Try to diagnose "cannot find type SourceFile" errors better

[dtolnay]

Cargo's logic for what cfg get passed to the build script build, what to the library build, and what to the documentation build is apparently super subtle. See the discussion under rust-lang/docs.rs#1580.

[dtolnay]

The cfg in this PR on the compile_error! is derived from:

proc-macro2/build.rs

Lines 107 to 109 in 3ff70da

	if version.nightly || !semver_exempt {
	println!("cargo:rustc-cfg=wrap_proc_macro");
	}

and:

proc-macro2/build.rs

Lines 115 to 117 in 3ff70da

	if semver_exempt && version.nightly {
	println!("cargo:rustc-cfg=super_unstable");
	}

which means:

all(procmacro2_semver_exempt, wrap_proc_macro, not(super_unstable))

= semver_exempt && wrap_proc_macro && !super_unstable

= semver_exempt && (nightly || !semver_exempt) && !(semver_exempt && nightly)

= true && (nightly || !true) && !(true && nightly) || false && (nightly || !false) && !(false && nightly)

= true && (nightly || false) && !nightly || false

= nightly && !nightly

= false