Commits on Feb 7, 2019

1. WIP: Add a flag to trace inference's heuristic limiting decisions …

   There's generally two reasons for sub-optimal type inference.
   1) The user did something that inference couldn't reason about (read from a global,
      get an arbitrary object from the network, deserialization, etc). We now have
      some pretty good tooling to detect this (e.g. interactively in Cthulhu.jl and
      some basic automation in XLA.jl).
   2) Inference decided that it wouldn't be worth further pursuing an inference avenue
      (e.g. because of recursion or because one of our other inference limits).
   
   Hitting the second case is frustrating, because the limits don't apply transitively
   (e.g. you may enter a method that infers fine if used as the starting method, but
   does not infer properly if called from another method). These cases are frustrating
   because our tooling gets confused (since at the leaf, it infers fine). They are also
   really hard to debug (esp to people not intimately familiar with inference).
   
   This attempts to add some tooling that allows users to ask inference to tell us when
   it hits this limits. The tracing is enabled by a special flag in the Params object and
   each kind of limit is supposed to be its own kind of object (in order for it to be
   automatically consumable by tooling). I also added a flag to code_typed that will just
   dump out each trace entry as an info log record. Example usage:
   
   ```
   julia> f(::Val{4000}) = 0
   f (generic function with 1 methods)
   
   julia> f(::Val{n}) where {n} = f(Val(n+1))
   f (generic function with 2 methods)
   
   julia> @eval @code_typed optimize=false trace=true f(Val(1))
   [ Info: Signature f(::Val{4}) was narrowed to f(::Val) due to recursion [EdgeCycleLimited]
   CodeInfo(
   1 ─ %1 = ($(Expr(:static_parameter, 1)) + 1)::Const(2, false)
   │   %2 = Main.Val(%1)::Const(Val{2}(), true)
   │   %3 = Main.f(%2)::Const(0, false)
   └──      return %3
   ) => Int64
   ```

   

   Keno committed Feb 7, 2019
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