Building servo with existing Rust installation

[AndreiBarsan]

Shouldn't building servo also work with a pre-existing Rust installation? Since Rust has to install llvm and itself (which includes lengthy downloads and long compile times) the whole process ends up lasting several hours on my machine, most of which being the Rust toolchain - which is already present on my computer.

Edit:

Note: configure seems to detect my existing installation, but proceeds to clone Rust's github repo anyway

Edit 2:

After some confusing compilation errors, it turns out that it was, in fact, using the version I had installed. Which was a tad old (0.5). I've removed it and compiling the head trunk version of rustc right now.

[jdm]

@brson Do you understand how it's set up? Servo's always built with my system rust install without doing anything special.

[brson]

The configure script has some old code for detecting or specifying the rust compiler to use (--local-rust-root), but I never bothered to update it when we pulled rust into the tree. As you both suggest it appears to properly detect and use the local rustc, but still builds the in-tree version anyway. It should be pretty easy to have configure set a flag to indicate it's using an out-of-tree Rust and then modify the build rule in Makefile.in.

I don't recommend building with an out-of-tree Rust though because servo typically requires an exact revision of Rust and it's easier to let the build system deal with that. The build times are long though and I have some plans to alleviate them, including modifying the Rust build to reuse the snapshot LLVM and the Servo build to only build Rust up through stage1 (of course, I have had these plans for a while so I'm not sure when they will bear fruit).

[brson]

I'll see if I can hack up the build a bit this afternoon to make this work. We'll still need to clone Rust, but not build it.

[brson]

OK I changed the configure script so that, by default, it will not look for a local rust installation, but if you do pass --local-rust-root it will use it and not build rust.