Suffixed pure function warning

crates/compiler/load/tests/test_reporting.rs

@@ -14751,4 +14751,32 @@ All branches in an `if` must have the same type!
     This will help readers identify it as a source of effects.
     "###
     );
+
+    test_report!(
+        function_def_leftover_bang,
+        indoc!(
+            r#"
+            app [main!] { pf: platform "../../../../../examples/cli/effects-platform/main.roc" }
+
+            import pf.Effect
+
+            main! = \{} ->
+                Effect.putLine! (hello! {})
+
+            hello! = \{} ->
+                "hello"
+            "#
+        ),
+        @r###"
+    ── UNNECESSARY EXCLAMATION in /code/proj/Main.roc ──────────────────────────────
+
+    This function is pure, but its name suggests otherwise:
+
+    8│  hello! = \{} ->
+        ^^^^^^
+
+    Remove the exclamation mark to give an accurate impression of its
+    behavior.
+    "###
+    );
 }

crates/compiler/lower_params/src/type_error.rs

@@ -101,7 +101,8 @@ pub fn remove_module_param_arguments(
             | TypeError::MissingModuleParams(_, _, _)
             | TypeError::ModuleParamsMismatch(_, _, _, _)
             | TypeError::PureStmt(_)
-            | TypeError::UnsuffixedEffectfulFunction(_, _) => {}
+            | TypeError::UnsuffixedEffectfulFunction(_, _)
+            | TypeError::SuffixedPureFunction(_, _) => {}
         }
     }
 }

crates/compiler/solve/src/solve.rs

@@ -1543,7 +1543,17 @@ fn check_symbol_suffix(
             }
         }
         IdentSuffix::Bang => {
-            // [purity-inference] TODO
+            if let Content::Structure(FlatType::Func(_, _, _, fx)) =
+                env.subs.get_content_without_compacting(loc_var.value)
+            {
+                match env.subs.get_content_without_compacting(*fx) {
+                    // [purity-inference] TODO: Should FlexVar actually be a case?
+                    Content::Pure | Content::FlexVar(_) => {
+                        problems.push(TypeError::SuffixedPureFunction(loc_var.region, symbol));
+                    }
+                    _ => {}
+                }
+            }
         }
     }
 }

crates/compiler/solve_problem/src/lib.rs

@@ -41,6 +41,7 @@ pub enum TypeError {
     ModuleParamsMismatch(Region, ModuleId, ErrorType, ErrorType),
     PureStmt(Region),
     UnsuffixedEffectfulFunction(Region, Symbol),
+    SuffixedPureFunction(Region, Symbol),
 }
 
 impl TypeError {
@@ -67,6 +68,7 @@ impl TypeError {
             TypeError::IngestedFileUnsupportedType(..) => Fatal,
             TypeError::PureStmt(..) => Warning,
             TypeError::UnsuffixedEffectfulFunction(_, _) => Warning,
+            TypeError::SuffixedPureFunction(_, _) => Warning,
         }
     }
 
@@ -84,7 +86,8 @@ impl TypeError {
             | TypeError::MissingModuleParams(region, ..)
             | TypeError::ModuleParamsMismatch(region, ..)
             | TypeError::PureStmt(region)
-            | TypeError::UnsuffixedEffectfulFunction(region, _) => Some(*region),
+            | TypeError::UnsuffixedEffectfulFunction(region, _)
+            | TypeError::SuffixedPureFunction(region, _) => Some(*region),
             TypeError::UnfulfilledAbility(ab, ..) => ab.region(),
             TypeError::Exhaustive(e) => Some(e.region()),
             TypeError::CircularDef(c) => c.first().map(|ce| ce.symbol_region),

crates/reporting/src/error/type.rs

@@ -347,6 +347,20 @@ pub fn type_problem<'b>(
                 severity,
             })
         }
+        SuffixedPureFunction(region, _symbol) => {
+            let stack = [
+                alloc.reflow("This function is pure, but its name suggests otherwise:"),
+                alloc.region(lines.convert_region(region), severity),
+                alloc.reflow("Remove the exclamation mark to give an accurate impression of its behavior."),
+            ];
+
+            Some(Report {
+                title: "UNNECESSARY EXCLAMATION".to_string(),
+                filename,
+                doc: alloc.stack(stack),
+                severity,
+            })
+        }
     }
 }