Report exposed-but-not-defined symbols

compiler/can/src/module.rs

@@ -240,8 +240,8 @@ pub fn canonicalize_module_defs<'a>(
             // exposed_symbols and added to exposed_vars_by_symbol. If any were
             // not, that means they were declared as exposed but there was
             // no actual declaration with that name!
-            if !exposed_symbols.is_empty() {
+            for symbol in exposed_symbols {
-                panic!("TODO gracefully handle invalid `exposes` entry (or entries) which had no corresponding definition: {:?}", exposed_symbols);
+                env.problem(Problem::ExposedButNotDefined(symbol));
             }
 
             // Incorporate any remaining output.lookups entries into references.

compiler/problem/src/can.rs

@@ -12,6 +12,7 @@ use roc_region::all::{Located, Region};
 pub enum Problem {
     UnusedDef(Symbol, Region),
     UnusedImport(ModuleId, Region),
+    ExposedButNotDefined(Symbol),
     /// First symbol is the name of the closure with that argument
     /// Second symbol is the name of the argument that is unused
     UnusedArgument(Symbol, Symbol, Region),

compiler/reporting/src/error/canonicalize.rs

@@ -37,6 +37,19 @@ pub fn can_problem<'b>(
             alloc.module(module_id),
             alloc.reflow(" isn't used, you don't need to import it."),
         ]),
+        Problem::ExposedButNotDefined(symbol) => {
+            alloc.stack(vec![
+                alloc
+                    .symbol_unqualified(symbol)
+                    .append(alloc.reflow(" is listed as exposed, but it isn't defined in this module.")),
+                alloc
+                    .reflow("You can fix this by adding a definition for ")
+                    .append(alloc.symbol_unqualified(symbol))
+                    .append(alloc.reflow(", or by removing it from "))
+                    .append(alloc.keyword("exposes"))
+                    .append(alloc.reflow("."))
+            ])
+        }
         Problem::UnusedArgument(closure_symbol, argument_symbol, region) => {
             let line = "\". Adding an underscore at the start of a variable name is a way of saying that the variable is not used.";