Make ExceptionStack a type alias only

Now we match on structure alone, which is argubly a bit ropey but
doesn't require a new type.  Also allows us to remove show for
ExceptionStack, without too many ill effects.

base/error.jl

@@ -91,36 +91,31 @@ function catch_backtrace()
     return _reformat_bt(bt[], bt2[])
 end
 
-struct ExceptionStack <: AbstractArray{Any,1}
-    stack
-end
-
 """
     current_exceptions(task=current_task(); [inclue_bt=true])
 
 Get the stack of exceptions currently being handled. For nested catch blocks
 there may be more than one current exception in which case the most recently
 thrown exception is last in the stack. The stack is returned as an
-`ExceptionStack` which is an AbstractVector of named tuples
-`(exception,backtrace)`. If `backtrace` is false, the backtrace in each pair
-will be set to `nothing`.
+`ExceptionStack` which is a Vector of named tuples `(exception,backtrace)`. If
+`backtrace` is false, the backtrace in each pair will be set to `nothing`.
 
 Explicitly passing `task` will return the current exception stack on an
 arbitrary task. This is useful for inspecting tasks which have failed due to
 uncaught exceptions.
 """
 function current_exceptions(task=current_task(); backtrace=true)
     raw = ccall(:jl_get_excstack, Any, (Any,Cint,Cint), task, backtrace, typemax(Cint))
-    formatted = Any[]
     stride = backtrace ? 3 : 1
-    for i = reverse(1:stride:length(raw))
-        exc = raw[i]
-        bt = backtrace ? Base._reformat_bt(raw[i+1],raw[i+2]) : nothing
-        push!(formatted, (exception=exc,backtrace=bt))
-    end
-    ExceptionStack(formatted)
+    [
+        (exception=raw[i],
+         backtrace=backtrace ? Base._reformat_bt(raw[i+1],raw[i+2]) : nothing)
+        for i = reverse(1:stride:length(raw))
+    ]
 end
 
+const ExceptionStack = Array{NamedTuple{(:exception, :backtrace),T}, 1} where {T<:Tuple}
+
 ## keyword arg lowering generates calls to this ##
 function kwerr(kw, args::Vararg{Any,N}) where {N}
     @_noinline_meta

base/errorshow.jl

@@ -628,23 +628,6 @@ function process_backtrace(t::Vector, limit::Int=typemax(Int); skipC = true)
     return ret
 end
 
-size(s::ExceptionStack) = size(s.stack)
-getindex(s::ExceptionStack, i) = s.stack[i]
-
-function show(io::IO, ::MIME"text/plain", stack::ExceptionStack)
-    nexc = length(stack)
-    printstyled(io, nexc, "-element ExceptionStack", nexc == 0 ? "" : ":\n")
-    for i = nexc:-1:1
-        if nexc != 1
-            printstyled(io, "caused by [exception ", i, "]\n", color=:light_black)
-        end
-        exc,bt = stack[i]
-        showerror(io, exc, bt, backtrace=bt!==nothing)
-        println(io)
-    end
-end
-show(io::IO, stack::ExceptionStack) = show(io, MIME"text/plain"(), stack)
-
 @noinline function throw_eachindex_mismatch(::IndexLinear, A...)
     throw(DimensionMismatch("all inputs to eachindex must have the same indices, got $(join(LinearIndices.(A), ", ", " and "))"))
 end

stdlib/Logging/src/ConsoleLogger.jl

@@ -48,6 +48,7 @@ function showvalue(io, e::Tuple{Exception,Any})
     ex,bt = e
     showerror(io, ex, bt; backtrace = bt!=nothing)
 end
+showvalue(io, stack::Base.ExceptionStack) = Base.display_error(io, stack)
 showvalue(io, ex::Exception) = showerror(io, ex)
 
 function default_logcolor(level)