Misleading Error message when missing the let keyword

[memoryleak47]

If you write x : i32 = 0;
instead of let x : i32 = 0;
The error message ''type ascription is experimental" pops up.
I know little about type ascription, but I don't see any reason to use type ascription on an unresolved name (in this case x).
Something like "Did you mean let x : i32 = 0; would be preferable".

I think you should know this issue, but I did not find any duplicates.

Meta:

`rustc --version --verbose`

rustc 1.12.1 (d4f39402a 2016-10-19)
binary: rustc
commit-hash: d4f39402a0c2c2b94ec0375cd7f7f6d7918113cd
commit-date: 2016-10-19
host: x86_64-unknown-linux-gnu
release: 1.12.1

[Mark-Simulacrum]

Closing in favor of #34255.