Rollup of 25 pull requests by alexcrichton · Pull Request #38697 · rust-lang/rust (original) (raw)

This commit switches the rustbuild build system to compiling the compiler twice for a normal bootstrap rather than the historical three times.

Rust is a bootstrapped language which means that a previous version of the compiler is used to build the next version of the compiler. Over time, however, we change many parts of compiler artifacts such as the metadata format, symbol names, etc. These changes make artifacts from one compiler incompatible from another compiler. Consequently if a compiler wants to be able to use some artifacts then it itself must have compiled the artifacts.

Historically the rustc build system has achieved this by compiling the compiler three times:

• An older compiler (stage0) is downloaded to kick off the chain.
• This compiler now compiles a new compiler (stage1)
• The stage1 compiler then compiles another compiler (stage2)
• Finally, the stage2 compiler needs libraries to link against, so it compiles all the libraries again.

This entire process amounts in compiling the compiler three times. Additionally, this process always guarantees that the Rust source tree can compile itself because the stage2 compiler (created by a freshly created compiler) would successfully compile itself again. This property, ensuring Rust can compile itself, is quite important!

In general, though, this third compilation is not required for general purpose development on the compiler. The third compiler (stage2) can reuse the libraries that were created during the second compile. In other words, the second compilation can produce both a compiler and the libraries that compiler will use. These artifacts must be compatible due to the way plugins work today anyway, and they were created by the same source code so they should be compatible as well.

So given all that, this commit switches the default build process to only compile the compiler three times, avoiding this third compilation by copying artifacts from the previous one. Along the way a new entry in the Travis matrix was also added to ensure that our full bootstrap can succeed. This entry does not run tests, though, as it should not be necessary.

To restore the old behavior of a full bootstrap (three compiles) you can either pass:

./configure --enable-full-bootstrap

or if you're using config.toml:

[build]
full-bootstrap = true

Overall this will hopefully be an easy 33% win in build times of the compiler. If we do 33% less work we should be 33% faster! This in turn should affect cycle times and such on Travis and AppVeyor positively as well as making it easier to work on the compiler itself.