Add world age hint for UndefVarError (#58572)

Similar to the existing world age hint for MethodError, this adds
a helpful message when an UndefVarError occurs because a binding
was defined in a newer world age than the code trying to access it.

The implementation checks if a binding that was undefined at the
error's world age is now defined in the current world, and displays:
"The binding may be too new: running in world age X, while current
world is Y."

Additionally, for all binding kinds, the error hint now notes when
the binding state has changed between the error's world and the
current world.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: Keno

base/errorshow.jl

@@ -1154,22 +1154,42 @@ function UndefVarError_hint(io::IO, ex::UndefVarError)
     if isdefined(ex, :scope)
         scope = ex.scope
         if scope isa Module
-            bpart = Base.lookup_binding_partition(ex.world, GlobalRef(scope, var))
-            kind = Base.binding_kind(bpart)
-            if kind === Base.PARTITION_KIND_GLOBAL || kind === Base.PARTITION_KIND_UNDEF_CONST || kind == Base.PARTITION_KIND_DECLARED
+            bpart = lookup_binding_partition(ex.world, GlobalRef(scope, var))
+            kind = binding_kind(bpart)
+
+            # Get the current world's binding partition for comparison
+            curworld = tls_world_age()
+            cur_bpart = lookup_binding_partition(curworld, GlobalRef(scope, var))
+            cur_kind = binding_kind(cur_bpart)
+
+            # Track if we printed the "too new" message
+            printed_too_new = false
+
+            # Check if the binding exists in the current world but was undefined in the error's world
+            if kind === PARTITION_KIND_GUARD
+                if isdefinedglobal(scope, var)
+                    print(io, "\nThe binding may be too new: running in world age $(ex.world), while current world is $(curworld).")
+                    printed_too_new = true
+                else
+                    print(io, "\nSuggestion: check for spelling errors or missing imports.")
+                end
+            elseif kind === PARTITION_KIND_GLOBAL || kind === PARTITION_KIND_UNDEF_CONST || kind == PARTITION_KIND_DECLARED
                 print(io, "\nSuggestion: add an appropriate import or assignment. This global was declared but not assigned.")
-            elseif kind === Base.PARTITION_KIND_FAILED
+            elseif kind === PARTITION_KIND_FAILED
                 print(io, "\nHint: It looks like two or more modules export different ",
                 "bindings with this name, resulting in ambiguity. Try explicitly ",
                 "importing it from a particular module, or qualifying the name ",
                 "with the module it should come from.")
-            elseif kind === Base.PARTITION_KIND_GUARD
-                print(io, "\nSuggestion: check for spelling errors or missing imports.")
-            elseif Base.is_some_explicit_imported(kind)
-                print(io, "\nSuggestion: this global was defined as `$(Base.partition_restriction(bpart).globalref)` but not assigned a value.")
-            elseif kind === Base.PARTITION_KIND_BACKDATED_CONST
+            elseif is_some_explicit_imported(kind)
+                print(io, "\nSuggestion: this global was defined as `$(partition_restriction(bpart).globalref)` but not assigned a value.")
+            elseif kind === PARTITION_KIND_BACKDATED_CONST
                 print(io, "\nSuggestion: define the const at top-level before running function that uses it (stricter Julia v1.12+ rule).")
             end
+
+            # Check if binding kind changed between the error's world and current world
+            if !printed_too_new && kind !== cur_kind
+                print(io, "\nNote: the binding state changed since the error occurred (was: $(kind), now: $(cur_kind)).")
+            end
         elseif scope === :static_parameter
             print(io, "\nSuggestion: run Test.detect_unbound_args to detect method arguments that do not fully constrain a type parameter.")
         elseif scope === :local

test/errorshow.jl

@@ -943,6 +943,24 @@ end
     @test_throws expected_message X.x
 end
 
+# Module for UndefVarError world age testing
+module TestWorldAgeUndef end
+
+@testset "UndefVarError world age hint" begin
+    ex = try
+        TestWorldAgeUndef.newvar
+    catch e
+        e
+    end
+    @test ex isa UndefVarError
+
+    Core.eval(TestWorldAgeUndef, :(newvar = 42))
+
+    err_str = sprint(Base.showerror, ex)
+    @test occursin("The binding may be too new: running in world age", err_str)
+    @test occursin("while current world is", err_str)
+end
+
 # test showing MethodError with type argument
 struct NoMethodsDefinedHere; end
 let buf = IOBuffer()