VSCode rustc problem matcher should use fileLocation autoDetect

[rickvanprim]

Per microsoft/vscode#449 and https://code.visualstudio.com/docs/editor/tasks#_defining-a-problem-matcher , problem matchers can now be defined that support both relative and absolute paths. I propose that https://github.com/rust-analyzer/rust-analyzer/blob/master/editors/code/package.json#L602-L634 be updated to specify "fileLocation": "autoDetect". Cargo seems to compiles path based dependencies with absolute paths when they are not located inside of the current crate/workspace root (rust-lang/cargo#8506 (comment)), and presently the problem matcher that ships with rust-analyzer attempts to treat those paths as relative.

Current workaround is to specify inside of tasks.json:

"problemMatcher": [
    {
        "base": "$rustc",
        "fileLocation": "autoDetect"
    }
]

[kjeremy]

Do you have an easily reproducible test case for this? PRs welcome!

[rickvanprim]

Here's a zip file of a basic workspace with two crates, executable and library. If you run cargo build inside of executable (or specify it from the root with --manifest-path executable/Cargo.toml), you'll notice that the compile error inside of library prints an absolute path.
RustAnalyzer_5482.zip