Better error message about [T] as a bare type (and in general)

[SimonSapin]

From twitter:

Compilers really need to make suggestions about what can be done to resolve an error. e.g. for rustc the compiler complains about using T[] as a bare type. Should suggest that you need to use ~, etc. otherwise the error is very opaque.

https://twitter.com/alex_gaynor/status/435446756264779777
https://twitter.com/alex_gaynor/status/435446869452263424

This bug is not only about giving a better error message for this specific case, but also about giving suggestions in general.

[adrientetar]

This sounds like #12144.

[SimonSapin]

It’s related to #12144 (the suggestions of how to fix a problem could be given by rustc --explain, although having a short version inline would be valuable IMO), but not quite the same.

[treeman]

Edited as I used an outdated rustc. Now with DST, cc #12938

fn foo(x: [uint]) { }
fn main() { }

tst.rs:1:8: 1:9 error: mismatched types: expected `[uint]`, found `<generic #0>` (expected vector, found inferred type)
tst.rs:1 fn foo(x: [uint]) { }
                ^
error: aborting due to previous error

Which still could be improved.

[thestinger]

It's now a valid bare type and the error message is fine:

foo.rs:1:8: 1:9 error: variable `x` has dynamically sized type `[uint]` [E0151]
foo.rs:1 fn foo(x: [uint]) { }