definitely 1.0.0

Codepaths that are statically unreachable according to the compiler's intraprocedural control flow analysis
Documentation 
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#[rustversion::attr(not(nightly), ignore = "requires nightly")]
#[cfg_attr(miri, ignore = "incompatible with miri")]
#[test]
fn ui() {
    let t = trybuild::TestCases::new();
    t.compile_fail("tests/ui/*.rs");
}