macro: helpers for error handling

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

proxmox-api-macro/src/api_macro.rs

@@ -3,8 +3,8 @@ use std::collections::HashMap;
 use proc_macro2::{Delimiter, Ident, Span, TokenStream, TokenTree};
 
 use failure::{bail, format_err, Error};
-use quote::{quote, quote_spanned, ToTokens};
-use syn::{Expr, Token};
+use quote::{quote, ToTokens};
+use syn::{spanned::Spanned, Expr, Token};
 
 use super::api_def::{CommonTypeDefinition, ParameterDefinition};
 use super::parsing::*;
@@ -17,7 +17,7 @@ pub fn api_macro(attr: TokenStream, item: TokenStream) -> Result<TokenStream, Er
 
     let definition = match definition {
         TokenTree::Group(ref group) if group.delimiter() == Delimiter::Brace => group.stream(),
-        _ => bail!("expected api definition in braces"),
+        _ => cbail!(definition.span() => "expected api definition in braces"),
     };
 
     let definition = parse_object(definition)?;
@@ -34,7 +34,7 @@ pub fn api_macro(attr: TokenStream, item: TokenStream) -> Result<TokenStream, Er
             Ok(output)
         }
         syn::Item::Fn(func) => handle_function(definition, func),
-        _ => bail!("api macro currently only applies to structs and functions"),
+        _ => cbail!(item.span() => "api macro currently only applies to structs and functions"),
     }
 }
 
@@ -43,9 +43,10 @@ fn handle_function(
     mut item: syn::ItemFn,
 ) -> Result<TokenStream, Error> {
     if item.decl.generics.lt_token.is_some() {
-        return Ok(quote_spanned! { item.decl.generics.lt_token.unwrap().span =>
-            compile_error!("cannot use generic functions for api macros currently");
-        }.into());
+        cbail!(
+            item.decl.generics.span(),
+            "cannot use generic functions for api macros currently",
+        );
         // Not until we stabilize our generated representation!
     }
 

proxmox-api-macro/src/error.rs

@@ -0,0 +1,24 @@
+#[derive(Debug)]
+pub struct CompileError {
+    pub tokens: proc_macro::TokenStream,
+}
+
+unsafe impl Send for CompileError {}
+unsafe impl Sync for CompileError {}
+
+impl std::fmt::Display for CompileError {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(f, "generic compile error")
+    }
+}
+
+impl std::error::Error for CompileError {}
+
+macro_rules! cbail {
+    ($span:expr => $($msg:tt)*) => {
+        return Err(::failure::Error::from(crate::error::CompileError {
+            tokens: ::quote::quote_spanned! { $span => compile_error!($($msg)*); }.into()
+        }))
+    };
+    ($span:expr, $($msg:tt)*) => { cbail!($span => $($msg)*) }
+}

proxmox-api-macro/src/lib.rs

@@ -5,6 +5,9 @@ extern crate proc_macro2;
 
 use proc_macro::TokenStream;
 
+#[macro_use]
+mod error;
+
 mod api_def;
 mod parsing;
 mod util;
@@ -12,6 +15,13 @@ mod util;
 mod api_macro;
 mod router_macro;
 
+fn handle_error(kind: &'static str, err: failure::Error) -> TokenStream {
+    match err.downcast::<error::CompileError>() {
+        Ok(err) => err.tokens,
+        Err(err) => panic!("error in {}: {}", kind, err),
+    }
+}
+
 /// This is the `#[api(api definition)]` attribute for functions. An Api definition defines the
 /// parameters and return type of an API call. The function will automatically be wrapped in a
 /// function taking and returning a json `Value`, while performing validity checks on both input
@@ -44,7 +54,7 @@ mod router_macro;
 pub fn api(attr: TokenStream, item: TokenStream) -> TokenStream {
     match api_macro::api_macro(attr.into(), item.into()) {
         Ok(output) => output.into(),
-        Err(err) => panic!("error in api definition: {}", err),
+        Err(err) => handle_error("api definition", err),
     }
 }
 
@@ -135,6 +145,6 @@ pub fn router(input: TokenStream) -> TokenStream {
     // TODO...
     match router_macro::router_macro(input.into()) {
         Ok(output) => output.into(),
-        Err(err) => panic!("error in router macro: {}", err),
+        Err(err) => handle_error("router", err),
     }
 }