diff --git a/src/pkgfiles.jl b/src/pkgfiles.jl
index 4854066..690f5e8 100644
--- a/src/pkgfiles.jl
+++ b/src/pkgfiles.jl
@@ -1,3 +1,57 @@
+"""
+`MapExprFile(mapexpr, filename::String)` stores the arguments needed for
+`include(mapexpr, filename)`. `mapexpr` is a function mapping `Expr` to an
+`Expr`, which is applied to the parsed expressions from `filename` before
+evaluation. This is sometimes used for preprocessing files before they are
+loaded.
+
+Otherwise, `MapExprFile` behaves like a string, allowing it to be used
+wherever a file path is expected.
+"""
+struct MapExprFile <: AbstractString
+    mapexpr
+    filename::String
+end
+MapExprFile(filename::String) = MapExprFile(identity, filename)
+
+Base.show(io::IO, mapfile::MapExprFile) =
+    print(io, "MapExprFile(", mapfile.mapexpr, ", \"", mapfile.filename, "\")")
+
+# AbstractString interface
+Base.iterate(mapfile::MapExprFile) = iterate(mapfile.filename)
+Base.iterate(mapfile::MapExprFile, state::Integer) = iterate(mapfile.filename, state)
+
+Base.getindex(mapfile::MapExprFile, i::Integer) = getindex(mapfile.filename, i)
+
+Base.ncodeunits(mapfile::MapExprFile) = ncodeunits(mapfile.filename)
+Base.codeunit(mapfile::MapExprFile, i::Integer) = codeunit(mapfile.filename, i)
+
+Base.:(==)(mapfile1::MapExprFile, mapfile2::MapExprFile) =
+    (mapfile1.mapexpr == mapfile2.mapexpr) & (mapfile1.filename == mapfile2.filename)
+Base.:(==)(mapfile1::MapExprFile, file2::AbstractString) = false
+Base.:(==)(file1::AbstractString, mapfile2::MapExprFile) = false
+
+# Don't lose the `mapexpr` from common path operations
+Base.:(*)(mapfile::MapExprFile, path::AbstractString) =
+    MapExprFile(mapfile.mapexpr, mapfile.filename * path)
+Base.:(*)(path::AbstractString, mapfile::MapExprFile) =
+    MapExprFile(mapfile.mapexpr, path * mapfile.filename)
+# The above would be enough for `joinpath` except for its return-type assertion ::String
+function Base.joinpath(mapfile::MapExprFile, path::AbstractString)
+    @assert !isa(path, MapExprFile) "Cannot join MapExprFile with another MapExprFile"
+    return MapExprFile(mapfile.mapexpr, joinpath(mapfile.filename, path))
+end
+function Base.joinpath(path::AbstractString, mapfile::MapExprFile)
+    @assert !isa(path, MapExprFile) "Cannot join MapExprFile with another MapExprFile"
+    return MapExprFile(mapfile.mapexpr, joinpath(path, mapfile.filename))
+end
+Base.normpath(mapfile::MapExprFile) = MapExprFile(mapfile.mapexpr, normpath(mapfile.filename))
+Base.abspath(mapfile::MapExprFile) = MapExprFile(mapfile.mapexpr, abspath(mapfile.filename))
+function Base.relpath(mapfile::MapExprFile, path::AbstractString)
+    @assert !isa(path, MapExprFile) "Cannot get relative path from MapExprFile to another MapExprFile"
+    return MapExprFile(mapfile.mapexpr, relpath(mapfile.filename, path))
+end
+
 """
 PkgFiles encodes information about the current location of a package.
 Fields:
@@ -10,7 +64,7 @@ Note that `basedir` may be subsequently updated by Pkg operations such as `add`
 mutable struct PkgFiles
     id::PkgId
     basedir::String
-    files::Vector{Any}
+    files::Vector{Any}    # might contain `filename::String`, `::MapExprFile`, or custom file types (https://github.com/timholy/Revise.jl/pull/680)
 end
 
 PkgFiles(id::PkgId, path::AbstractString) = PkgFiles(id, path, Any[])
diff --git a/test/runtests.jl b/test/runtests.jl
index d3d3d04..d3adf5e 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -4,7 +4,7 @@ using CodeTracking
 using Test, InteractiveUtils, REPL, LinearAlgebra, SparseArrays
 # Note: ColorTypes needs to be installed, but note the intentional absence of `using ColorTypes`
 
-using CodeTracking: line_is_decl, MethodInfoKey
+using CodeTracking: line_is_decl, MethodInfoKey, MapExprFile
 
 if !isempty(ARGS) && "revise" ∈ ARGS
     # For running tests with and without Revise
@@ -294,6 +294,27 @@ isdefined(Main, :Revise) ? Main.Revise.includet("script.jl") : include("script.j
     @test line == 148
 end
 
+@testset "MapExprFile" begin
+    mapfile = MapExprFile(identity, "testfile.jl")
+    @test String(mapfile) == "testfile.jl"
+    @test sprint(show, mapfile) == "MapExprFile(identity, \"testfile.jl\")"
+    mappath = joinpath("base", mapfile)
+    @test mappath == MapExprFile(identity, joinpath("base", "testfile.jl"))
+    @test abspath(mapfile) == MapExprFile(identity, abspath("testfile.jl"))
+    @test normpath(mapfile) == MapExprFile(identity, normpath("testfile.jl"))
+    @test relpath(mappath, "base") == mapfile
+    @test isabspath(mapfile) == false
+    @test ispath(mapfile) == false
+    @test isfile(mapfile) == false
+    open(mapfile, "w") do io
+        write(io, "test content")
+    end
+    @test isabspath(mapfile) == false
+    @test ispath(mapfile) == true
+    @test isfile(mapfile) == true
+    rm(mapfile)
+end
+
 @testset "With Revise" begin
     if isdefined(Main, :Revise)
         m = @which gcd(10, 20)