Qualify function when printing MethodInstance (JuliaLang#38608)

Also print file & line for the case `l.def::Module`. This is already done
for stackframes but it is useful to implement it more generally.

base/show.jl

@@ -984,18 +984,29 @@ function sourceinfo_slotnames(src::CodeInfo)
  return printnames
 end
 
-function show(io::IO, l::Core.MethodInstance)
+show(io::IO, l::Core.MethodInstance) = show_mi(io, l)
+
+function show_mi(io::IO, l::Core.MethodInstance, from_stackframe::Bool=false)
  def = l.def
  if isa(def, Method)
  if isdefined(def, :generator) && l === def.generator
  print(io, "MethodInstance generator for ")
  show(io, def)
  else
  print(io, "MethodInstance for ")
- show_tuple_as_call(io, def.name, l.specTypes)
+ show_tuple_as_call(io, def.name, l.specTypes, false, nothing, nothing, true)
  end
  else
  print(io, "Toplevel MethodInstance thunk")
+ # `thunk` is not very much information to go on. If this
+ # MethodInstance is part of a stacktrace, it gets location info
+ # added by other means. But if it isn't, then we should try
+ # to print a little more identifying information.
+ if !from_stackframe
+ linetable = l.uninferred.linetable
+ line = isempty(linetable) ? "unknown" : (lt = linetable[1]; string(lt.file) * ':' * string(lt.line))
+ print(io, " from ", def, " starting at ", line)
+ end
  end
 end
 

base/stacktraces.jl

@@ -241,7 +241,7 @@ function show_spec_linfo(io::IO, frame::StackFrame)
  Base.show_tuple_as_call(io, def.name, sig, true, nothing, argnames)
  end
  else
- Base.show(io, linfo)
+ Base.show_mi(io, linfo, true)
  end
  elseif linfo isa CodeInfo
  print(io, "top-level scope")

test/backtrace.jl

@@ -254,7 +254,7 @@ let code = """
  """
 
  bt_str = read(`$(Base.julia_cmd()) --startup-file=no --compile=min -e $code`, String)
- @test occursin("InterpreterIP in MethodInstance for foo", bt_str)
+ @test occursin(r"InterpreterIP in MethodInstance for .*A\.foo", bt_str)
  @test occursin("InterpreterIP in top-level CodeInfo for Main.A", bt_str)
 end