example apps: Update Rust example for Rust 1.19

[nvb]

This includes a few changes:

• Use the ? operator in place of the try! macro. This new operator
  stabilized a few versions ago and is now the preferred syntax.
• Use loop break value functionality, which stabilized in Rust
  1.19.
• Reformat using rustfmt, which seems to have changed since it was last
  run on this code.

[cockroach-teamcity]

This change is 

[cockroach-teamcity]

http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/cc7afc5d403403f60230609389ea04c241b4b663/

[tamird]

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Reviewed 2 of 2 files at r1.
Review status: all files reviewed at latest revision, 2 unresolved discussions, all commit checks successful.

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_includes/app/basic-sample.rs, line 7 at r1 (raw file):

fn main() {
    let conn = Connection::connect("postgresql://maxroach@localhost:26257/bank", TlsMode::None)
        .unwrap();

while you're here, want to replace these unwraps with expects?

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_includes/app/txn-sample.rs, line 21 at r1 (raw file):

            r => break r,
        }
    };

for bonus points, you can remove res entirely, right?

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Comments from Reviewable

[cockroach-teamcity]

http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/de3fec11fc87663f9e6c0fe71e3d17a13ea86ce6/

[nvb]

Review status: 1 of 2 files reviewed at latest revision, 2 unresolved discussions, some commit checks pending.

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_includes/app/basic-sample.rs, line 7 at r1 (raw file):

Previously, tamird (Tamir Duberstein) wrote…

while you're here, want to replace these unwraps with expects?

Ehh, I'd rather not clutter the example application with a bunch of .expect("it should connect, duh") lines. This is just for documentation, and I don't think there are any cases in here where a message would help with comprehension.

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_includes/app/txn-sample.rs, line 21 at r1 (raw file):

Previously, tamird (Tamir Duberstein) wrote…

for bonus points, you can remove res entirely, right?

Done. Good call!

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Comments from Reviewable

[tamird]

Reviewed 1 of 1 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[jseldess]

LGTM. Thanks for the update, @nvanbenschoten.