Certainly, SPARK's GPLv3 license is incompatible with Curl's license. Curl's license is MIT-ish, but adds a prohibition for those who use it from using the author's name to promote their business. That restriction is not compatible with GPLv3. Since the author of curl took the time to write that restriction, I would imagine it is more important to them than SPARK. Since the copyleft licenses are hostile to restricting free speech, it's safe to assume that all copyleft options would be similarly unacceptable.
Rust is dual APL2/MIT, and Curl's modified MIT is compatible with MIT, so no such issue would exist for Rust.
(Standard disclaimer, I'm not your lawyer, no citations offered, seek legal counsel.)
I do not know much about Wuffs, but it seems to be completely safe. No arithmetic overflows, no bound checking failures, no None unwrapping panics, no memory allocation failure panics.
Rust is dual APL2/MIT, and Curl's modified MIT is compatible with MIT, so no such issue would exist for Rust.
(Standard disclaimer, I'm not your lawyer, no citations offered, seek legal counsel.)