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fix(derive-encode): emit friendly compile errors instead of panicking #307

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123 changes: 79 additions & 44 deletions derive-encode/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,12 @@ use proc_macro2::TokenStream as TokenStream2;
use quote::quote;
use syn::DeriveInput;

fn error_spanned<T: quote::ToTokens>(tokens: &T, msg: &str) -> TokenStream {
syn::Error::new_spanned(tokens, msg)
.to_compile_error()
.into()
}

/// Derive `prometheus_client::encoding::EncodeLabelSet`.
#[proc_macro_derive(EncodeLabelSet, attributes(prometheus))]
pub fn derive_encode_label_set(input: TokenStream) -> TokenStream {
Expand All @@ -19,55 +25,82 @@ pub fn derive_encode_label_set(input: TokenStream) -> TokenStream {

let body: TokenStream2 = match ast.clone().data {
syn::Data::Struct(s) => match s.fields {
syn::Fields::Named(syn::FieldsNamed { named, .. }) => named
.into_iter()
.map(|f| {
let attribute = f
.attrs
.iter()
.find(|a| a.path().is_ident("prometheus"))
.map(|a| a.parse_args::<syn::Ident>().unwrap().to_string());
let flatten = match attribute.as_deref() {
Some("flatten") => true,
Some(other) => {
panic!("Provided attribute '{other}', but only 'flatten' is supported")
}
None => false,
};
let ident = f.ident.unwrap();
if flatten {
quote! {
EncodeLabelSet::encode(&self.#ident, encoder)?;
}
} else {
let ident_string = KEYWORD_IDENTIFIERS
syn::Fields::Named(syn::FieldsNamed { named, .. }) => {
let body = named
.into_iter()
.map(|f| {
let ident = f.ident.unwrap();
let flatten = match f
.attrs
.iter()
.find(|pair| ident == pair.1)
.map(|pair| pair.0.to_string())
.unwrap_or_else(|| ident.to_string());

quote! {
let mut label_encoder = encoder.encode_label();
let mut label_key_encoder = label_encoder.encode_label_key()?;
EncodeLabelKey::encode(&#ident_string, &mut label_key_encoder)?;

let mut label_value_encoder = label_key_encoder.encode_label_value()?;
EncodeLabelValue::encode(&self.#ident, &mut label_value_encoder)?;

label_value_encoder.finish()?;
.find(|a| a.path().is_ident("prometheus"))
{
None => false,
Some(a) => match a.parse_args::<syn::Ident>() {
Ok(ident) if ident == "flatten" => true,
Ok(other) => {
return Err(error_spanned(
&other,
&format!(
"Provided attribute '{other}', but only 'flatten' is supported"
),
));
}
Err(_) => {
return Err(error_spanned(
a,
"Attribute on `#[prometheus(...)]` must be an identifier, e.g. `#[prometheus(flatten)]`",
));
}
},
};

if flatten {
Ok(quote! {
EncodeLabelSet::encode(&self.#ident, encoder)?;
})
} else {
let ident_string = KEYWORD_IDENTIFIERS
.iter()
.find(|pair| ident == pair.1)
.map(|pair| pair.0.to_string())
.unwrap_or_else(|| ident.to_string());

Ok(quote! {
let mut label_encoder = encoder.encode_label();
let mut label_key_encoder = label_encoder.encode_label_key()?;
EncodeLabelKey::encode(&#ident_string, &mut label_key_encoder)?;

let mut label_value_encoder = label_key_encoder.encode_label_value()?;
EncodeLabelValue::encode(&self.#ident, &mut label_value_encoder)?;

label_value_encoder.finish()?;
})
}
}
})
.collect(),
})
.collect::<Result<TokenStream2, TokenStream>>();

match body {
Ok(body) => body,
Err(err) => return err,
}
}
syn::Fields::Unnamed(_) => {
panic!("Can not derive Encode for struct with unnamed fields.")
return error_spanned(
&ast,
"Can not derive Encode for struct with unnamed fields.",
);
}
syn::Fields::Unit => {
return error_spanned(&ast, "Can not derive Encode for struct with unit field.");
}
syn::Fields::Unit => panic!("Can not derive Encode for struct with unit field."),
},
syn::Data::Enum(syn::DataEnum { .. }) => {
panic!("Can not derive Encode for enum.")
return error_spanned(&ast, "Can not derive Encode for enum.");
}
syn::Data::Union(_) => {
return error_spanned(&ast, "Can not derive Encode for union.");
}
syn::Data::Union(_) => panic!("Can not derive Encode for union."),
};

let gen = quote! {
Expand Down Expand Up @@ -95,7 +128,7 @@ pub fn derive_encode_label_value(input: TokenStream) -> TokenStream {

let body = match ast.clone().data {
syn::Data::Struct(_) => {
panic!("Can not derive EncodeLabel for struct.")
return error_spanned(&ast, "Can not derive EncodeLabel for struct.");
}
syn::Data::Enum(syn::DataEnum { variants, .. }) => {
let match_arms: TokenStream2 = variants
Expand All @@ -114,7 +147,9 @@ pub fn derive_encode_label_value(input: TokenStream) -> TokenStream {
}
}
}
syn::Data::Union(_) => panic!("Can not derive Encode for union."),
syn::Data::Union(_) => {
return error_spanned(&ast, "Can not derive Encode for union.");
}
};

let gen = quote! {
Expand Down
37 changes: 37 additions & 0 deletions derive-encode/tests/build/friendly-compilation-error-msg.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,37 @@
use prometheus_client::encoding::EncodeLabelSet;
use prometheus_client::encoding::EncodeLabelValue;

// Unnamed struct fields: not supported by `EncodeLabelSet`.
#[derive(EncodeLabelSet)]
struct Unnamed(String);

// Unit struct: not supported by `EncodeLabelSet`.
#[derive(EncodeLabelSet)]
struct UnitStruct;

// Enum: not supported by `EncodeLabelSet`.
#[derive(EncodeLabelSet)]
enum NotAllowedSet {
A,
B,
}

// Unknown attribute: only `#[prometheus(flatten)]` is accepted.
#[derive(EncodeLabelSet)]
struct BadAttr {
#[prometheus(unknown)]
a: u64,
}

// Struct: not supported by `EncodeLabelValue` (only enums are).
#[derive(EncodeLabelValue)]
struct StructForValue;

// Union: not supported by either derive.
#[derive(EncodeLabelSet)]
union UnionSet {
a: u32,
b: f32,
}

fn main() {}
41 changes: 41 additions & 0 deletions derive-encode/tests/build/friendly-compilation-error-msg.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,41 @@
error: Can not derive Encode for struct with unnamed fields.
--> tests/build/friendly-compilation-error-msg.rs:6:1
|
6 | struct Unnamed(String);
| ^^^^^^^^^^^^^^^^^^^^^^^

error: Can not derive Encode for struct with unit field.
--> tests/build/friendly-compilation-error-msg.rs:10:1
|
10 | struct UnitStruct;
| ^^^^^^^^^^^^^^^^^^

error: Can not derive Encode for enum.
--> tests/build/friendly-compilation-error-msg.rs:14:1
|
14 | / enum NotAllowedSet {
15 | | A,
16 | | B,
17 | | }
| |_^

error: Provided attribute 'unknown', but only 'flatten' is supported
--> tests/build/friendly-compilation-error-msg.rs:22:18
|
22 | #[prometheus(unknown)]
| ^^^^^^^

error: Can not derive EncodeLabel for struct.
--> tests/build/friendly-compilation-error-msg.rs:28:1
|
28 | struct StructForValue;
| ^^^^^^^^^^^^^^^^^^^^^^

error: Can not derive Encode for union.
--> tests/build/friendly-compilation-error-msg.rs:32:1
|
32 | / union UnionSet {
33 | | a: u32,
34 | | b: f32,
35 | | }
| |_^
3 changes: 2 additions & 1 deletion derive-encode/tests/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -205,5 +205,6 @@ fn flatten() {
#[test]
fn build() {
let t = trybuild::TestCases::new();
t.pass("tests/build/redefine-prelude-symbols.rs")
t.pass("tests/build/redefine-prelude-symbols.rs");
t.compile_fail("tests/build/friendly-compilation-error-msg.rs");
}