Support null metadata, represent as nothing. (#307)

Author: maleadt

src/core/metadata.jl

@@ -108,7 +108,7 @@ function operands(md::MDNode)
     nops = API.LLVMExtraGetMDNodeNumOperands2(md)
     ops = Vector{API.LLVMMetadataRef}(undef, nops)
     API.LLVMExtraGetMDNodeOperands2(md, ops)
-    return [Metadata(op) for op in ops]
+    return [op == C_NULL ? nothing : Metadata(op) for op in ops]
 end
 
 # TODO: setindex?
@@ -131,3 +131,10 @@ MDNode(mds::Vector{<:Metadata}; ctx::Context) =
     MDTuple(API.LLVMMDNodeInContext2(ctx, mds, length(mds)))
 MDNode(vals::Vector; ctx::Context) =
     MDNode(convert(Vector{Metadata}, vals); ctx)
+
+# we support passing `nothing`, but convert it to a non-exported `MDNull` instance
+# so that we can keep everything as a subtype of `Metadata`
+struct MDNull <: Metadata end
+Base.convert(::Type{Metadata}, ::Nothing) = MDNull()
+Base.unsafe_convert(::Type{API.LLVMMetadataRef}, md::MDNull) =
+    convert(API.LLVMMetadataRef, C_NULL)

test/core.jl

@@ -851,6 +851,23 @@ Context() do ctx
     @test ops[1] == str
 end
 
+# null metadata, represented as null pointers in the API, by `nothing` in Julia
+Context() do ctx
+    ir = """
+            !0 = !{i32 42, null, !"string"}
+            !foo = !{!0}
+        """
+    mod = parse(LLVM.Module, ir; ctx)
+
+    foo_md = operands(metadata(mod)["foo"])[1]
+    @test operands(foo_md)[1] !== nothing
+    @test operands(foo_md)[2] === nothing
+    @test operands(foo_md)[3] !== nothing
+
+    bar_md = MDNode([ConstantInt(Int32(42); ctx), nothing, MDString("string"; ctx)]; ctx)
+    @test foo_md == bar_md
+end
+
 @testset "debuginfo" begin
 
 Context() do ctx