"Explicit lifetime bound required" message should give suggestions

[nikomatsakis]

See comments on discuss:

• http://discuss.rust-lang.org/t/confused-by-lifetime-error-messages-tell-me-about-it/358/20?u=nikomatsakis
• http://discuss.rust-lang.org/t/confused-by-lifetime-error-messages-tell-me-about-it/358/22?u=nikomatsakis

Nominating for P-high, not 1.0.

[huonw]

cc #16874

[eddyb]

"trait object requires explicit lifetime bound" would be less confusing (when it occurs on a trait) - I've seen many cases where a trait was accidentally used instead of a regular type and there was no obvious hint as to why a lifetime bound would be required at all.

[steveklabnik]

It seems that the span for one of these errors has gotten better:

       "the lifetime bound for this object type cannot be deduced \
                           from context; please supply an explicit bound");

The Discuss thread is difficult because there aren't a lot of contained examples, and we don't have an extended error explanation for this message, nor any tests that I can find?

$ git grep "explicit lifetime bound required"
src/librustc_typeck/astconv.rs:                  "ambiguous lifetime bound, explicit lifetime bound required");
src/librustc_typeck/diagnostics.rs:    E0227, // ambiguous lifetime bound, explicit lifetime bound required
src/librustc_typeck/diagnostics.rs:    E0228, // explicit lifetime bound required

[brson]

Close. Old and unloved.