attributes: add compile error on const fn instrumentation (#2418)

## Motivation

The `#[instrument]` macro cannot be used on `const fn`s, because the
generated code will perform runtime tracing behavior. However, when
adding the attribute to a `const fn`, the compiler errors generated
currently are somewhat unclear (see #2414). It would be better if we
generated a less verbose error that simply states that `#[instrument]`
is not supported on `const fn`s.

## Solution

This branch changes the `#[instrument]` macro to detect when the
annotated function is a `const fn`, and emit a simpler, more descritpive
error message. The new error simply states that the `#[instrument]`
attribute cannot be used on `const fn`s, and should be much less
confusing to the user.

Fixes #2414

tracing-attributes/src/lib.rs

@@ -82,7 +82,7 @@
 extern crate proc_macro;
 
 use proc_macro2::TokenStream;
-use quote::ToTokens;
+use quote::{quote, ToTokens};
 use syn::parse::{Parse, ParseStream};
 use syn::{Attribute, ItemFn, Signature, Visibility};
 
@@ -535,6 +535,14 @@ mod expand;
 /// }
 /// ```
 ///
+/// `const fn` cannot be instrumented, and will result in a compilation failure:
+///
+/// ```compile_fail
+/// # use tracing_attributes::instrument;
+/// #[instrument]
+/// const fn my_const_function() {}
+/// ```
+///
 /// [span]: https://docs.rs/tracing/latest/tracing/span/index.html
 /// [name]: https://docs.rs/tracing/latest/tracing/struct.Metadata.html#method.name
 /// [target]: https://docs.rs/tracing/latest/tracing/struct.Metadata.html#method.target
@@ -585,6 +593,13 @@ fn instrument_precise(
     let input = syn::parse::<ItemFn>(item)?;
     let instrumented_function_name = input.sig.ident.to_string();
 
+    if input.sig.constness.is_some() {
+        return Ok(quote! {
+            compile_error!("the `#[instrument]` attribute may not be used with `const fn`s")
+        }
+        .into());
+    }
+
     // check for async_trait-like patterns in the block, and instrument
     // the future instead of the wrapper
     if let Some(async_like) = expand::AsyncInfo::from_fn(&input) {

tracing-attributes/tests/ui.rs

@@ -5,3 +5,10 @@ fn async_instrument() {
     let t = trybuild::TestCases::new();
     t.compile_fail("tests/ui/async_instrument.rs");
 }
+
+#[rustversion::stable]
+#[test]
+fn const_instrument() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/const_instrument.rs");
+}

tracing-attributes/tests/ui/const_instrument.rs

@@ -0,0 +1,8 @@
+#![allow(unreachable_code)]
+
+#[tracing::instrument]
+const fn unit() {
+    ""
+}
+
+fn main() {}

tracing-attributes/tests/ui/const_instrument.stderr

@@ -0,0 +1,15 @@
+error: macros that expand to items must be delimited with braces or followed by a semicolon
+ --> tests/ui/const_instrument.rs:3:1
+  |
+3 | #[tracing::instrument]
+  | ^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in the attribute macro `tracing::instrument` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: the `#[instrument]` attribute may not be used with `const fn`s
+ --> tests/ui/const_instrument.rs:3:1
+  |
+3 | #[tracing::instrument]
+  | ^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in the attribute macro `tracing::instrument` (in Nightly builds, run with -Z macro-backtrace for more info)