how-to-build-and-run.md: fix some text regarding bootstraping

[tshepang]

See https://github.com/rust-lang/rustc-dev-guide/pull/2599/files#r2403841373

r? @jieyouxu

[jieyouxu]

Nit: I would say to build the in-tree compiler and std. Which is exactly why in practice this is usually more useful for contributors if they actually need to produce std-using programs or run tests.

[jieyouxu]

I probably also would use ./x build library, I think I never used ./x build std myself. Though I guess I might even drop ./x build rustc or ./x build compiler for that matter, and only say something like:

• ./x build library -> produce stage 1 rustc/std toolchain
• ./x build library --keep-stage-std=1 -> if there's only compiler changes that do not need to rebuild std

[tshepang]

I prefer rustc/std over compiler/library since the latter feels less precise, especially since they are also directory names. Am also concerned of expanding this intro section more (and it already does point to --keep section, though I see now it was removed 257e73f).

[Noratrieb]

I don't think "std" is an alias for "library". std will not build proc_macro and test, for example.

[tshepang]

./x build library --keep-stage-std=1 -> if there's only compiler changes that do not need to rebuild std

why would you not ./x build compiler for this

[jieyouxu]

Because that only builds the compiler. If you want to run ui tests or build any non-no_std programs, having only the compiler is approximately useless. Or do you mean not using --keep-stage-std + ./x build library?

[tshepang]

it looked like ./x build compiler is equivalent to ./x build library --keep-stage-std 1, but maybe not

[tshepang]

@jieyouxu made further changes which include your suggestion (and that of @Noratrieb)