Make the "second Base copy" trick actually work

As was mentioned in #25988, there is a handy trick where you can load
a second copy of Base on top of an existing copy. This is useful for
at least two reasons:
1. Base printing is available, so things like MethodErrors print nicely
2. Even if the load fails, the resulting (broken) copy of base is inspectable
   by standard introspection tools from the REPL, as long as you're a bit
   careful not to mix types from the two copies of Base.

However, as I mentioned in #26079, this only actually works until about version.jl,
at which point things crash. This is because at that point it tries to use PCRE
which uses `Ref(0)`, which is actually an abstract type in Core, even though the
type of the constructed object (`RefValue`) is in Base. As a result, the new Base
gets the wrong kind of `RefValue` (the one from the original `Base`) and things break.

Luckily this is easily fixed by using an explicit `RefValue` call in the relevant places.

A second problem we run into is that `module`s nested under our new `Base`, get a default
import of the old `Base` (unless we declare the new Base to be the global top module, but
that would break the REPL subsequent to loading the new Base, which breaks reason 2 above).
I suggest (and implement in this PR) to have the default import be the next topmodule along
the parent link chain (as we already do for syntax defined in Base), which makes this work.
A small related detail is that in all such modules `import Base: x`, needs to be replaced by
`import .Base: x`, to make sure we resolve the identifier `Base` (as imported from our
new top module) rather than the global name `Base` (which still refers to the old module).

I changed sysimg.jl to avoid loading stdlibs in second Base mode, to avoid having to implement
the same changes there. Since the stdlibs are already decoupled from Base, they can already
be developed separately fairly easily, so there's not much reason to include them in this trick.

For completeness, there's a couple of ways to use this trick, but perhaps the simplest is:
```
cd("base")
baremodule NotBase
    Core.include(NotBase, "sysimg.jl")
end
```
from the REPL.

base/atomics.jl

@@ -2,8 +2,8 @@
 
 using Core.Intrinsics: llvmcall
 
-import Base: setindex!, getindex, unsafe_convert
-import Base.Sys: ARCH, WORD_SIZE
+import .Base: setindex!, getindex, unsafe_convert
+import .Base.Sys: ARCH, WORD_SIZE
 
 export
     Atomic,

base/broadcast.jl

@@ -2,11 +2,11 @@
 
 module Broadcast
 
-using Base.Cartesian
-using Base: Indices, OneTo, linearindices, tail, to_shape,
+using .Base.Cartesian
+using .Base: Indices, OneTo, linearindices, tail, to_shape,
             _msk_end, unsafe_bitgetindex, bitcache_chunks, bitcache_size, dumpbitcache,
             isoperator, promote_typejoin
-import Base: broadcast, broadcast!
+import .Base: broadcast, broadcast!
 export BroadcastStyle, broadcast_indices, broadcast_similar,
        broadcast_getindex, broadcast_setindex!, dotview, @__dot__
 

base/docs/Docs.jl

@@ -60,8 +60,9 @@ function.
 
 include("bindings.jl")
 
-import Base.Meta: quot, isexpr
-import Base: Callable, with_output_color
+import .Base.Meta: quot, isexpr
+import .Base: Callable, with_output_color
+using .Base: RefValue
 import ..CoreDocs: lazy_iterpolate
 
 export doc
@@ -390,7 +391,7 @@ end
 function multidoc(__source__, __module__, meta, ex, define)
     out = Expr(:toplevel)
     str = docexpr(__source__, __module__, lazy_iterpolate(meta), metadata(__source__, __module__, ex, false))
-    ref = Ref{DocStr}()
+    ref = RefValue{DocStr}()
     for (n, arg) in enumerate(ex.args)
         # The first `arg` to be documented needs to also create the docstring for the group.
         # Subsequent `arg`s just need `ref` to be able to find the docstring without having

base/docs/utils.jl

@@ -2,7 +2,7 @@
 
 # Text / HTML objects
 
-import Base: print, show, ==, hash
+import .Base: print, show, ==, hash
 
 export HTML, @html_str
 

base/file.jl

@@ -27,6 +27,8 @@ export
     unlink,
     walkdir
 
+import .Base.RefValue
+
 # get and set current directory
 
 """
@@ -36,7 +38,7 @@ Get the current working directory.
 """
 function pwd()
     b = Vector{UInt8}(uninitialized, 1024)
-    len = Ref{Csize_t}(length(b))
+    len = RefValue{Csize_t}(length(b))
     uv_error(:getcwd, ccall(:uv_cwd, Cint, (Ptr{UInt8}, Ptr{Csize_t}), b, len))
     String(b[1:len[]])
 end

base/filesystem.jl

@@ -38,14 +38,14 @@ export File,
        S_IRGRP, S_IWGRP, S_IXGRP, S_IRWXG,
        S_IROTH, S_IWOTH, S_IXOTH, S_IRWXO
 
-import Base:
+import .Base:
     UVError, _sizeof_uv_fs, check_open, close, eof, eventloop, fd, isopen,
     bytesavailable, position, read, read!, readavailable, seek, seekend, show,
     skip, stat, unsafe_read, unsafe_write, write, transcode, uv_error,
     rawhandle, OS_HANDLE, INVALID_OS_HANDLE
 
 if Sys.iswindows()
-    import Base: cwstring
+    import .Base: cwstring
 end
 
 include("path.jl")

base/gmp.jl

@@ -4,7 +4,7 @@ module GMP
 
 export BigInt
 
-import Base: *, +, -, /, <, <<, >>, >>>, <=, ==, >, >=, ^, (~), (&), (|), xor,
+import .Base: *, +, -, /, <, <<, >>, >>>, <=, ==, >, >=, ^, (~), (&), (|), xor,
              binomial, cmp, convert, div, divrem, factorial, fld, gcd, gcdx, lcm, mod,
              ndigits, promote_rule, rem, show, isqrt, string, powermod,
              sum, trailing_zeros, trailing_ones, count_ones, base, tryparse_internal,
@@ -110,7 +110,7 @@ module MPZ
 # - a method modifying its input has a "!" appendend to its name, according to Julia's conventions
 # - some convenient methods are added (in addition to the pure MPZ ones), e.g. `add(a, b) = add!(BigInt(), a, b)`
 #   and `add!(x, a) = add!(x, x, a)`.
-using Base.GMP: BigInt, Limb
+using .Base.GMP: BigInt, Limb
 
 const mpz_t = Ref{BigInt}
 const bitcnt_t = Culong

base/locks.jl

@@ -1,6 +1,6 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
-import Base: _uv_hook_close, unsafe_convert,
+import .Base: _uv_hook_close, unsafe_convert,
     lock, trylock, unlock, islocked
 
 export SpinLock, RecursiveSpinLock, Mutex

base/math.jl

@@ -15,17 +15,17 @@ export sin, cos, sincos, tan, sinh, cosh, tanh, asin, acos, atan,
        clamp, clamp!, modf, ^, mod2pi, rem2pi,
        beta, lbeta, @evalpoly
 
-import Base: log, exp, sin, cos, tan, sinh, cosh, tanh, asin,
+import .Base: log, exp, sin, cos, tan, sinh, cosh, tanh, asin,
              acos, atan, asinh, acosh, atanh, sqrt, log2, log10,
              max, min, minmax, ^, exp2, muladd, rem,
              exp10, expm1, log1p
 
-using Base: sign_mask, exponent_mask, exponent_one,
+using .Base: sign_mask, exponent_mask, exponent_one,
             exponent_half, uinttype, significand_mask
 
 using Core.Intrinsics: sqrt_llvm
 
-using Base: IEEEFloat
+using .Base: IEEEFloat
 
 @noinline function throw_complex_domainerror(f, x)
     throw(DomainError(x, string("$f will only return a complex result if called with a ",

base/mpfr.jl

@@ -7,7 +7,7 @@ export
     setprecision
 
 import
-    Base: *, +, -, /, <, <=, ==, >, >=, ^, ceil, cmp, convert, copysign, div,
+    .Base: *, +, -, /, <, <=, ==, >, >=, ^, ceil, cmp, convert, copysign, div,
         inv, exp, exp2, exponent, factorial, floor, fma, hypot, isinteger,
         isfinite, isinf, isnan, ldexp, log, log2, log10, max, min, mod, modf,
         nextfloat, prevfloat, promote_rule, rem, rem2pi, round, show, float,
@@ -17,15 +17,15 @@ import
         cosh, sinh, tanh, sech, csch, coth, acosh, asinh, atanh, atan2,
         cbrt, typemax, typemin, unsafe_trunc, realmin, realmax, rounding,
         setrounding, maxintfloat, widen, significand, frexp, tryparse, iszero,
-        isone, big, beta
+        isone, big, beta, RefValue
 
-import Base.Rounding: rounding_raw, setrounding_raw
+import .Base.Rounding: rounding_raw, setrounding_raw
 
-import Base.GMP: ClongMax, CulongMax, CdoubleMax, Limb
+import .Base.GMP: ClongMax, CulongMax, CdoubleMax, Limb
 
-import Base.Math.lgamma_r
+import .Base.Math.lgamma_r
 
-import Base.FastMath.sincos_fast
+import .Base.FastMath.sincos_fast
 
 version() = VersionNumber(unsafe_string(ccall((:mpfr_get_version,:libmpfr), Ptr{Cchar}, ())))
 patches() = split(unsafe_string(ccall((:mpfr_get_patches,:libmpfr), Ptr{Cchar}, ())),' ')
@@ -40,8 +40,8 @@ function __init__()
     end
 end
 
-const ROUNDING_MODE = Ref{Cint}(0)
-const DEFAULT_PRECISION = Ref(256)
+const ROUNDING_MODE = RefValue{Cint}(0)
+const DEFAULT_PRECISION = RefValue(256)
 
 # Basic type and initialization definitions
 

base/multidimensional.jl

@@ -2,14 +2,14 @@
 
 ### Multidimensional iterators
 module IteratorsMD
-    import Base: eltype, length, size, start, done, next, first, last, in, getindex,
+    import .Base: eltype, length, size, start, done, next, first, last, in, getindex,
                  setindex!, IndexStyle, min, max, zero, one, isless, eachindex,
                  ndims, IteratorSize, convert, show
 
-    import Base: +, -, *
-    import Base: simd_outer_range, simd_inner_length, simd_index
-    using Base: IndexLinear, IndexCartesian, AbstractCartesianIndex, fill_to_length, tail
-    using Base.Iterators: Reverse
+    import .Base: +, -, *
+    import .Base: simd_outer_range, simd_inner_length, simd_index
+    using .Base: IndexLinear, IndexCartesian, AbstractCartesianIndex, fill_to_length, tail
+    using .Base.Iterators: Reverse
 
     export CartesianIndex, CartesianIndices, LinearIndices
 

base/multimedia.jl

@@ -14,8 +14,8 @@ export AbstractDisplay, display, pushdisplay, popdisplay, displayable, redisplay
 # defined in sysimg.jl for bootstrapping:
 # struct MIME{mime} end
 # macro MIME_str(s)
-import Base: MIME, @MIME_str
-import Base: show, print, string, convert, repr
+import .Base: MIME, @MIME_str
+import .Base: show, print, string, convert, repr
 MIME(s) = MIME{Symbol(s)}()
 show(io::IO, ::MIME{mime}) where {mime} = print(io, "MIME type ", string(mime))
 print(io::IO, ::MIME{mime}) where {mime} = print(io, mime)

base/path.jl

@@ -62,7 +62,7 @@ Return the current user's home directory.
 function homedir()
     path_max = 1024
     buf = Vector{UInt8}(uninitialized, path_max)
-    sz = Ref{Csize_t}(path_max + 1)
+    sz = RefValue{Csize_t}(path_max + 1)
     while true
         rc = ccall(:uv_os_homedir, Cint, (Ptr{UInt8}, Ptr{Csize_t}), buf, sz)
         if rc == 0

base/pcre.jl

@@ -4,12 +4,14 @@
 
 module PCRE
 
+import ..RefValue
+
 include(string(length(Core.ARGS) >= 2 ? Core.ARGS[2] : "", "pcre_h.jl"))  # include($BUILDROOT/base/pcre_h.jl)
 
 const PCRE_LIB = "libpcre2-8"
 
-const JIT_STACK = Ref{Ptr{Cvoid}}(C_NULL)
-const MATCH_CONTEXT = Ref{Ptr{Cvoid}}(C_NULL)
+const JIT_STACK = RefValue{Ptr{Cvoid}}(C_NULL)
+const MATCH_CONTEXT = RefValue{Ptr{Cvoid}}(C_NULL)
 
 function __init__()
     try
@@ -70,7 +72,7 @@ const OPTIONS_MASK = COMPILE_MASK | EXECUTE_MASK
 const UNSET = ~Csize_t(0)  # Indicates that an output vector element is unset
 
 function info(regex::Ptr{Cvoid}, what::Integer, ::Type{T}) where T
-    buf = Ref{T}()
+    buf = RefValue{T}()
     ret = ccall((:pcre2_pattern_info_8, PCRE_LIB), Int32,
                 (Ptr{Cvoid}, Int32, Ptr{Cvoid}),
                 regex, what, buf)
@@ -92,8 +94,8 @@ function get_ovec(match_data)
 end
 
 function compile(pattern::AbstractString, options::Integer)
-    errno = Ref{Cint}(0)
-    erroff = Ref{Csize_t}(0)
+    errno = RefValue{Cint}(0)
+    erroff = RefValue{Csize_t}(0)
     re_ptr = ccall((:pcre2_compile_8, PCRE_LIB), Ptr{Cvoid},
                    (Ptr{UInt8}, Csize_t, UInt32, Ref{Cint}, Ref{Csize_t}, Ptr{Cvoid}),
                    pattern, sizeof(pattern), options, errno, erroff, C_NULL)
@@ -146,15 +148,15 @@ function substring_number_from_name(re, name)
 end
 
 function substring_length_bynumber(match_data, number)
-    s = Ref{Csize_t}()
+    s = RefValue{Csize_t}()
     rc = ccall((:pcre2_substring_length_bynumber_8, PCRE_LIB), Cint,
           (Ptr{Cvoid}, UInt32, Ref{Csize_t}), match_data, number, s)
     rc < 0 && error("PCRE error: $(err_message(rc))")
     convert(Int, s[])
 end
 
 function substring_copy_bynumber(match_data, number, buf, buf_size)
-    s = Ref{Csize_t}(buf_size)
+    s = RefValue{Csize_t}(buf_size)
     rc = ccall((:pcre2_substring_copy_bynumber_8, PCRE_LIB), Cint,
                (Ptr{Cvoid}, UInt32, Ptr{UInt8}, Ref{Csize_t}),
                match_data, number, buf, s)

base/printf.jl

@@ -1,8 +1,8 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
 module Printf
-using Base.Grisu
-using Base.GMP
+using .Base.Grisu
+using .Base.GMP
 
 ### printf formatter generation ###
 const SmallFloatingPoint = Union{Float64,Float32,Float16}

base/refpointer.jl

@@ -50,14 +50,8 @@ end
 RefValue(x::T) where {T} = RefValue{T}(x)
 isassigned(x::RefValue) = isdefined(x, :x)
 
-Ref(x::Any) = RefValue(x)
-Ref{T}() where {T} = RefValue{T}() # Ref{T}()
-Ref{T}(x) where {T} = RefValue{T}(x) # Ref{T}(x)
 convert(::Type{Ref{T}}, x) where {T} = RefValue{T}(x)
 
-Ref(x::Ref, i::Integer) = (i != 1 && error("Ref only has one element"); x)
-Ref(x::Ptr{T}, i::Integer) where {T} = x + (i - 1) * Core.sizeof(T)
-
 function unsafe_convert(P::Type{Ptr{T}}, b::RefValue{T}) where T
     if datatype_pointerfree(RefValue{T})
         p = pointer_from_objref(b)
@@ -87,7 +81,6 @@ end
 RefArray(x::AbstractArray{T}, i::Int, roots::Any) where {T} = RefArray{T,typeof(x),Any}(x, i, roots)
 RefArray(x::AbstractArray{T}, i::Int=1, roots::Nothing=nothing) where {T} = RefArray{T,typeof(x),Nothing}(x, i, nothing)
 convert(::Type{Ref{T}}, x::AbstractArray{T}) where {T} = RefArray(x, 1)
-Ref(x::AbstractArray, i::Integer) = RefArray(x, i)
 
 function unsafe_convert(P::Type{Ptr{T}}, b::RefArray{T}) where T
     if datatype_pointerfree(RefValue{T})
@@ -105,26 +98,38 @@ function unsafe_convert(P::Type{Ptr{Any}}, b::RefArray{Any})
 end
 unsafe_convert(::Type{Ptr{Cvoid}}, b::RefArray{T}) where {T} = convert(Ptr{Cvoid}, unsafe_convert(Ptr{T}, b))
 
-# convert Arrays to pointer arrays for ccall
-function Ref{P}(a::Array{<:Union{Ptr,Cwstring,Cstring}}) where P<:Union{Ptr,Cwstring,Cstring}
-    return RefArray(a) # effectively a no-op
-end
-function Ref{P}(a::Array{T}) where P<:Union{Ptr,Cwstring,Cstring} where T
-    if (!isbits(T) && T <: eltype(P))
-        # this Array already has the right memory layout for the requested Ref
-        return RefArray(a,1,false) # root something, so that this function is type-stable
-    else
-        ptrs = Vector{P}(uninitialized, length(a)+1)
-        roots = Vector{Any}(uninitialized, length(a))
-        for i = 1:length(a)
-            root = cconvert(P, a[i])
-            ptrs[i] = unsafe_convert(P, root)::P
-            roots[i] = root
+###
+if parentmodule(@__MODULE__) === Main
+    Ref(x::Any) = RefValue(x)
+    Ref{T}() where {T} = RefValue{T}() # Ref{T}()
+    Ref{T}(x) where {T} = RefValue{T}(x) # Ref{T}(x)
+
+    Ref(x::Ref, i::Integer) = (i != 1 && error("Ref only has one element"); x)
+    Ref(x::Ptr{T}, i::Integer) where {T} = x + (i - 1) * Core.sizeof(T)
+
+    # convert Arrays to pointer arrays for ccall
+    function Ref{P}(a::Array{<:Union{Ptr,Cwstring,Cstring}}) where P<:Union{Ptr,Cwstring,Cstring}
+        return RefArray(a) # effectively a no-op
+    end
+    function Ref{P}(a::Array{T}) where P<:Union{Ptr,Cwstring,Cstring} where T
+        if (!isbits(T) && T <: eltype(P))
+            # this Array already has the right memory layout for the requested Ref
+            return RefArray(a,1,false) # root something, so that this function is type-stable
+        else
+            ptrs = Vector{P}(uninitialized, length(a)+1)
+            roots = Vector{Any}(uninitialized, length(a))
+            for i = 1:length(a)
+                root = cconvert(P, a[i])
+                ptrs[i] = unsafe_convert(P, root)::P
+                roots[i] = root
+            end
+            ptrs[length(a)+1] = C_NULL
+            return RefArray(ptrs,1,roots)
         end
-        ptrs[length(a)+1] = C_NULL
-        return RefArray(ptrs,1,roots)
     end
+    Ref(x::AbstractArray, i::Integer) = RefArray(x, i)
 end
+
 cconvert(::Type{Ptr{P}}, a::Array{<:Ptr}) where {P<:Ptr} = a
 cconvert(::Type{Ref{P}}, a::Array{<:Ptr}) where {P<:Ptr} = a
 cconvert(::Type{Ptr{P}}, a::Array) where {P<:Union{Ptr,Cwstring,Cstring}} = Ref{P}(a)

base/sort.jl

@@ -2,16 +2,16 @@
 
 module Sort
 
-using Base.Order, Base.Checked
-using Base: copymutable, linearindices, IndexStyle, viewindexing, IndexLinear, _length
-
-import
-    Base.sort,
-    Base.sort!,
-    Base.issorted,
-    Base.sortperm,
-    Base.Slice,
-    Base.to_indices
+using .Base.Order, .Base.Checked
+using .Base: copymutable, linearindices, IndexStyle, viewindexing, IndexLinear, _length
+
+import .Base:
+    sort,
+    sort!,
+    issorted,
+    sortperm,
+    Slice,
+    to_indices
 
 export # also exported by Base
     # order-only: