Test: prune old LA based on ENV variable

.ci/run_tests.jl

@@ -6,6 +6,5 @@ end
 
 proj = abspath(joinpath(@__DIR__, ".."))
 cmd = """Base.runtests(["LinearAlgebra"]; propagate_project=true, ncores=$ncores)"""
-withenv("JULIA_NUM_THREADS" => 1) do
-    run(`$(Base.julia_cmd()) --project=$proj --compiled-modules=existing -e $cmd`)
-end
+run(addenv(`$(Base.julia_cmd()) --project=$proj --compiled-modules=existing -e $cmd`,
+    "JULIA_NUM_THREADS" => 1, "JULIA_PRUNE_OLD_LA" => true))

README.md

@@ -37,10 +37,24 @@ This package performs some type piracy and is also included in the sysimage, whi
 To use a development version of this package, you can choose one of the following methods:
 
 1. **Change the UUID in the project file and load the package:**
-   This approach will produce warnings and may lead to method ambiguities between the development version and the one in the sysimage, but it can be used for basic experimentation.
+
+   Start `julia` as
+   ```julia
+   JULIA_PRUNE_OLD_LA=true julia +nightly --compiled-modules=existing --project
+   ```
+   where it is assumed that one is already within the `LinearAlgebra` directory. Otherwise, adjust
+   the project path accordingly. The `julia +nightly` command above assumes that `juliaup` is being used
+   to launch `julia`, but one may substitute this with the path to the julia executable.
+
+   Within the `julia` session, load the `LinearAlgebra` module after pruning the one in the sysimage. This may be done as
+   ```julia
+   include("test/prune_old_LA.jl") && using LinearAlgebra
+   ```
 
 2. **Build Julia with the custom `LinearAlgebra` commit:**
-   Modify the commit in `stdlib/LinearAlgebra.version` and build Julia.
+
+   Modify the commit in [`stdlib/LinearAlgebra.version`](https://github.com/JuliaLang/julia/blob/master/stdlib/LinearAlgebra.version) and build Julia. This requires one to push the development branch
+   to `GitHub` or an equivalent platform.
 
 3. **Build a custom sysimage with the new `LinearAlgebra`:**
    - Install `PackageCompiler`.

docs/make.jl

@@ -1,3 +1,4 @@
+ENV["JULIA_PRUNE_OLD_LA"] = "true"
 include("../test/prune_old_LA.jl")
 
 using LinearAlgebra

test/bitarray.jl

@@ -1,3 +1,7 @@
+# This file is a part of Julia. License is MIT: https://julialang.org/license
+
+module TestBitArray
+
 isdefined(Main, :pruned_old_LA) || @eval Main include("prune_old_LA.jl")
 
 using LinearAlgebra, Test, Random
@@ -95,3 +99,5 @@ b2 = bitrand(v1)
 
 b1 = bitrand(n1, n1)
 @check_bit_operation diag(b1)
+
+end # module

test/prune_old_LA.jl

@@ -48,9 +48,11 @@ methods_to_delete =
 :acsch
 ]
 
+prune_old_LA = parse(Bool, get(ENV, "JULIA_PRUNE_OLD_LA", "false"))
+
 let
     LA = get(Base.loaded_modules, Base.PkgId(Base.UUID("37e2e46d-f89d-539d-b4ee-838fcccc9c8e"), "LinearAlgebra"), nothing)
-    if LA !== nothing
+    if LA !== nothing && prune_old_LA
         @assert hasmethod(*, Tuple{Matrix{Float64}, Matrix{Float64}})
         for methss in methods_to_delete
             meths = getglobal(Base, methss)