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Implement Debug for C-like enums with a concatenated string #155452

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efa134c
Implement `Debug` for C-like enums with a concatenated string
makai410 Apr 17, 2026
aa988cb
Add `debug_c_like_enum_write_str` to avoid duplicating similar code
makai410 Apr 18, 2026
d9870e7
39 is my favorite number
makai410 Jun 9, 2026
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135 changes: 128 additions & 7 deletions compiler/rustc_builtin_macros/src/deriving/debug.rs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
use rustc_ast::{self as ast, EnumDef, MetaItem, Safety};
use rustc_ast::{self as ast, EnumDef, ExprKind, MetaItem, Safety, TyKind, token};
use rustc_expand::base::{Annotatable, ExtCtxt};
use rustc_session::config::FmtDebug;
use rustc_span::{Ident, Span, Symbol, sym};
Expand Down Expand Up @@ -166,15 +166,13 @@ fn show_substructure(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) ->
let path_debug = cx.path_global(span, cx.std_path(&[sym::fmt, sym::Debug]));
let ty_dyn_debug = cx.ty(
span,
ast::TyKind::TraitObject(
TyKind::TraitObject(
vec![cx.trait_bound(path_debug, false)],
ast::TraitObjectSyntax::Dyn,
),
);
let ty_slice = cx.ty(
span,
ast::TyKind::Slice(cx.ty_ref(span, ty_dyn_debug, None, ast::Mutability::Not)),
);
let ty_slice =
cx.ty(span, TyKind::Slice(cx.ty_ref(span, ty_dyn_debug, None, ast::Mutability::Not)));
let values_let = cx.stmt_let_ty(
span,
false,
Expand Down Expand Up @@ -230,6 +228,10 @@ fn show_fieldless_enum(
substr: &Substructure<'_>,
) -> BlockOrExpr {
let fmt = substr.nonselflike_args[0].clone();
if let Some((stmts, expr)) = show_fieldless_enum_concat_str(cx, span, def, fmt.clone()) {
return BlockOrExpr::new_mixed(stmts, Some(expr));
}
let fn_path_write_str = cx.std_path(&[sym::fmt, sym::Formatter, sym::write_str]);
let arms = def
.variants
.iter()
Expand All @@ -250,6 +252,125 @@ fn show_fieldless_enum(
})
.collect::<ThinVec<_>>();
let name = cx.expr_match(span, cx.expr_self(span), arms);
let fn_path_write_str = cx.std_path(&[sym::fmt, sym::Formatter, sym::write_str]);
BlockOrExpr::new_expr(cx.expr_call_global(span, fn_path_write_str, thin_vec![fmt, name]))
}

/// Special case for fieldless enums with no discriminants. Builds
/// ```text
/// impl ::core::fmt::Debug for A {
/// fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
/// const __NAMES: &str = "ABBBCC";
/// const __OFFSET: [usize; 4] =[0, 1, 4, 6];
/// let __d = ::core::intrinsics::discriminant_value(self) as usize;
/// ::core::fmt::Formatter::debug_c_like_enums_write_str(f, __NAMES, &__OFFSET, __d)
/// }
/// }
/// ```
fn show_fieldless_enum_concat_str(
cx: &ExtCtxt<'_>,
span: Span,
def: &EnumDef,
fmt: Box<ast::Expr>,
) -> Option<(ThinVec<ast::Stmt>, Box<ast::Expr>)> {
let variant_count = def.variants.len();
if variant_count < 39 {
return None;
}

let variant_names = def
.variants
.iter()
.map(|v| v.disr_expr.is_none().then_some(v.ident.name.as_str()))
.collect::<Option<ThinVec<_>>>()?;
Comment on lines +280 to +284

@makai410 makai410 Apr 18, 2026 •
edited by rustbot
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Hmmm, I think I missed a valid case, considering:

enum Uwu {
    QwQ = 0,
    AwA = 1,
}

which has explicit discriminants but is actually dense.

View changes since the review

@makai410 makai410 Apr 18, 2026

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Also, we can use an offset when some variants have negative discriminants while the overall variants remain dense.

I'll do a follow-up PR to implement these.


let total_bytes: usize = variant_names.iter().map(|n| n.len()).sum();
let mut concatenated_names = String::with_capacity(total_bytes);
let mut offset_indices = Vec::with_capacity(variant_names.len() + 1);
offset_indices.push(0);

for name in variant_names.iter() {
concatenated_names.push_str(name);
offset_indices.push(concatenated_names.len());
}

// Create the constant concatenated string
let names_ident = Ident::from_str_and_span("__NAMES", span);
let str_ty = cx.ty(
span,
TyKind::Ref(
None,
ast::MutTy {
ty: cx.ty(
span,
TyKind::Path(None, ast::Path::from_ident(Ident::new(sym::str, span))),
),
mutbl: ast::Mutability::Not,
},
),
);
let names_str_expr =
ast::ConstItemRhsKind::new_body(cx.expr_str(span, Symbol::intern(&concatenated_names)));
let names_const_item = cx.item_const(span, names_ident, str_ty, names_str_expr);

// Create the constant offset array
let offset_ident = Ident::from_str_and_span("__OFFSET", span);
let offset_index_exprs =
offset_indices.iter().map(|s| cx.expr_usize(span, *s)).collect::<ThinVec<_>>();
let starts_array_expr =
ast::ConstItemRhsKind::new_body(cx.expr_array(span, offset_index_exprs));
let usize_ty =
cx.ty(span, TyKind::Path(None, ast::Path::from_ident(Ident::new(sym::usize, span))));
let offset_array_len_expr = cx.anon_const(
span,
ExprKind::Lit(token::Lit::new(
token::LitKind::Integer,
Symbol::intern(&(variant_count + 1).to_string()),
None,
)),
);
let offset_const_item = cx.item_const(
span,
offset_ident,
cx.ty(span, TyKind::Array(usize_ty, offset_array_len_expr)),
starts_array_expr,
);

// let __d = ::core::intrinsics::discriminant_value(self) as usize;
let discriminant_ident = Ident::from_str_and_span("__d", span);
let discriminant_intrinsic_path = cx.std_path(&[sym::intrinsics, sym::discriminant_value]);
let discriminant_cast_expr = cx.expr(
span,
ast::ExprKind::Cast(
cx.expr_call_global(span, discriminant_intrinsic_path, thin_vec![cx.expr_self(span)]),
cx.ty_path(ast::Path::from_ident(Ident::new(sym::usize, span))),
),
);
let discriminant_let_stmt =
cx.stmt_let(span, false, discriminant_ident, discriminant_cast_expr);

// __d expression
let discriminant_expr = cx.expr_ident(span, discriminant_ident);

// __NAMES expression
let names_expr = cx.expr_ident(span, names_ident);

// &__OFFSET expression
let offset_ref_expr = cx.expr_addr_of(span, cx.expr_ident(span, offset_ident));

// ::core::fmt::Formatter::debug_c_like_enum_write_str(f, __NAMES, &__OFFSET, __d)
let fn_path = cx.std_path(&[sym::fmt, sym::Formatter, sym::debug_c_like_enum_write_str]);
let call_expr = cx.expr_call_global(
span,
fn_path,
thin_vec![fmt, names_expr, offset_ref_expr, discriminant_expr],
);

Some((
thin_vec![
cx.stmt_item(span, names_const_item),
cx.stmt_item(span, offset_const_item),
discriminant_let_stmt,
],
call_expr,
))
}
1 change: 1 addition & 0 deletions compiler/rustc_span/src/symbol.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -767,6 +767,7 @@ symbols! {
debug_assert_macro,
debug_assert_ne_macro,
debug_assertions,
debug_c_like_enum_write_str,
debug_struct_fields_finish,
debug_tuple_fields_finish,
debugger_visualizer,
Expand Down
15 changes: 15 additions & 0 deletions library/core/src/fmt/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2577,6 +2577,21 @@ impl<'a> Formatter<'a> {
builder.finish()
}

/// Shrinks `derive(Debug)` code, for faster compilation and smaller binaries.
/// For C-like enums with concatenated variant name strings.
#[doc(hidden)]
#[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
pub fn debug_c_like_enum_write_str<'b>(
&'b mut self,
names: &str,
offset: &[usize],
discr: usize,
) -> Result {
let start = offset[discr];
let end = offset[discr + 1];
self.write_str(&names[start..end])
}

/// Creates a `DebugTuple` builder designed to assist with creation of
/// `fmt::Debug` implementations for tuple structs.
///
Expand Down
53 changes: 53 additions & 0 deletions tests/ui/derives/deriving-all-codegen.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -154,6 +154,59 @@ enum Fieldless {
C,
}

// A C-like, fieldless enum with variants of varying name lengths.
#[derive(Debug)]
enum Fieldless0 {
A,
BBB,
CC,
}


// A C-like, fieldless enum with 39 variants.
#[derive(Debug)]
enum Fieldless39 {
AAAAA,
BBBB,
CC,
DDDDDDDD,
E,
FFFFFFFFFFFFF,
GGGGGG,
Hatsune,
IIIIIII,
JJJJJJJJJ,
KKK,
LLLLLL,
Miku,
N,
OO,
P,
Q,
R,
SSSSSS,
TTT,
U,
VV,
W,
X,
Y,
ZZZZZZZZZZZZZZZZZ,
IDONT,
KNOW,
WHAT,
TO,
SAY,
HERE,
SO,
JUST,
SOME,
RANDOM,
STUFF,
GUGUGAGA,
LOL,
}

// An enum with multiple fieldless and fielded variants.
#[derive(Clone, Copy, Debug, Default, Hash, PartialEq, Eq, PartialOrd, Ord)]
enum Mixed {
Expand Down
78 changes: 78 additions & 0 deletions tests/ui/derives/deriving-all-codegen.stdout
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Original file line number Diff line number Diff line change
Expand Up @@ -1239,6 +1239,84 @@ impl ::core::cmp::Ord for Fieldless {
}
}

// A C-like, fieldless enum with variants of varying name lengths.
enum Fieldless0 { A, BBB, CC, }
#[automatically_derived]
impl ::core::fmt::Debug for Fieldless0 {
#[inline]
fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
::core::fmt::Formatter::write_str(f,
match self {
Fieldless0::A => "A",
Fieldless0::BBB => "BBB",
Fieldless0::CC => "CC",
})
}
}


// A C-like, fieldless enum with 39 variants.
enum Fieldless39 {
AAAAA,
BBBB,
CC,
DDDDDDDD,
E,
FFFFFFFFFFFFF,
GGGGGG,
Hatsune,
IIIIIII,
JJJJJJJJJ,
KKK,
LLLLLL,
Miku,
N,
OO,
P,
Q,
R,
SSSSSS,
TTT,
U,
VV,
W,
X,
Y,
ZZZZZZZZZZZZZZZZZ,
IDONT,
KNOW,
WHAT,
TO,
SAY,
HERE,
SO,
JUST,
SOME,
RANDOM,
STUFF,
GUGUGAGA,
LOL,
}
#[automatically_derived]
impl ::core::fmt::Debug for Fieldless39 {
#[inline]
fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
const __NAMES: &str =
"AAAAABBBBCCDDDDDDDDEFFFFFFFFFFFFFGGGGGGHatsuneIIIIIIIJJJJJJJJJKKKLLLLLLMikuNOOPQRSSSSSSTTTUVVWXYZZZZZZZZZZZZZZZZZIDONTKNOWWHATTOSAYHERESOJUSTSOMERANDOMSTUFFGUGUGAGALOL";
const __OFFSET: [usize; 40] =
[0usize, 5usize, 9usize, 11usize, 19usize, 20usize, 33usize,
39usize, 46usize, 53usize, 62usize, 65usize, 71usize,
75usize, 76usize, 78usize, 79usize, 80usize, 81usize,
87usize, 90usize, 91usize, 93usize, 94usize, 95usize,
96usize, 113usize, 118usize, 122usize, 126usize, 128usize,
131usize, 135usize, 137usize, 141usize, 145usize, 151usize,
156usize, 164usize, 167usize];
let __d = ::core::intrinsics::discriminant_value(self) as usize;
::core::fmt::Formatter::debug_c_like_enum_write_str(f, __NAMES,
&__OFFSET, __d)
}
}

// An enum with multiple fieldless and fielded variants.
enum Mixed {

Expand Down
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