replace MethodInfoKey Pair alias with struct to avoid type piracy (#149)

Changes `MethodInfoKey` from a `Pair` type alias to a proper struct,
avoiding type piracy that would be introduced by defining
`Pair{...}(::Method)` conversion methods. The struct implements
`Base.iterate` to maintain compatibility with destructuring patterns.

This is a breaking change as it changes the representation of method
info keys, which packages like LCU and Revise depend heavily on. So we
unfotunately need to bump the major version from 2 to 3.

As an alternative, removing `Pair{...}(::Method)` would also be
possible, but that would also break LCU and Revise, so I'm taking this
approach which appears to be a better and moer complete fix.

Author: aviatesk

Project.toml

@@ -1,7 +1,7 @@
 name = "CodeTracking"
 uuid = "da1fd8a2-8d9e-5ec2-8556-3022fb5608a2"
 authors = ["Tim Holy <[email protected]>"]
-version = "2.0.2"
+version = "3.0.0"
 
 [deps]
 InteractiveUtils = "b77e0a4c-d291-57a0-90e8-8db25a27a240"

src/CodeTracking.jl

@@ -29,13 +29,27 @@ include("utils.jl")
 
 # These values get populated by Revise
 
-# `method_info[mt => sig]` is either:
+# `method_info[MethodInfoKey(mt, sig)]` is either:
 #   - `missing`, to indicate that the method cannot be located
 #   - a list of `(lnn,ex)` pairs. In almost all cases there will be just one of these,
 #     but "mistakes" in moving methods from one file to another can result in more than
 #     one definition. The last pair in the list is the currently-active definition.
 
-const MethodInfoKey = Pair{Union{Nothing, MethodTable}, Type}
+struct MethodInfoKey
+    mt::Union{Nothing, MethodTable}
+    sig::Type
+    MethodInfoKey(mt::Union{Nothing, MethodTable}, @nospecialize(sig::Type)) = new(mt, sig)
+end
+
+function Base.iterate(mt_sig::MethodInfoKey, s::Union{Int,Nothing}=nothing)
+    if s === nothing
+        return mt_sig.mt, 1
+    elseif s == 1
+        return mt_sig.sig, 2
+    else
+        return nothing
+    end
+end
 
 const method_info = IdDict{MethodInfoKey,Union{Missing,Vector{Tuple{LineNumberNode,Expr}}}}()