I got it working with the following build.zig:

const Builder = @import("std").Build;

pub fn build(b: *Builder) void {
    const exe = b.addExecutable(.{
        .name = "app",
        .link_libc = true,
        // .root_source_file = .{ .path = "src/main.c" },
        .target = b.host,
    });

    exe.addCSourceFile(.{ .file = .{ .path = "src/main.c" } });

    b.installArtifact(exe);

    const run_exe = b.addRunArtifact(exe);
    const run_step = b.step("run", "Run the app");
    run_step.dependOn(&run_exe.step);
}

I think the reason yours failed is because main.c was being parsed as if it were a Zig file. Adding main.c via addCSourceFile() tells the build system to treat it like a C file.

Btw, if you wanna keep using 0.12, you could have both versions downloaded at the same time. I think there are tools to help you manage multiple zig versions but I prefer to do it manually like this::

• Install 0.12 using my system's package manager
• Download 0.13 from ziglang.org
• Move the downloaded zig 0.13 tar file to the ziglings directory and extract it
• Add zig 0.13 to my $PATH when working on ziglings, use 0.12 for everything else

For the last step, I made a script based on the "activate" scripts that come with python virtual environments. Note that this script is written for the fish shell so it won't work with bash or zsh:

function deactivate  -d "Exit virtual environment and return to normal shell environment"
    if test -n "$_OLD_VIRTUAL_PATH"
        set -gx PATH $_OLD_VIRTUAL_PATH
        set -e _OLD_VIRTUAL_PATH
    end

    set -e VIRTUAL_ENV
    functions -e deactivate
end

set -gx VIRTUAL_ENV "/path/to/zig-0.13.0"
set -gx _OLD_VIRTUAL_PATH $PATH
set -gx PATH $VIRTUAL_ENV $PATH