Give a bit more informative error message related to deleting isle_generated_code/

[larry0x]

PR #4143 added this error if a isle_generated_code/ directory exists and an "isle-in-source-tree" feature isn't enabled:

#[cfg(not(feature = "isle-in-source-tree"))]
{
    if explicit_isle_dir.is_dir() {
        eprintln!(concat!(
            "Error: directory isle_generated_code/ exists but is only used when\n",
            "`--feature isle-in-source-tree` is specified. To prevent confusion,\n",
            "this build script requires the directory to be removed when reverting\n",
            "to the usual generated code in target/. Please delete the directory and\n",
            "re-run this build.\n",
        ));
        std::process::exit(1);
    }
}

It says to delete the directory. However, for users who are not familiar with the inner workings of cranelift, it's not clear where this directory is. It took me a long while to find it:

~/.cargo/registry/src/index.crates.io-{hash}/cranelift-codegen-{version}/isle_generated_code

It may be helpful to add the explicit_isle_dir value to the error message so that users are informed exactly which directory they are supposed to delete.

[cfallin]

Hmm, something is going wrong here -- you shouldn't ever need to delete a directory in ~/.cargo. The intent of this feature was to allow Cranelift developers to see a version of the code in-tree when developing Cranelift, and should not be used when Cranelift is pulled in as a dependency from crates.io. Could you say more about your environment and how you're using Cranelift?

[larry0x]

@cfallin I don't use cranelift directly so not sure what's supposed to happen... I use wasmer. At wasmer 5.0.0 my program compiles with no problem. Once bumped to wasmer 5.0.1, compiling my program results in this error:

error: failed to run custom build command for `cranelift-codegen v0.110.2`

Caused by:
  process didn't exit successfully: `/Users/[...]/target/release/build/cranelift-codegen-725296997bf7694e/build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-changed=build.rs

  --- stderr
  Error: directory isle_generated_code/ exists but is only used when
  `--feature isle-in-source-tree` is specified. To prevent confusion,
  this build script requires the directory to be removed when reverting
  to the usual generated code in target/. Please delete the directory and
  re-run this build.

I followed the instruction and deleted:

~/.cargo/registry/src/index.crates.io-{hash}/cranelift-codegen-{version}/isle_generated_code

Now the program compiles just fine.

[cfallin]

@larry0x OK, thanks for the additional info. This error does not happen when using Cranelift by itself, or with Wasmtime (in this repo). We do not provide support for Wasmer -- that is a different project. I would recommend asking them for help if the issue occurs again; they may be setting options in some way that is causing this issue.