bootstrap: improve 'build --help' by explaining what exactly the last example does

[RalfJung]

I recently found myself confused about what exactly gets built how often when I run which command; I think this would have helped me.

One thing I did not touch, but I feel could also be improved, is the wording just above: "For a quick build with a usable compile, you can pass". I am not a native speaker, but this sounds odd to me. Do you mean "For a quick build of a usable compiler" (but then it should say where the usable compiler is produced)? Or do you mean "For a quick build testing if the compiler is usable"? I can reword this, but I'd like to make sure I understand the intent of the message.

What about

    For a quick build of a usable compiler, you can pass:

        ./x.py build --stage 1 src/libtest

    This will first build everything once (like --stage 0 without further
    arguments would), and then use the compiler built in stage 0 to build
    src/libtest and its dependencies.
    Once this is done, build/$ARCH/stage1 contains a usable compiler.

However, I am not sure this is actually true. In particular, why even bother building the libstd in stage 1? AFAIK that ends up in build/*/stage1-std, not in build/*/stage1 (which is filled from build/*/stage0-*).

[rust-highfive]

r? @brson

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[aidanhs]

r? @alexcrichton

"For a quick build with a usable compile, you can pass".

I looked at your diff before reading your comment, and this confused me independently. Pretty sure it should be 'compiler'.

However, I am not sure this is actually true. In particular, why even bother building the libstd in stage 1?

The compiler built during stage0 (stage0-out) cannot necessarily compile with the snapshot libstd used to build stage0 (stage0-in) because metadata may differ. To ensure libstd is compatible, it needs to be built with the same compiler it gets used with.

Targeting stage 1 libtest is an indirect way of saying "I need both a stage 1 compiler and a libstd compatible with it present in the sysroot".

[alexcrichton]

Looks good to me! The wording in the PR description also looks great to me, want to insert that as well?

[RalfJung]

Targeting stage 1 libtest is an indirect way of saying "I need both a stage 1 compiler and a libstd compatible with it present in the sysroot".

I guess my question is, where is that sysroot? Nothing seems to actually assemble that compiler and its compatible libstd in a single folder hierarchy when I just say --stage 1 src/libtest, does it?

@alexcrichton will do.

[RalfJung]

I inserted the wording from the PR description, and confirmed that it is actually true -- i.e., that the given folder (starting with a clean checkout) actually contains a usable compiler.

[alexcrichton]

@bors: r+

Thanks!

I guess my question is, where is that sysroot?

Technically build/$target/stage1 is the "sysroot" (e.g. it's a valid --sysroot argument). This is assembled by rustbuild in the various "Copying ..." messages it emits.

[bors]

📌 Commit 7359c79 has been approved by alexcrichton

[bors]

⌛ Testing commit 7359c79 with merge 9454dd5...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 9454dd5 to master...