How do I transition a major mode (vhdl-mode) to use the new
compile.el? From reading the compilation-error-regexp-alist
description it sounds like it should still work with the old alist
but then it doesn't work right.
It should work. Give us more details of the problems you encountered.
Only error messages with a certain syntax ("<filename>:<linenumber>:") are
found and highlighted, but not others which should also match regexps
defined in the alist. Just as if only one regexp from (the middle) of the
alist is active. The same alist highlighted all errors with the old
compile.el.
How do you set the alist? With the new code, the alist is not used
directly, but instead it's "compiled" to a set of font-lock-keywords
rules, so if you change it after "compilation" it won't take effect.