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Output better error messages when deriving ContentHash for an enum fails

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Consider this code:
```
struct NoContentHash {}

#[derive(ContentHash)]
enum Hashable {
    NoCanHash(NoContentHash),
    Empty,
}
```

Before this commit, it generates an error like this:
```
error[E0277]: the trait bound `NoContentHash: ContentHash` is not satisfied
   --> lib/src/content_hash.rs:150:10
    |
150 | #[derive(ContentHash)]
    |          ^^^^^^^^^^^ the trait `ContentHash` is not implemented for `NoContentHash`
151 | enum Hashable {
152 |     NoCanHash(NoContentHash),
    |     --------- required by a bound introduced by this call
    |
    = help: the following other types implement trait `ContentHash`:
              bool
              i32
              i64
              u8
              u32
              u64
              std::collections::HashMap<K, V>
              BTreeMap<K, V>
            and 35 others

For more information about this error, try `rustc --explain E0277`.
```

After this commit, it generates a better error message:
```
error[E0277]: the trait bound `NoContentHash: ContentHash` is not satisfied
   --> lib/src/content_hash.rs:152:15
    |
152 |     NoCanHash(NoContentHash),
    |               ^^^^^^^^^^^^^ the trait `ContentHash` is not implemented for `NoContentHash`
    |
    = help: the following other types implement trait `ContentHash`:
              bool
              i32
              i64
              u8
              u32
              u64
              std::collections::HashMap<K, V>
              BTreeMap<K, V>
            and 35 others

For more information about this error, try `rustc --explain E0277`.
error: could not compile `jj-lib` (lib) due to 1 previous error
```

It also works for enum variants with named fields:
```
error[E0277]: the trait bound `NoContentHash: ContentHash` is not satisfied
   --> lib/src/content_hash.rs:152:23
    |
152 |     NoCanHash { named: NoContentHash },
    |                       ^^^^^^^^^^^^^ the trait `ContentHash` is not implemented for `NoContentHash`
    |
    = help: the following other types implement trait `ContentHash`:
              bool
              i32
              i64
              u8
              u32
              u64
              std::collections::HashMap<K, V>
              BTreeMap<K, V>
            and 35 others

For more information about this error, try `rustc --explain E0277`.
```
This commit is contained in:
Evan Mesterhazy 2024-02-20 09:44:03 -05:00 committed by Evan Mesterhazy
parent e8f324ffde
commit 79518eafce
1 changed files with 36 additions and 9 deletions
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45
lib/proc-macros/src/content_hash.rs
View file

@ -1,7 +1,7 @@
use proc_macro2::{Ident, TokenStream};
use quote::{format_ident, quote, quote_spanned};
use syn::spanned::Spanned;
use syn::{parse_quote, Data, Field, Fields, GenericParam, Generics, Index};
use syn::{parse_quote, Data, Field, Fields, GenericParam, Generics, Index, Type};
pub fn add_trait_bounds(mut generics: Generics) -> Generics {
for param in &mut generics.params {
@ -54,21 +54,21 @@ pub fn generate_hash_impl(data: &Data) -> TokenStream {
match &v.fields {
Fields::Named(fields) => {
let bindings = enum_bindings(fields.named.iter());
let ix = index_to_ordinal(i);
let hash_statements =
hash_statements_for_enum_fields(i, fields.named.iter());
quote_spanned! {v.span() =>
Self::#variant_id{ #(#bindings),* } => {
::jj_lib::content_hash::ContentHash::hash(&#ix, state);
#( ::jj_lib::content_hash::ContentHash::hash(#bindings, state); )*
#(#hash_statements)*
}
}
}
Fields::Unnamed(fields) => {
let bindings = enum_bindings(fields.unnamed.iter());
let ix = index_to_ordinal(i);
let hash_statements =
hash_statements_for_enum_fields(i, fields.unnamed.iter());
quote_spanned! {v.span() =>
Self::#variant_id( #(#bindings),* ) => {
::jj_lib::content_hash::ContentHash::hash(&#ix, state);
#( ::jj_lib::content_hash::ContentHash::hash(#bindings, state); )*
#(#hash_statements)*
}
}
}
@ -99,13 +99,40 @@ fn index_to_ordinal(ix: usize) -> u32 {
u32::try_from(ix).expect("The number of enum variants overflows a u32.")
}
fn enum_bindings<'a>(fields: impl IntoIterator<Item = &'a Field>) -> Vec<Ident> {
fn enum_bindings_with_type<'a>(fields: impl IntoIterator<Item = &'a Field>) -> Vec<(Type, Ident)> {
fields
.into_iter()
.enumerate()
.map(|(i, f)| {
// If the field is named, use the name, otherwise generate a placeholder name.
f.ident.clone().unwrap_or(format_ident!("field_{}", i))
(
f.ty.clone(),
f.ident.clone().unwrap_or(format_ident!("field_{}", i)),
)
})
.collect::<Vec<_>>()
}
fn enum_bindings<'a>(fields: impl IntoIterator<Item = &'a Field>) -> Vec<Ident> {
enum_bindings_with_type(fields)
.into_iter()
.map(|(_, b)| b)
.collect()
}
fn hash_statements_for_enum_fields<'a>(
index: usize,
fields: impl IntoIterator<Item = &'a Field>,
) -> Vec<TokenStream> {
let ix = index_to_ordinal(index);
let typed_bindings = enum_bindings_with_type(fields);
let mut hash_statements = Vec::with_capacity(typed_bindings.len() + 1);
hash_statements.push(quote! {::jj_lib::content_hash::ContentHash::hash(&#ix, state);});
for (ty, b) in typed_bindings.iter() {
hash_statements.push(quote_spanned! {b.span() =>
<#ty as ::jj_lib::content_hash::ContentHash>::hash(#b, state);
});
}
hash_statements
}