context: fix non-reference pattern used to match a reference

[tmatth]

Fixes:

error[E0658]: non-reference pattern used to match a reference (see issue #42640)
    --> src/context.rs:1590:13
     |
1590 |         for (name, start, end) in &self.map {
     |             ^^^^^^^^^^^^^^^^^^ help: consider using a reference: `&(name, start, end)`

[tmatth]

Note: this was with cargo 0.26.0-nightly (1d6dfea44 2018-01-26)

[tmatth]

Refs rust-lang/rust#42640

[rzumer]

Is this necessary? Latest stable rustc seemed to work.

[tmatth]

@rzumer failed on the above version of nightly, but seems non-fatal for e.g. rustc 1.28.0-nightly (1d4dbf488 2018-06-11) + cargo 1.28.0-nightly (e2348c2db 2018-06-07)

[rzumer]

Yes, but this is an unsupported version, stable 0.26 should compile. I am not against fixing compilation on older versions of rustc if readability does not suffer, however.

[tmatth]

@rzumer yeah I don't have strong opinions about it either.

[tmatth]

Closing as we can assume that it's unlikely for people to hit this unless they are on old nightly.