cargo check --message-format json regression of error span data on empty main.rs file

[Veetaha]

Problem
cargo check --message-format json returns invalid span data, particularly line_end and line_start are set to 0, though it is guaranteed to be 1-based.
This happens with a completly empty main.rs file.

Tested on the following cargo version:
cargo 1.42.0-nightly (9d32b7b01 2020-01-26)

Whilst on the stable version the returned span array is empty:
cargo 1.41.0 (626f0f40e 2019-12-03)

Steps

1. Clone this repo
2. Run rustup default nightly
3. Run cargo check --message-format json > check.json

Now you can see invalid span in check.json file.

I've already created and formatted check-1.41.json and check-1.42.json files with the output of different cargo versions.
Here are the crutial differences 1.41 and 1.42

Context
The issue was initially reported at rust-analyzer project repo here.

[ehuss]

Moved to rust-lang/rust, where diagnostics live.

Seems to be caused by #67770, cc @Centril.

[Veetaha]

@Centril additional info:
The regression is also present on the older version of nightly rustc:
rustc 1.42.0-nightly (b5a3341f1 2020-01-20)

This was the version where I initially spotted the issue, then updated and rechecked, but it was still present in cargo 1.42.0-nightly (9d32b7b01 2020-01-26).

[Centril]

Probably caused by 20ebb80.

[pnkfelix]

triage: P-high. Assigning to self.