Report anonymous functions as "this function" rather than symbol name

Closes #2453

Thanks for the assist @chris-packett

crates/compiler/can/src/expr.rs

@@ -1184,7 +1184,10 @@ fn canonicalize_closure_body<'a>(
 ) -> (ClosureData, Output) {
     // The globally unique symbol that will refer to this closure once it gets converted
     // into a top-level procedure for code gen.
-    let symbol = opt_def_name.unwrap_or_else(|| scope.gen_unique_symbol());
+    let (symbol, is_anonymous) = match opt_def_name {
+        Some(name) => (name, false),
+        None => (scope.gen_unique_symbol(), true),
+    };
 
     let mut can_args = Vec::with_capacity(loc_arg_patterns.len());
     let mut output = Output::default();
@@ -1245,7 +1248,12 @@ fn canonicalize_closure_body<'a>(
     for (sub_symbol, region) in bound_by_argument_patterns {
         if !output.references.has_value_lookup(sub_symbol) {
             // The body never referenced this argument we declared. It's an unused argument!
-            env.problem(Problem::UnusedArgument(symbol, sub_symbol, region));
+            env.problem(Problem::UnusedArgument(
+                symbol,
+                is_anonymous,
+                sub_symbol,
+                region,
+            ));
         } else {
             // We shouldn't ultimately count arguments as referenced locals. Otherwise,
             // we end up with weird conclusions like the expression (\x -> x + 1)

crates/compiler/problem/src/can.rs

@@ -35,8 +35,9 @@ pub enum Problem {
     ExposedButNotDefined(Symbol),
     UnknownGeneratesWith(Loc<Ident>),
     /// First symbol is the name of the closure with that argument
+    /// Bool is whether the closure is anonymous
     /// Second symbol is the name of the argument that is unused
-    UnusedArgument(Symbol, Symbol, Region),
+    UnusedArgument(Symbol, bool, Symbol, Region),
     PrecedenceProblem(PrecedenceProblem),
     // Example: (5 = 1 + 2) is an unsupported pattern in an assignment; Int patterns aren't allowed in assignments!
     UnsupportedPattern(BadPattern, Region),