Use let model=model in variable macro to improve type stability (#3500)

src/macros.jl

@@ -1204,6 +1204,17 @@ end
 
 # TODO: update 3-argument @constraint macro to pass through names like @variable
 
+function _wrap_let(model, code)
+    if Meta.isexpr(model, :escape) && model.args[1] isa Symbol
+        return quote
+            let $model = $model
+                $code
+            end
+        end
+    end
+    return code
+end
+
 """
     _constraint_macro(
         args, macro_name::Symbol, parsefun::Function, source::LineNumberNode
@@ -1331,10 +1342,11 @@ function _constraint_macro(
         $parsecode
         $constraintcall
     end
-
     creation_code =
         Containers.container_code(idxvars, indices, code, requested_container)
-
+    # Wrap the entire code block in a let statement to make the model act as
+    # a type stable local variable.
+    creation_code = _wrap_let(model, creation_code)
     if anonvar
         # Anonymous constraint, no need to register it in the model-level
         # dictionary nor to assign it to a variable in the user scope.
@@ -1939,6 +1951,9 @@ macro expression(args...)
     end
     code =
         Containers.container_code(idxvars, indices, code, requested_container)
+    # Wrap the entire code block in a let statement to make the model act as
+    # a type stable local variable.
+    code = _wrap_let(m, code)
     # don't do anything with the model, but check that it's valid anyway
     if anonvar
         macro_code = code
@@ -3016,7 +3031,9 @@ 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 = _wrap_let(model, creation_code)
     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.
+m = JuMP.Model()
+JuMP.@variable(m, x[1:0])
+Test.@test x == JuMP.VariableRef[]
+
 end

test/test_macros.jl

@@ -2034,4 +2034,53 @@ function test_unsupported_ternary_operator()
     return
 end
 
+# This code needs to be evaluated in a top-level scope to prevent inference from
+# knowing the type of `model`.
+function test_wrap_let_non_symbol_models()
+    module_name = @eval module $(gensym())
+    using JuMP, Test
+    data = (; model = Model())
+    end
+    @eval module_name begin
+        @variable(data.model, x)
+        @test x isa VariableRef
+        @objective(data.model, Min, x^2)
+        @test isequal_canonical(objective_function(data.model), x^2)
+        @expression(data.model, expr[i = 1:2], x + i)
+        @test expr == [x + 1, x + 2]
+        @constraint(data.model, c[i = 1:2], i * expr[i] <= i)
+        @test c isa Vector{<:ConstraintRef}
+        @variable(data.model, bad_var[1:0])
+        @test bad_var isa Vector{<:Any}
+        @test !(bad_var isa Vector{VariableRef})  # Cannot prove type
+        @expression(data.model, bad_expr[i = 1:0], x + i)
+        @test bad_expr isa Vector{Any}
+    end
+    return
+end
+
+# This code needs to be evaluated in a top-level scope to prevent inference from
+# knowing the type of `model`.
+function test_wrap_let_symbol_models()
+    module_name = @eval module $(gensym())
+    using JuMP, Test
+    model = Model()
+    end
+    @eval module_name begin
+        @variable(model, x)
+        @test x isa VariableRef
+        @objective(model, Min, x^2)
+        @test isequal_canonical(objective_function(model), x^2)
+        @expression(model, expr[i = 1:2], x + i)
+        @test expr == [x + 1, x + 2]
+        @constraint(model, c[i = 1:2], i * expr[i] <= i)
+        @test c isa Vector{<:ConstraintRef}
+        @variable(model, bad_var[1:0])
+        @test bad_var isa Vector{VariableRef}
+        @expression(model, bad_expr[i = 1:0], x + i)
+        @test bad_expr isa Vector{Any}
+    end
+    return
+end
+
 end  # module