Constrain string interpolation

compiler/reporting/src/error/canonicalize.rs

@@ -262,6 +262,12 @@ pub fn can_problem<'b>(
                 alloc.reflow(" can occur in this position."),
             ]),
         ]),
+        Problem::InvalidInterpolation(region) => {
+            todo!(
+                "TODO report an invalid string interpolation at region {:?}",
+                region
+            );
+        }
         Problem::RuntimeError(runtime_error) => pretty_runtime_error(alloc, runtime_error),
     };
 
@@ -524,6 +530,12 @@ fn pretty_runtime_error<'b>(
             alloc.region(region),
             alloc.reflow("Only variables can be updated with record update syntax."),
         ]),
+        RuntimeError::InvalidInterpolation(region) => {
+            todo!(
+                "TODO runtime error for an invalid string interpolation at region {:?}",
+                region
+            );
+        }
         RuntimeError::NoImplementation => todo!("no implementation, unreachable"),
         RuntimeError::NonExhaustivePattern => {
             unreachable!("not currently reported (but can blow up at runtime)")

compiler/reporting/src/error/type.rs

@@ -781,7 +781,7 @@ fn to_expr_report<'b>(
                 unreachable!("I don't think these can be reached")
             }
 
-            Reason::InterpolatedStringVar => {
+            Reason::StrInterpolation => {
                 unimplemented!("string interpolation is not implemented yet")
             }
 
@@ -875,6 +875,10 @@ fn add_category<'b>(
         Int => alloc.concat(vec![this_is, alloc.text(" an integer of type:")]),
         Float => alloc.concat(vec![this_is, alloc.text(" a float of type:")]),
         Str => alloc.concat(vec![this_is, alloc.text(" a string of type:")]),
+        StrInterpolation => alloc.concat(vec![
+            this_is,
+            alloc.text(" a value in a string interpolation, which was of type:"),
+        ]),
 
         Lambda => alloc.concat(vec![this_is, alloc.text(" an anonymous function of type:")]),