Use let model=model in variable macro to improve type stability

src/macros.jl

@@ -3016,7 +3016,13 @@ macro variable(args...)
             requested_container,
         )
     end
-
+    # Wrap the entire code block in a let statement to make the model act as
+    # a type stable local variable.
+    creation_code = quote
+        let $model = $model
+            $creation_code
+        end
+    end
     if anonvar
         # Anonymous variable, no need to register it in the model-level
         # dictionary nor to assign it to a variable in the user scope.

test/test_hygiene.jl

@@ -57,4 +57,10 @@ Test.@test ex[3] == 6
 Test.@test i == 10
 Test.@test j == 10
 
+# Test that `model` is inferred correctly inside macros, despite being a
+# non-const global.
+model = JuMP.Model()
+JuMP.@variable(model, x[1:0])
+Test.@test x == JuMP.VariableRef[]
+
 end