Cannot run doctests with `cargo test`

[rfwatson]

Running cargo test with a wasm target:

cargo test --target wasm32-unknown-unknown

... does not run the doctests at all. This should be fixable with unstable feature:

cargo test --target wasm32-unknown-unknown -Zdoctest-xcompile

but it results in an error:

   Compiling weblog v0.2.0 (/home/rob/dev/weblog)
    Finished test [unoptimized + debuginfo] target(s) in 0.07s
   Doc-tests weblog

running 4 tests
test src/lib.rs - (line 23) ... FAILED
test src/lib.rs - (line 15) ... FAILED
test src/lib.rs - (line 31) ... FAILED
test src/lib.rs - (line 41) ... FAILED

failures:

---- src/lib.rs - (line 23) stdout ----
Test executable failed (exit code 126).

stderr:
/tmp/rustdoctestrcp9CA/rust_out: /tmp/rustdoctestrcp9CA/rust_out: cannot execute binary file


---- src/lib.rs - (line 15) stdout ----
Test executable failed (exit code 126).

stderr:
/tmp/rustdoctesteKqwSz/rust_out: /tmp/rustdoctesteKqwSz/rust_out: cannot execute binary file


---- src/lib.rs - (line 31) stdout ----
Test executable failed (exit code 126).

stderr:
/tmp/rustdoctest1vEO6e/rust_out: /tmp/rustdoctest1vEO6e/rust_out: cannot execute binary file


---- src/lib.rs - (line 41) stdout ----
Test executable failed (exit code 126).

stderr:
/tmp/rustdoctestjrqw3o/rust_out: /tmp/rustdoctestjrqw3o/rust_out: cannot execute binary file



failures:
    src/lib.rs - (line 15)
    src/lib.rs - (line 23)
    src/lib.rs - (line 31)