Enable switching to the previous environment

src/API.jl

@@ -12,7 +12,7 @@ using Serialization
 
 import ..depots, ..depots1, ..logdir, ..devdir, ..printpkgstyle
 import ..Operations, ..GitTools, ..Pkg, ..Registry
-import ..can_fancyprint, ..pathrepr, ..isurl
+import ..can_fancyprint, ..pathrepr, ..isurl, ..PREV_ENV_PATH
 using ..Types, ..TOML
 using ..Types: VersionTypes
 using Base.BinaryPlatforms
@@ -1527,9 +1527,11 @@ function status(ctx::Context, pkgs::Vector{PackageSpec}; diff::Bool=false, mode=
 end
 
 
-function activate(;temp=false, shared=false, io::IO=stderr_f())
+function activate(;temp=false, shared=false, prev=false, io::IO=stderr_f())
     shared && pkgerror("Must give a name for a shared environment")
     temp && return activate(mktempdir(); io=io)
+    prev && return activate(PREV_ENV_PATH[]; io=io)
+    PREV_ENV_PATH[] = Base.active_project()
     Base.ACTIVE_PROJECT[] = nothing
     p = Base.active_project()
     p === nothing || printpkgstyle(io, :Activating, "project at $(pathrepr(dirname(p)))")
@@ -1585,6 +1587,7 @@ function activate(path::AbstractString; shared::Bool=false, temp::Bool=false, io
             fullpath = joinpath(Pkg.envdir(Pkg.depots1()), path)
         end
     end
+    PREV_ENV_PATH[] = Base.active_project()
     Base.ACTIVE_PROJECT[] = Base.load_path_expand(fullpath)
     p = Base.active_project()
     if p !== nothing

src/Pkg.jl

@@ -37,6 +37,7 @@ const OFFLINE_MODE = Ref(false)
 const DEFAULT_IO = Ref{Union{IO,Nothing}}(nothing)
 stderr_f() = something(DEFAULT_IO[], stderr)
 stdout_f() = something(DEFAULT_IO[], stdout)
+const PREV_ENV_PATH = Ref{String}("")
 
 can_fancyprint(io::IO) = (io isa Base.TTY) && (get(ENV, "CI", nothing) != "true")
 
@@ -393,7 +394,7 @@ any packages listed as arguments, the output will be limited to those packages.
 Setting `diff=true` will, if the environment is in a git repository, limit
 the output to the difference as compared to the last git commit.
 
-See [`Pkg.project`](@ref) and [`Pkg.dependencies`](@ref) to get the project/manifest 
+See [`Pkg.project`](@ref) and [`Pkg.dependencies`](@ref) to get the project/manifest
 status as a Julia object instead of printing it.
 
 !!! compat "Julia 1.1"

src/REPLMode/argument_parsers.jl

@@ -195,7 +195,10 @@ function parse_activate(args::Vector{QString}, options)
             return [x.raw]
         end
         x = x.raw
-        if first(x) == '@'
+        if x == "-"
+            options[:prev] = true
+            return []
+        elseif first(x) == '@'
             options[:shared] = true
             return [x[2:end]]
         else

test/new.jl

@@ -325,6 +325,10 @@ end
         api, opts = first(Pkg.pkg"activate --temp")
         @test api == Pkg.activate
         @test opts == Dict(:temp => true)
+        # - activating the previous project
+        api, opts = first(Pkg.pkg"activate -")
+        @test api == Pkg.activate
+        @test opts == Dict(:prev => true)
     end
 end
 
@@ -337,6 +341,23 @@ end
         Pkg.activate(; io=io, temp=true)
         output = String(take!(io))
         @test occursin(r"Activating new project at `.*`", output)
+        prev_env = Base.active_project()
+
+        # - activating the previous project
+        Pkg.activate(; temp=true)
+        @test prev_env != Base.active_project()
+        Pkg.activate(; prev=true)
+        @test prev_env == Base.active_project()
+
+        Pkg.activate(; temp=true)
+        @test prev_env != Base.active_project()
+        Pkg.activate(; prev=true)
+        @test prev_env == Base.active_project()
+
+        Pkg.activate("")
+        @test prev_env != Base.active_project()
+        Pkg.activate(; prev=true)
+        @test prev_env == Base.active_project()
     end
 end