force interpolated variables to be of type string

crates/compiler/can/src/expr.rs

@@ -2600,10 +2600,38 @@ fn flatten_str_lines<'a>(
 fn desugar_str_segments(var_store: &mut VarStore, segments: Vec<StrSegment>) -> Expr {
     use StrSegment::*;
 
+    let n = segments.len();
     let mut iter = segments.into_iter().rev();
     let mut loc_expr = match iter.next() {
         Some(Plaintext(string)) => Loc::at(Region::zero(), Expr::Str(string)),
-        Some(Interpolation(loc_expr)) => loc_expr,
+        Some(Interpolation(loc_expr)) => {
+            if n == 1 {
+                // We concat with the empty string to ensure a type error when loc_expr is not a string
+                let empty_string = Loc::at(Region::zero(), Expr::Str("".into()));
+
+                let fn_expr = Loc::at(
+                    Region::zero(),
+                    Expr::Var(Symbol::STR_CONCAT, var_store.fresh()),
+                );
+                let expr = Expr::Call(
+                    Box::new((
+                        var_store.fresh(),
+                        fn_expr,
+                        var_store.fresh(),
+                        var_store.fresh(),
+                    )),
+                    vec![
+                        (var_store.fresh(), empty_string),
+                        (var_store.fresh(), loc_expr),
+                    ],
+                    CalledVia::StringInterpolation,
+                );
+
+                Loc::at(Region::zero(), expr)
+            } else {
+                loc_expr
+            }
+        }
         None => {
             // No segments? Empty string!
 

crates/compiler/test_mono/generated/lambda_capture_niches_with_other_lambda_capture.txt

@@ -1,10 +1,14 @@
+procedure Str.3 (#Attr.2, #Attr.3):
+    let Str.251 : Str = lowlevel StrConcat #Attr.2 #Attr.3;
+    ret Str.251;
+
 procedure Test.1 (Test.5):
     let Test.16 : [C {}, C U64, C Str] = TagId(0) Test.5;
     ret Test.16;
 
 procedure Test.1 (Test.5):
-    let Test.30 : [C {}, C U64, C Str] = TagId(1) Test.5;
+    let Test.31 : [C {}, C U64, C Str] = TagId(1) Test.5;
-    ret Test.30;
+    ret Test.31;
 
 procedure Test.2 (Test.7):
     let Test.23 : [C {}, C U64, C Str] = TagId(2) Test.7;
@@ -16,13 +20,16 @@ procedure Test.6 (Test.17, #Attr.12):
     ret Test.19;
 
 procedure Test.6 (Test.17, #Attr.12):
-    let Test.34 : U64 = UnionAtIndex (Id 1) (Index 0) #Attr.12;
+    let Test.35 : U64 = UnionAtIndex (Id 1) (Index 0) #Attr.12;
-    let Test.33 : Str = "";
+    let Test.34 : Str = "";
-    ret Test.33;
+    ret Test.34;
 
 procedure Test.8 (Test.24, #Attr.12):
-    let Test.27 : Str = UnionAtIndex (Id 2) (Index 0) #Attr.12;
+    let Test.28 : Str = UnionAtIndex (Id 2) (Index 0) #Attr.12;
-    ret Test.27;
+    let Test.27 : Str = "";
+    let Test.26 : Str = CallByName Str.3 Test.27 Test.28;
+    dec Test.28;
+    ret Test.26;
 
 procedure Test.0 ():
     let Test.3 : U8 = 0u8;
@@ -55,7 +62,7 @@ procedure Test.0 ():
             jump Test.13 Test.21;
     
         default:
-            let Test.29 : U64 = 1i64;
+            let Test.30 : U64 = 1i64;
-            let Test.28 : [C {}, C U64, C Str] = CallByName Test.1 Test.29;
+            let Test.29 : [C {}, C U64, C Str] = CallByName Test.1 Test.30;
-            jump Test.13 Test.28;
+            jump Test.13 Test.29;
     

crates/repl_test/src/tests.rs

@@ -813,6 +813,32 @@ fn list_get_negative_index() {
     );
 }
 
+#[cfg(not(feature = "wasm"))] // TODO: mismatch is due to terminal control codes!
+#[test]
+fn invalid_string_interpolation() {
+    expect_failure(
+        "\"$(123)\"",
+        indoc!(
+            r#"
+            ── TYPE MISMATCH ───────────────────────────────────────────────────────────────
+
+            This argument to this string interpolation has an unexpected type:
+
+            4│      "$(123)"
+                       ^^^
+
+            The argument is a number of type:
+
+                Num *
+
+            But this string interpolation needs its argument to be:
+
+                Str
+            "#
+        ),
+    );
+}
+
 #[test]
 fn issue_2149_i8_ok() {
     expect_success(r#"Str.toI8 "127""#, "Ok 127 : Result I8 [InvalidNumStr]");