Add more detailed explanations for uncacheable flags

[Jake-Shadle]

While it's great that uncacheable flags have a high level "TooHard/Flag" attached to them, it would be really helpful to have a link to an issue for each one that explains why, as right now it's kind of disappointing to hit on an uncacheable compiler call, but not really have a detailed reason for why the flag is "hard". I opened #378 because it was apparent from the code why it was considered hard, but other ones aren't as clear immediately.

[luser]

Not a bad idea. I guess we could re-enable the project wiki here and make a page with sections describing why we don't handle particular flags or scenarios and then print links to the proper section in the stats output. Currently the non-cacheable reasons are just stored as HashMap<String, usize>:

sccache/src/server.rs

Line 1038 in 12008f6

pub not_cached: HashMap<String, usize>,

so we'd need to change that to have a place to store the URLs. Printing them in the --show-stats output would be straightforward:

sccache/src/server.rs

Line 1198 in 12008f6

if !self.not_cached.is_empty() {

[Jake-Shadle]

That sounds even better than what I was thinking. 😄

[TriplEight]

That would be very helpful.