Make AssertionError more informative by printing the expression

src/Prometheus.jl

@@ -20,16 +20,19 @@ function Base.showerror(io::IO, err::ArgumentError)
     print(io, "Prometheus.", nameof(typeof(err)), ": ", err.msg)
 end
 
-struct AssertionError <: PrometheusException end
+struct AssertionError <: PrometheusException
+    msg::String
+end
 macro assert(cond)
-    return :($(esc(cond)) || throw(AssertionError()))
+    msg = string(cond)
+    return :($(esc(cond)) || throw(AssertionError($msg)))
 end
 
-function Base.showerror(io::IO, ::AssertionError)
+function Base.showerror(io::IO, err::AssertionError)
     print(
         io,
-        "Prometheus.AssertionError: this is unexpected. Please file an issue at " *
-        "https://github.com/fredrikekre/Prometheus.jl/issues/new.",
+        "Prometheus.AssertionError: `", err.msg, "`. This is unexpected, please file an " *
+        "issue at https://github.com/fredrikekre/Prometheus.jl/issues/new.",
     )
 end
 

test/runtests.jl

@@ -770,8 +770,12 @@ end
 end
 
 @testset "Utilities" begin
-    @test_throws Prometheus.AssertionError Prometheus.@assert false
-    @test occursin("Please file an issue", sprint(showerror, Prometheus.AssertionError()))
+    x = 1
+    err = try Prometheus.@assert x === nothing; catch e; e; end
+    @test err isa Prometheus.AssertionError
+    @test err.msg == "x === nothing"
+    @test occursin("`x === nothing`", sprint(showerror, err))
+    @test occursin("please file an issue", sprint(showerror, err))
     @test occursin(
         "Prometheus.ArgumentError: err",
         sprint(showerror, Prometheus.ArgumentError("err")),