From eca101ef0f006fd644a309699aca5a7167942c75 Mon Sep 17 00:00:00 2001
From: Oscar Puente <156957451+orpuente-MS@users.noreply.github.com>
Date: Wed, 23 Oct 2024 16:28:12 -0700
Subject: [PATCH] `Measurement` attribute on function issues error

---
 compiler/qsc_frontend/src/lower.rs       | 43 +++++++++++++++++-------
 compiler/qsc_frontend/src/lower/tests.rs | 24 +++++++++++++
 2 files changed, 54 insertions(+), 13 deletions(-)

diff --git a/compiler/qsc_frontend/src/lower.rs b/compiler/qsc_frontend/src/lower.rs
index da09044210..42dd0ceae5 100644
--- a/compiler/qsc_frontend/src/lower.rs
+++ b/compiler/qsc_frontend/src/lower.rs
@@ -30,8 +30,12 @@ pub(super) enum Error {
     #[error("invalid attribute arguments: expected {0}")]
     #[diagnostic(code("Qsc.LowerAst.InvalidAttrArgs"))]
     InvalidAttrArgs(String, #[label] Span),
+    #[error("invalid use of the Measurement attribute on a function")]
+    #[diagnostic(code("Qsc.LowerAst.InvalidMeasurementAttrOnFunction"))]
+    InvalidMeasurementAttrOnFunction(#[label] Span),
     #[error("missing callable body")]
     #[diagnostic(code("Qsc.LowerAst.MissingBody"))]
+    #[diagnostic(help("try declaring the callable as an operation"))]
     MissingBody(#[label] Span),
     #[error("duplicate specialization")]
     #[diagnostic(code("Qsc.LowerAst.DuplicateSpec"))]
@@ -380,7 +384,7 @@ impl With<'_> {
         attrs: &[qsc_hir::hir::Attr],
     ) -> hir::CallableDecl {
         let id = self.lower_id(decl.id);
-        let kind = lower_callable_kind(decl.kind, attrs);
+        let kind = self.lower_callable_kind(decl.kind, attrs, decl.name.span);
         let name = self.lower_ident(&decl.name);
         let mut input = self.lower_pat(&decl.input);
         let output = convert::ty_from_ast(self.names, &decl.output).0;
@@ -428,6 +432,30 @@ impl With<'_> {
         }
     }
 
+    fn lower_callable_kind(
+        &mut self,
+        kind: ast::CallableKind,
+        attrs: &[qsc_hir::hir::Attr],
+        span: Span,
+    ) -> hir::CallableKind {
+        if attrs.contains(&qsc_hir::hir::Attr::Measurement) {
+            match kind {
+                ast::CallableKind::Operation => hir::CallableKind::Measurement,
+                ast::CallableKind::Function => {
+                    self.lowerer
+                        .errors
+                        .push(Error::InvalidMeasurementAttrOnFunction(span));
+                    hir::CallableKind::Function
+                }
+            }
+        } else {
+            match kind {
+                ast::CallableKind::Operation => hir::CallableKind::Operation,
+                ast::CallableKind::Function => hir::CallableKind::Function,
+            }
+        }
+    }
+
     fn find_spec(
         &mut self,
         specs: &[Box<ast::SpecDecl>],
@@ -623,7 +651,7 @@ impl With<'_> {
                     FunctorSetValue::Empty
                 };
                 let lambda = Lambda {
-                    kind: lower_callable_kind(*kind, &[]),
+                    kind: self.lower_callable_kind(*kind, &[], expr.span),
                     functors,
                     input: self.lower_pat(input),
                     body: self.lower_expr(body),
@@ -934,17 +962,6 @@ impl With<'_> {
     }
 }
 
-fn lower_callable_kind(kind: ast::CallableKind, attrs: &[qsc_hir::hir::Attr]) -> hir::CallableKind {
-    if attrs.contains(&qsc_hir::hir::Attr::Measurement) {
-        hir::CallableKind::Measurement
-    } else {
-        match kind {
-            ast::CallableKind::Function => hir::CallableKind::Function,
-            ast::CallableKind::Operation => hir::CallableKind::Operation,
-        }
-    }
-}
-
 fn lower_mutability(mutability: ast::Mutability) -> hir::Mutability {
     match mutability {
         ast::Mutability::Immutable => hir::Mutability::Immutable,
diff --git a/compiler/qsc_frontend/src/lower/tests.rs b/compiler/qsc_frontend/src/lower/tests.rs
index e2d2a14ea5..f35ba17a66 100644
--- a/compiler/qsc_frontend/src/lower/tests.rs
+++ b/compiler/qsc_frontend/src/lower/tests.rs
@@ -2208,6 +2208,30 @@ fn invalid_spec_pat() {
     );
 }
 
+#[test]
+fn test_measurement_attr_on_function_issues_error() {
+    check_errors(
+        indoc! {r#"
+            namespace Test {
+                @Measurement()
+                function Foo(q: Qubit) : Result {
+                    body intrinsic;
+                }
+            }
+        "#},
+        &expect![[r#"
+            [
+                InvalidMeasurementAttrOnFunction(
+                    Span {
+                        lo: 49,
+                        hi: 52,
+                    },
+                ),
+            ]
+        "#]],
+    );
+}
+
 #[test]
 fn item_docs() {
     check_hir(