diff --git a/compiler/can/src/expr.rs b/compiler/can/src/expr.rs
index 490d90e2cb..ad62f52ee5 100644
--- a/compiler/can/src/expr.rs
+++ b/compiler/can/src/expr.rs
@@ -1,4 +1,5 @@
 use crate::annotation::IntroducedVariables;
+use crate::builtins::builtin_defs;
 use crate::def::{can_defs_with_return, Def};
 use crate::env::Env;
 use crate::num::{
@@ -1186,9 +1187,62 @@ pub fn inline_calls(var_store: &mut VarStore, scope: &mut Scope, expr: Expr) ->
             let (fn_var, loc_expr, expr_var) = *boxed_tuple;
 
             match loc_expr.value {
-                Var(symbol) if symbol.is_builtin() => {
-                    todo!("Inline this builtin: {:?}", symbol);
-                }
+                Var(symbol) if symbol.is_builtin() => match builtin_defs(var_store).get(&symbol) {
+                    Some(Closure(_var, _, recursive, params, boxed_body)) => {
+                        debug_assert_eq!(*recursive, Recursive::NotRecursive);
+
+                        // Since this is a canonicalized Expr, we should have
+                        // already detected any arity mismatches and replaced this
+                        // with a RuntimeError if there was a mismatch.
+                        debug_assert_eq!(params.len(), args.len());
+
+                        // Start with the function's body as the answer.
+                        let (mut loc_answer, _body_var) = *boxed_body.clone();
+
+                        // Wrap the body in one LetNonRec for each argument,
+                        // such that at the end we have all the arguments in
+                        // scope with the values the caller provided.
+                        for ((_param_var, loc_pattern), (expr_var, loc_expr)) in
+                            params.iter().cloned().zip(args.into_iter()).rev()
+                        {
+                            // TODO get the correct vars into here.
+                            // Not sure if param_var should be involved.
+                            let pattern_vars = SendMap::default();
+
+                            // TODO get the actual correct aliases
+                            let aliases = SendMap::default();
+
+                            let def = Def {
+                                loc_pattern,
+                                loc_expr,
+                                expr_var,
+                                pattern_vars,
+                                annotation: None,
+                            };
+
+                            loc_answer = Located {
+                                region: Region::zero(),
+                                value: LetNonRec(
+                                    Box::new(def),
+                                    Box::new(loc_answer),
+                                    var_store.fresh(),
+                                    aliases,
+                                ),
+                            };
+                        }
+
+                        loc_answer.value
+                    }
+                    Some(_) => {
+                        unreachable!("Tried to inline a non-function");
+                    }
+                    None => {
+                        unreachable!(
+                            "Tried to inline a builtin that wasn't registered: {:?}",
+                            symbol
+                        );
+                    }
+                },
                 _ => {
                     // For now, we only inline calls to builtins. Leave this alone!
                     Call(Box::new((fn_var, loc_expr, expr_var)), args, called_via)
diff --git a/compiler/can/tests/can_inline.rs b/compiler/can/tests/can_inline.rs
index 7275bc971e..bcf2c944e5 100644
--- a/compiler/can/tests/can_inline.rs
+++ b/compiler/can/tests/can_inline.rs
@@ -14,13 +14,16 @@ mod helpers;
 mod can_inline {
     use crate::helpers::{can_expr_with, test_home};
     use bumpalo::Bump;
+    use roc_can::def::Def;
     use roc_can::expr::inline_calls;
     use roc_can::expr::Expr::{self, *};
+    use roc_can::pattern::Pattern;
     use roc_can::scope::Scope;
+    use roc_collections::all::SendMap;
     use roc_module::operator::CalledVia;
     use roc_module::symbol::Symbol;
     use roc_region::all::{Located, Region};
-    use roc_types::subs::VarStore;
+    use roc_types::subs::{VarStore, Variable};
 
     fn assert_inlines_to(input: &str, expected: Expr, var_store: &mut VarStore) {
         let arena = Bump::new();
@@ -34,6 +37,7 @@ mod can_inline {
     #[test]
     fn inline_list_len() {
         let var_store = &mut VarStore::default();
+        let aliases = SendMap::default();
 
         assert_inlines_to(
             indoc!(
@@ -41,23 +45,55 @@ mod can_inline {
                     Int.isZero 5
                 "#
             ),
-            Expr::Call(
-                Box::new((
-                    var_store.fresh(),
-                    Located {
+            LetNonRec(
+                Box::new(Def {
+                    loc_pattern: Located {
                         region: Region::zero(),
-                        value: Expr::Var(Symbol::FLOAT_EQ),
+                        value: Pattern::Identifier(Symbol::INT_IS_ZERO_ARG),
                     },
-                    var_store.fresh(),
-                )),
-                vec![(
-                    var_store.fresh(),
-                    Located {
-                        region: Region::zero(),
-                        value: Int(var_store.fresh(), 5),
+                    pattern_vars: SendMap::default(),
+                    loc_expr: Located {
+                        region: Region::new(0, 0, 11, 12),
+                        value: Num(unsafe { Variable::unsafe_test_debug_variable(7) }, 5),
                     },
-                )],
-                CalledVia::Space,
+                    expr_var: unsafe { Variable::unsafe_test_debug_variable(8) },
+                    annotation: None,
+                }),
+                Box::new(Located {
+                    region: Region::zero(),
+                    value: Expr::Call(
+                        Box::new((
+                            unsafe { Variable::unsafe_test_debug_variable(138) },
+                            Located {
+                                region: Region::zero(),
+                                value: Expr::Var(Symbol::INT_EQ_I64),
+                            },
+                            unsafe { Variable::unsafe_test_debug_variable(139) },
+                        )),
+                        vec![
+                            (
+                                unsafe { Variable::unsafe_test_debug_variable(140) },
+                                Located {
+                                    region: Region::zero(),
+                                    value: Var(Symbol::INT_IS_ZERO_ARG),
+                                },
+                            ),
+                            (
+                                unsafe { Variable::unsafe_test_debug_variable(141) },
+                                Located {
+                                    region: Region::zero(),
+                                    value: Int(
+                                        unsafe { Variable::unsafe_test_debug_variable(137) },
+                                        0,
+                                    ),
+                                },
+                            ),
+                        ],
+                        CalledVia::Space,
+                    ),
+                }),
+                unsafe { Variable::unsafe_test_debug_variable(198) },
+                aliases,
             ),
             var_store,
         )