Add a test for #2526

tests/ui/proc_macro.rs

@@ -0,0 +1,8 @@
+//! Check that we correctly lint procedural macros.
+
+#![crate_type = "proc-macro"]
+
+#[allow(dead_code)]
+fn f() {
+    let _x = 3.14;
+}

tests/ui/proc_macro.stderr

@@ -0,0 +1,10 @@
+error: approximate value of `f{32, 64}::consts::PI` found. Consider using it directly
+  --> $DIR/proc_macro.rs:7:14
+   |
+LL |     let _x = 3.14;
+   |              ^^^^
+   |
+   = note: #[deny(clippy::approx_constant)] on by default
+
+error: aborting due to previous error
+