feat: set temporary env whenever Julia is started with --project=--temp

- Update relevant docs.
- Add tests for `--project=--temp` and `JULIA_PROJECT="--temp"`.

This makes it convenient to start and run Julia in a temporary environment. This brings `Pkg.activate(; temp=true)` functionality to cmdline.

Author: ven-k

base/initdefs.jl

@@ -238,7 +238,8 @@ function init_active_project()
     set_active_project(
         project === nothing ? nothing :
         project == "" ? nothing :
-        startswith(project, "@") ? load_path_expand(project) : abspath(expanduser(project))
+        startswith(project, "@") ? load_path_expand(project) :
+        project == "--temp" ? mktempdir() : abspath(expanduser(project))
     )
 end
 

doc/src/manual/command-line-interface.md

@@ -95,7 +95,7 @@ The following is a complete list of command-line switches available when launchi
 |`-v`, `--version`                      |Display version information|
 |`-h`, `--help`                         |Print command-line options (this message).|
 |`--help-hidden`                        |Uncommon options not shown by `-h`|
-|`--project[={<dir>\|@.}]`              |Set `<dir>` as the active project/environment. The default `@.` option will search through parent directories until a `Project.toml` or `JuliaProject.toml` file is found.|
+|`--project[={<dir>\|--temp\|@.}]`      |Set `<dir>` as the active project/environment. Or, create a temporary environment with `--temp`. The default `@.` option will search through parent directories until a `Project.toml` or `JuliaProject.toml` file is found.|
 |`-J`, `--sysimage <file>`              |Start up with the given system image file|
 |`-H`, `--home <dir>`                   |Set location of `julia` executable|
 |`--startup-file={yes*\|no}`            |Load `JULIA_DEPOT_PATH/config/startup.jl`; if `JULIA_DEPOT_PATH` environment variable is unset, load `~/.julia/config/startup.jl`|

src/jloptions.c

@@ -102,7 +102,9 @@ static const char opts[]  =
     " --help-hidden              Uncommon options not shown by `-h`\n\n"
 
     // startup options
-    " --project[={<dir>|@.}]     Set <dir> as the active project/environment\n"
+    " --project[={<dir>|--temp|@.}]\n"
+    "                            Set <dir> as the active project/environment.\n"
+    "                            Or, create a temporary environment with `--temp`\n"
     " -J, --sysimage <file>      Start up with the given system image file\n"
     " -H, --home <dir>           Set location of `julia` executable\n"
     " --startup-file={yes*|no}   Load `JULIA_DEPOT_PATH/config/startup.jl`; if `JULIA_DEPOT_PATH`\n"

test/cmdlineargs.jl

@@ -225,6 +225,10 @@ let exename = `$(Base.julia_cmd()) --startup-file=no --color=no`
         @test expanded == readchomp(addenv(`$exename -e 'println(Base.active_project())'`, "JULIA_PROJECT" => "@foo", "HOME" => homedir()))
     end
 
+    # handling of temp in --project and JULIA_PROJECT
+    @test occursin(tempdir(), readchomp(`$exename --project=--temp -e 'println(Base.active_project())'`))
+    @test occursin(tempdir(), readchomp(addenv(`$exename -e 'println(Base.active_project())'`, "JULIA_PROJECT" => "--temp", "HOME" => homedir())))
+
     # --quiet, --banner
     let p = "print((Base.JLOptions().quiet, Base.JLOptions().banner))"
         @test read(`$exename                   -e $p`, String) == "(0, -1)"