Is refusing treatment, say for cancer, which prolongs life (and ? suffering) considered suicide?
It is impossible for the arahant to commit suicide due to mental issues, the figurative mental arrow. As for the physical pain, it is entirely possible IMO that that arahant can take measures to stop that feeling in some cases - done not out of aversion or dukkha.
An example would be to take morphine to relieve severe breathlessness in a terminally ill at the expense of shortening the life.
For the non-arahant considering suicide, there is always the mental arrow present.
"But in the case of a well-taught noble disciple, O monks, when he is touched by a painful feeling, he will not worry nor grieve and lament, he will not beat his breast and weep, nor will he be distraught. It is one kind of feeling he experiences, a bodily one, but not a mental feeling. It is as if a man were pierced by a dart, but was not hit by a second dart following the first one. So this person experiences feelings caused by a single dart only. It is similar with a well-taught noble disciple: when touched by a painful feeling, he will no worry nor grieve and lament, he will not beat his breast and weep, nor will he be distraught. He experiences one single feeling, a bodily one.
"Having been touched by that painful feeling, he does not resist (and resent) it. Hence, in him no underlying tendency of resistance against that painful feeling comes to underlie (his mind).
https://www.accesstoinsight.org/tipitak ... .nypo.html
While sat on one side venerable Dabba Mallaputta said this to the Gracious One: “Now is the time for my Complete Emancipation, Fortunate One.”
“Now is the time for whatever you are thinking, Dabba.”
Then venerable Dabba Mallaputta, after rising from his seat, worshipping and circumambulating the Gracious One, after going up into the sky, and sitting in cross-legged posture in the air, in the firmament, entering the fire-element, and emerging, attained Complete Emancipation.
Then while venerable Dabba Mallaputta—after going up into the sky, and sitting in cross-legged posture in the air, in the firmament, entering the fire-element, and emerging—was attaining Complete Emancipation, his body burning and being consumed, there was no charcoal and no ash evident.
https://suttacentral.net/ud8.9/en/anandajoti
"Now I am frail, Ananda, old, aged, far gone in years. This is my eightieth year, and my life is spent. Even as an old cart, Ananda, is held together with much difficulty, so the body of the Tathagata is kept going only with supports. It is, Ananda, only when the Tathagata, disregarding external objects, with the cessation of certain feelings [not certain dukkha], attains to and abides in the signless concentration of mind, that his body is more comfortable."
And what is right speech? Abstaining from lying, from divisive speech, from abusive speech, & from idle chatter: This is called right speech.