowned, with a Tree in his hand**.
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> What is this, that rises like the issue of a King,
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> And weares vpon his Baby-brow, the round
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> And top of Soueraignty?
>  All. Listen, but speake not too't
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> 3 Appar. Be Lyon metled, proud, and take no care:
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> Who chafes, who frets, or where Conspirers are:
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> Macbeth shall neuer vanquish'd be, vntill
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> **Great Byrnam Wood, to high Dunsmane Hill**
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> **Shall come against him.**
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[*Macbeth* (Act IV, Scene 1, emphasis mine)](http://www.gutenberg.org/cache/epub/2264/pg2264.html)