Introduce -m/--module flag to execute a main function in a package

This aims to bring similar functionality to Julia as the `-m` flag for Python which exists to directly run some function in a package and being able to pass arguments to that function.

While in Python, `python -m package args` runs the file `<package>.__main__.py`, the equivalent Julia command (`julia -m Package args`) instead runs `Package.main(args)`. A package can also have a `main` function in a submodule runnable with `julia -m Package.SubModule args`. The package is assumed to be installed in the environment `julia` is run in.

An example usage could be:

Add the package:
```
(@v1.11) pkg> add https://github.com/KristofferC/Rot13.jl
     Cloning git-repo `https://github.com/KristofferC/Rot13.jl`
    Updating git-repo `https://github.com/KristofferC/Rot13.jl`
   Resolving package versions...
    Updating `~/.julia/environments/v1.11/Project.toml`
  [43ef800a] + Rot13 v0.1.0 `https://github.com/KristofferC/Rot13.jl#master`
    Updating `~/.julia/environments/v1.11/Manifest.toml`
  [43ef800a] + Rot13 v0.1.0 `https://github.com/KristofferC/Rot13.jl#master`
```

And then it can be run (since it has a `main` function) via:

```
❯ ./julia/julia -m Rot13 "encrypt this for me" "and this as well"
rapelcg guvf sbe zr
naq guvf nf jryy
```

NEWS.md

@@ -35,6 +35,7 @@ Command-line option changes
 This is intended to unify script and compilation workflows, where code loading may happen
 in the compiler and execution of `Main.main` may happen in the resulting executable. For interactive use, there is no semantic
 difference between defining a `main` function and executing the code directly at the end of the script ([50974]).
+* The `-m/--module` flag can be passed to run the `main` function inside a package with a set of arguments.
 
 Multi-threading changes
 -----------------------

base/client.jl

@@ -240,7 +240,7 @@ function exec_options(opts)
         if cmd_suppresses_program(cmd)
             arg_is_program = false
             repl = false
-        elseif cmd == 'L'
+        elseif cmd == 'L' || cmd == 'm'
             # nothing
         elseif cmd == 'B' # --bug-report
             # If we're doing a bug report, don't load anything else. We will
@@ -292,6 +292,30 @@ function exec_options(opts)
         elseif cmd == 'E'
             invokelatest(show, Core.eval(Main, parse_input_line(arg)))
             println()
+        elseif cmd == 'm'
+            mods = split(arg, ".")
+            pkg = popfirst!(mods)
+            # Some more validation of `mods` and `pkg`?
+            m = Base.require(Main, Symbol(pkg))
+            while !isempty(mods)
+                if !isdefined(m, Symbol(mods[1])) &&
+                    error("`$(mods[1])` not defined")
+                end
+                m = getfield(m, Symbol(popfirst!(mods)))
+                if !(m isa Module)
+                    error("`$(mods[1])` is not a module")
+                end
+            end
+            if !isdefined(m, :main)
+                error("no `main` method defined in module")
+            end
+            main = getfield(m, Symbol("main"))
+            retval = @invokelatest main(ARGS)
+            retval === nothing && (retval = 0)
+            if !(retval isa Integer)
+                error("`main` method did not return an integer")
+            end
+            exit(retval)
         elseif cmd == 'L'
             # load file immediately on all processors
             if !distributed_mode

doc/man/julia.1

@@ -100,6 +100,10 @@ Enable or disable incremental precompilation of modules
 -e, --eval <expr>
 Evaluate <expr>
 
+.TP
+-m, --module <Package>[.SubModule] [args]
+Run function <Package>.[SubModule.]main(args)
+
 .TP
 -E, --print <expr>
 Evaluate <expr> and display the result

src/jloptions.c

@@ -118,6 +118,8 @@ static const char opts[]  =
     // actions
     " -e, --eval <expr>          Evaluate <expr>\n"
     " -E, --print <expr>         Evaluate <expr> and display the result\n"
+    " -m, --module <Package>[.SubModule] [args]\n"
+    "                            Run function `<Package>.[SubModule.]main(args))\n"
     " -L, --load <file>          Load <file> immediately on all processors\n\n"
 
     // parallel options
@@ -261,7 +263,7 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
            opt_gc_threads,
            opt_permalloc_pkgimg
     };
-    static const char* const shortopts = "+vhqH:e:E:L:J:C:it:p:O:g:";
+    static const char* const shortopts = "+vhqH:e:E:L:J:C:it:p:O:g:m:";
     static const struct option longopts[] = {
         // exposed command line options
         // NOTE: This set of required arguments need to be kept in sync
@@ -274,6 +276,7 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
         { "banner",          required_argument, 0, opt_banner },
         { "home",            required_argument, 0, 'H' },
         { "eval",            required_argument, 0, 'e' },
+        { "module",          required_argument, 0, 'm' },
         { "print",           required_argument, 0, 'E' },
         { "load",            required_argument, 0, 'L' },
         { "bug-report",      required_argument, 0, opt_bug_report },
@@ -412,6 +415,7 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
         case 'e': // eval
         case 'E': // print
         case 'L': // load
+        case 'm': // module
         case opt_bug_report: // bug
         {
             size_t sz = strlen(optarg) + 1;
@@ -425,6 +429,10 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
             ncmds++;
             cmds[ncmds] = 0;
             jl_options.cmds = cmds;
+            if (c == 'm') {
+                optind -= 1;
+                goto parsing_args_done;
+            }
             break;
         }
         case 'J': // sysimage
@@ -860,6 +868,7 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
                       "This is a bug, please report it.", c);
         }
     }
+    parsing_args_done:
     if (codecov || malloclog) {
         if (pkgimage_explicit && jl_options.use_pkgimages) {
             jl_errorf("julia: Can't use --pkgimages=yes together "

test/loading.jl

@@ -1331,3 +1331,8 @@ end
         @test filesize(cache_path) != cache_size
     end
 end
+
+@testset "-m" begin
+    rot13proj = joinpath(@__DIR__, "project", "Rot13")
+    @test readchomp(`$(Base.julia_cmd()) --startup-file=no --project=$rot13proj -m Rot13 --project nowhere ABJURER`) == "--cebwrpg\nabjurer\nNOWHERE"
+end

test/project/Rot13/Project.toml

@@ -0,0 +1,3 @@
+name = "Rot13"
+uuid = "43ef800a-eac4-47f4-949b-25107b932e8f"
+version = "0.1.0"

test/project/Rot13/src/Rot13.jl

@@ -0,0 +1,17 @@
+module Rot13
+
+function rot13(c::Char)
+    shft = islowercase(c) ? 'a' : 'A'
+    isletter(c) ? c = shft + (c - shft + 13) % 26 : c
+end
+
+rot13(str::AbstractString) = map(rot13, str)
+
+function main(ARGS)
+    for arg in ARGS
+        println(rot13(arg))
+    end
+    return 0
+end
+
+end # module Rot13