cargo check doesn't check documentation code fences

[thegranddesign]

Considering a lot of people would use the output of cargo check in their editors to verify correctness of their code before running their tests, one would assume that cargo check --all would run compiler checks of the code in the docs tests, unfortunately that doesn't seem to be the case.

Current Behavior

It's possible for cargo check --all to run clean but cargo test to fail with compiler errors.

Expected Behavior

When docs have code fences that have compiler errors:

cargo check --all

and a new command option:

cargo check --docs

would fail.

[carols10cents]

Actually this looks like a duplicate of #3431 -- that issue is still open because cargo check --all does not include tests or doc tests. Closing as a duplicate, thank you for the report! ❤️