allow artifact string macro to take an explicit path

can be used by JLLs to avoid scour throgh the file system for the Artifacts.toml when it already knows where it is

Author: KristofferC

stdlib/Artifacts/src/Artifacts.jl

@@ -652,13 +652,18 @@ access a single file/directory within an artifact.  Example:
 !!! compat "Julia 1.6"
     Slash-indexing requires at least Julia 1.6.
 """
-macro artifact_str(name, platform=nothing)
+macro artifact_str(name, platform=nothing, artifact_path=nothing)
     # Find Artifacts.toml file we're going to load from
     srcfile = string(__source__.file)
     if ((isinteractive() && startswith(srcfile, "REPL[")) || (!isinteractive() && srcfile == "none")) && !isfile(srcfile)
         srcfile = pwd()
     end
-    local artifacts_toml = find_artifacts_toml(srcfile)
+    # Sometimes we know the exact path to the Artifacts.toml file, so we can save some lookups
+    local artifacts_toml = if artifact_path === nothing
+        find_artifacts_toml(srcfile)
+    else
+        artifact_path
+    end
     if artifacts_toml === nothing
         error(string(
             "Cannot locate '(Julia)Artifacts.toml' file when attempting to use artifact '",

stdlib/Artifacts/test/runtests.jl

@@ -91,6 +91,9 @@ end
         HelloWorldC_exe_path = joinpath(HelloWorldC_dir, "bin", "hello_world$(exeext)")
         @test isfile(HelloWorldC_exe_path)
 
+        HelloWorldC_dir_explicit_artifact = eval(Expr(:macrocall, Symbol("@artifact_str"), nothing, "HelloWorldC", nothing, joinpath(@__DIR__, "Artifacts.toml")))
+        @test isdir(HelloWorldC_dir_explicit_artifact)
+
         # Simple slash-indexed lookup
         HelloWorldC_bin_path = artifact"HelloWorldC/bin"
         @test isdir(HelloWorldC_bin_path)