[-Zbuild-std] Only build libtest when libstd is built

[phil-opp]

Currently libtest is always compiled when a compilation unit uses a test harness. This implicitly adds builds the standard library too because libtest depends on it. This breaks the use of custom test frameworks in no_std crates as reported in #7216 (comment).

This pull request fixes the issue by only building libtest if libstd is built. This makes sense in my opinion because when the user explicitly specified -Zbuild-std=core, they probably don't want to build the full standard library and rather get a compilation error when they accidentally use libtest.

[rust-highfive]

r? @ehuss

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[alexcrichton]

@bors: r+

Seems like a reasonable workaround for now!

[bors]

📌 Commit 6b4fd44 has been approved by alexcrichton

[bors]

⌛ Testing commit 6b4fd44 with merge a41885e9b40ac3801a48b355a0f95b84c935266a...

[bors]

💔 Test failed - checks-azure

[ehuss]

---- freshness::rebuild_on_mid_build_file_modification stdout ----
running d:\a\1\s\target\debug\cargo.exe build
running d:\a\1\s\target\debug\cargo.exe build
thread 'freshness::rebuild_on_mid_build_file_modification' panicked at '
Expected: execs
but: exited with exit code: 101
--- stdout

--- stderr
Compiling root v0.1.0 (D:\a\1\s\target\cit\t703\foo\root)
error: proc-macro derive panicked
--> root\src\lib.rs:5:22
|
5 | #[derive(Noop)]
| ^^^^
|
= help: message: called Result::unwrap() on an Err value: Os { code: 10054, kind: ConnectionReset, message: "An existing connection was forcibly closed by the remote host." }

I have never seen that error before, weird!

@bors retry

[bors]

⌛ Testing commit 6b4fd44 with merge 7ac51b7...

[bors]

☀️ Test successful - checks-azure
Approved by: alexcrichton
Pushing 7ac51b7 to master...