stop calling realpath in the compiler

[andrewrk]

realpath is problematic for several reasons:

1. it is not available for many targets, including WASI and BSDs
2. it just straight up doesn't work for long file paths, if you have /long1/long2/foo for example where long1 and long2 together exceed MAX_PATH, then the OS will give an error for that path when asked for the realpath.
3. it fails to respect the lazy nature of symlinks
4. race conditions with moving files around

Robust software avoids realpath.

/home/andy/dev/zig/lib/std/os.zig:5079:15: error: realpath is forbidden
    if (true) @compileError("realpath is forbidden");
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
referenced by:
    init: /home/andy/dev/zig/src/codegen/llvm.zig:802:38
    create: /home/andy/dev/zig/src/codegen/llvm.zig:754:27
    createEmpty: /home/andy/dev/zig/src/link/NvPtx.zig:41:39
    openPath: /home/andy/dev/zig/src/link/NvPtx.zig:63:12
    openPath: /home/andy/dev/zig/src/link.zig:363:41
    update: /home/andy/dev/zig/src/Compilation.zig:2003:38
    updateModule: /home/andy/dev/zig/src/main.zig:3864:17
    cmdBuild: /home/andy/dev/zig/src/main.zig:4510:9
    mainArgs: /home/andy/dev/zig/src/main.zig:285:16
    main: /home/andy/dev/zig/src/main.zig:213:12

Looks like this regressed in #13843.

Maybe we can even add a std lib option to disallow realpath on a per-application basis and then opt into that restriction in the compiler.

Related:

• Cross-Compiling for Windows breaks error stack traces #12729 (comment)
• unable to build zig from source on NetBSD #13680

[semarie]

Regarding availability of realpath on BSDs, the problem isn't really std.os.realpath() function (it is POSIX and wildly defined), but std.fs.Dir.realpath() counterpart which use a directory descriptor (no POSIX equivalent) and is implemented using std.os.getFdPath() (which isn't available for many BSDs).

[andrewrk]

There can be exactly one call to realpath in the compiler, in the CLI. It can set up the Package.Path corresponding to the current working directory. Depending on a few things, probably including user configuration, it may call realpath to populate the string for that path object, or it might leave it as relative to the cwd. Then, the rest of the codebase must never call realpath() and instead respect that cwd Path object.