Using rustc as a library hits "can't find crate for std" error

[huonw]

The following is the parse, expand & analyse steps of the compiler (cribbed from rustdoc). Running on any non-#[no_std] file causes the following error:

rustc_lib_example.rs:1:1: 1:1 error: can't find crate for `std`
rustc_lib_example.rs:1 #[feature(managed_boxes)];
                       ^

#[feature(managed_boxes)];
extern mod rustc;
extern mod syntax;

use std::os;
use rustc::driver::{driver, session};
use rustc::metadata::creader::Loader;
use syntax::diagnostic;

/// Extract the expanded ast of a crate, along with the codemap which
/// connects source code locations to the actual code.
fn main() {
    let path = Path::new(os::args()[1]);

    let parsesess = syntax::parse::new_parse_sess(None);
    let input = driver::FileInput(path);

    let sessopts = @session::Options {
        binary: ~"example",
        .. (*session::basic_options()).clone()
    };


    let diagnostic_handler = diagnostic::mk_handler(None);
    let span_diagnostic_handler =
        diagnostic::mk_span_handler(diagnostic_handler, parsesess.cm);

    let sess = driver::build_session_(sessopts, parsesess.cm,
                                      @diagnostic::DefaultEmitter as @diagnostic::Emitter,
                                      span_diagnostic_handler);

    let cfg = driver::build_configuration(sess);

    let crate = driver::phase_1_parse_input(sess, cfg.clone(), &input);

    let loader = &mut Loader::new(sess);
    let (crate, ast_map) = driver::phase_2_configure_and_expand(sess, cfg, loader, crate);
    driver::phase_3_run_analysis_passes(sess, &crate, ast_map);
}

This makes rustc completely unusable as a library for out-of-source-tree tools.

[alexcrichton]

One possible solution to this is #11068. The problem is that librustc will attempt to guess the current "sysroot" based on the path to the executable.

I wouldn't be so harsh as to say that rustc is completely unusable, you'll just need to explicitly specify a path or two for now, more a minor annoyance than a fundamental blocker.

[huonw]

I think it's more than minor: I don't know of a feasible way of getting the correct path cross-platform, and there's not even a sensible way of doing it on a single platform... some people install to /usr/local/..., others install to $HOME/..., sometimes just directly compile with $BUILD_DIR/x86-64_.../stage2/bin/rustc directly, without putting it in the PATH.

[jaredly]

@alexcrichton where would I specify that path? I'm running into this problem now (code here https://gist.github.com/jaredly/089b60e0649c4bda3cd9)

[alexcrichton]

I believe it's the maybe_sysroot option of Options

[steveklabnik]

Triage: this code has changed significantly, but I'm not sure if anything else has changed in the meantime to fix this.

[aturon]

Closing due to inactivity. Feel free to comment/reopen if this problem is actively causing you pain.