Use compile_error!

[killercup]

Rust 1.20 stabilized the compile_error! macro, which means we can finally output useful errors from macros and unusable feature-flag combinations!

Here are the places with code comments just waiting to use this macro:

diesel/diesel/src/macros/mod.rs

Lines 406 to 408 in b26472f

	// Remove this workaround as soon as rust issue 40872 is implemented
	// https://github.com/rust-lang/rust/issues/40872
	const ERROR: i32 = env!("invalid table! syntax");

diesel/diesel/src/test_helpers.rs

Line 50 in 453d8ff

// compile_error!("At least one backend must be enabled to run tests");

diesel/diesel_codegen/tests/test_helpers.rs

Line 50 in 453d8ff

// compile_error!("At least one backend must be enabled to run tests");

diesel/diesel/src/doctest_setup.rs

Line 144 in 4f87b47

// compile_error!("At least one backend must be enabled to run tests");

diesel/diesel/src/macros/insertable.rs

Line 501 in 148d0e4

// compile_error!("At least one backend must be enabled to run tests");

[notryanb]

@killercup - I'd like to tackle this issue. I think I may be able to also solve #624 while doing this?

[killercup]

@notryanb it's yours!

#624 might be a bit more tricky. Do you have a good idea how to solve it? I could only come up with one medium and one hard way… (see #624 (comment))

[notryanb]

👍 I'll look into #624 separately.

[sgrif]

Diesel CLI and Infer Schema should also use this in the places they currently fail to compile when built with 0 features.