Set temporary env whenever Julia is started with --project=@temp (#51149)

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

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

Author: ven-k

NEWS.md

@@ -42,6 +42,7 @@ Command-line option changes
 * Enabling or disabling color text in Julia can now be controlled with the
 [`NO_COLOR`](https://no-color.org/) or [`FORCE_COLOR`](https://force-color.org/) environment
 variables. ([#53742]).
+* `--project=@temp` starts Julia with a temporary environment.
 
 Multi-threading changes
 -----------------------

base/initdefs.jl

@@ -277,6 +277,7 @@ function load_path_expand(env::AbstractString)::Union{String, Nothing}
         # if you put a `@.` in LOAD_PATH manually, it's expanded late
         env == "@" && return active_project(false)
         env == "@." && return current_project()
+        env == "@temp" && return mktempdir()
         env == "@stdlib" && return Sys.STDLIB
         if startswith(env, "@script")
             if @isdefined(PROGRAM_FILE)

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

@@ -166,7 +166,7 @@ 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`                        |Print 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`](@ref JULIA_DEPOT_PATH) environment variable is unset, load `~/.julia/config/startup.jl`|

src/jloptions.c

@@ -115,7 +115,8 @@ static const char opts[]  =
     " --help-hidden                                 Print uncommon options not shown by `-h`\n\n"
 
     // startup options
-    " --project[={<dir>|@.}]                        Set <dir> as the active project/environment.\n"
+    " --project[={<dir>|@temp|@.}]                  Set <dir> as the active project/environment.\n"
+    "                                               Or, create a temporary environment with `@temp`\n"
     "                                               The default @. option will search through parent\n"
     "                                               directories until a Project.toml or JuliaProject.toml\n"
     "                                               file is found.\n"

test/cmdlineargs.jl

@@ -257,6 +257,10 @@ 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 tempdir() == readchomp(`$exename --project=@temp -e 'println(Base.active_project())'`)[1:lastindex(tempdir())]
+    @test tempdir() == readchomp(addenv(`$exename -e 'println(Base.active_project())'`, "JULIA_PROJECT" => "@temp", "HOME" => homedir()))[1:lastindex(tempdir())]
+
     # --quiet, --banner
     let p = "print((Base.JLOptions().quiet, Base.JLOptions().banner))"
         @test read(`$exename                   -e $p`, String) == "(0, -1)"