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I diminish my minor modes into single Unicode characters to save space. However, this obviously breaks down in a Linux TTY for instance. How can I detect if the current display method supports Unicode (or better yet, can display a specific character)?
Note: Simply detecting a graphical frame is not enough, as many graphical terminal emulators support Unicode, but emacs does not run in a graphical frame.
You cannot.
If Emacs is speaking to a terminal, it only knows which coding system is spoken by the terminal. If the coding system is a locale-specific one (say, ISO 8859-1 or Shift-JIS), then it is a good bet that the terminal supports most or all of the characters in the coding system. If the coding system is Unicode-based, then there is no way to know which particular subset of Unicode the terminal is able to display.
On a Unicode terminal, char-displayable-p answers unicode for all Unicode characters.
How can I detect if the current display method supports Unicode (or better yet, can display a specific character)?
You can detect if emacs can display a specific character with the char-displayable-p function.
(char-displayable-p ?☺)
You can insert such unicode chars with C-x 8, or use their hex code preceded by a \x.
display-graphic-palways returnsnilwhen Emacs is running in a terminal, whether it's “graphical” or not.display-graphic-pbeingnilis not strong enough to guarantee a lack of Unicode.