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rustdoc: Stop unconditionally evaluating the initializer of associated consts

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This commit is contained in:
León Orell Valerian Liehr 2026-01-16 21:08:01 +01:00
parent b08c11df4b
commit ac8e8505b7
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GPG key ID: D17A07215F68E713
9 changed files with 75 additions and 14 deletions
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9
src/librustdoc/html/render/mod.rs
View file

@ -1050,14 +1050,11 @@ fn assoc_const(
ty = print_type(ty, cx),
)?;
if let AssocConstValue::TraitDefault(konst) | AssocConstValue::Impl(konst) = value {
// FIXME: `.value()` uses `clean::utils::format_integer_with_underscore_sep` under the
// hood which adds noisy underscores and a type suffix to number literals.
// This hurts readability in this context especially when more complex expressions
// are involved and it doesn't add much of value.
// Find a way to print constants here without all that jazz.
let repr = konst.value(tcx).unwrap_or_else(|| konst.expr(tcx));
let repr = konst.expr(tcx);
if match value {
AssocConstValue::TraitDefault(_) => true, // always show
// FIXME: Comparing against the special string "_" denoting overly complex const exprs
// is rather hacky; `ConstKind::expr` should have a richer return type.
AssocConstValue::Impl(_) => repr != "_", // show if there is a meaningful value to show
AssocConstValue::None => unreachable!(),
} {

2
tests/rustdoc-html/anchors/anchors.no_const_anchor2.html
View file

@ -1 +1 @@
<section id="associatedconstant.X" class="associatedconstant"><a class="src rightside" href="../src/foo/anchors.rs.html#42">Source</a><h4 class="code-header">pub const <a href="#associatedconstant.X" class="constant">X</a>: <a class="primitive" href="{{channel}}/std/primitive.i32.html">i32</a> = 0i32</h4></section>
<section id="associatedconstant.X" class="associatedconstant"><a class="src rightside" href="../src/foo/anchors.rs.html#42">Source</a><h4 class="code-header">pub const <a href="#associatedconstant.X" class="constant">X</a>: <a class="primitive" href="{{channel}}/std/primitive.i32.html">i32</a> = 0</h4></section>

2
tests/rustdoc-html/attributes.rs
View file

@ -59,7 +59,7 @@ pub enum Enum {
pub trait Trait {
//@ has 'foo/trait.Trait.html'
//@ has - '//*[@id="associatedconstant.BAR"]/*[@class="code-header"]/*[@class="code-attribute"]' '#[unsafe(link_section = "bar")]'
//@ has - '//*[@id="associatedconstant.BAR"]/*[@class="code-header"]' 'const BAR: u32 = 0u32'
//@ has - '//*[@id="associatedconstant.BAR"]/*[@class="code-header"]' 'const BAR: u32 = 0'
#[unsafe(link_section = "bar")]
const BAR: u32 = 0;

31
tests/rustdoc-html/constant/assoc-const-has-projection-ty.rs Normal file
View file

@ -0,0 +1,31 @@
// Ensure that we properly print the value `1` as `1` in the initializer of associated constants
// that have user type "projection".
//
// We once used to evaluate the initializer in rustdoc and use rustc's MIR pretty-printer to
// render the resulting MIR const value. This pretty printer matches on the type to interpret
// the data and falls back to a cryptic `"{transmute(0x$data): $ty}"` for types it can't handle.
// Crucially, when constructing the MIR const we passed the unnormalized type of the initializer,
// i.e., the projection `<Struct as Trait>::Ty` instead of the normalized `u32` which the
// pretty printer obviously can't handle.
//
// Now we no longer evaluate it and use a custom printer for the const expr.
//
// issue: <https://github.com/rust-lang/rust/issues/150312>
#![crate_name = "it"]
pub trait Trait {
type Ty;
const CT: Self::Ty;
}
pub struct Struct;
impl Trait for Struct {
type Ty = u32;
//@ has it/struct.Struct.html
//@ has - '//*[@id="associatedconstant.CT"]' 'const CT: Self::Ty = 1'
const CT: Self::Ty = 1;
}

2
tests/rustdoc-html/constant/assoc-consts.rs
View file

@ -1,6 +1,6 @@
pub trait Foo {
//@ has assoc_consts/trait.Foo.html '//pre[@class="rust item-decl"]' \
// 'const FOO: usize = 13usize;'
// 'const FOO: usize = _;'
//@ has - '//*[@id="associatedconstant.FOO"]' 'const FOO: usize'
const FOO: usize = 12 + 1;
//@ has - '//*[@id="associatedconstant.FOO_NO_DEFAULT"]' 'const FOO_NO_DEFAULT: bool'

2
tests/rustdoc-html/deref/deref-to-primitive.rs
View file

@ -3,7 +3,7 @@
//@ has 'foo/struct.Foo.html'
//@ has - '//*[@id="deref-methods-i32"]' 'Methods from Deref<Target = i32>'
//@ has - '//*[@id="deref-methods-i32-1"]//*[@id="associatedconstant.BITS"]/h4' \
// 'pub const BITS: u32 = 32u32'
// 'pub const BITS: u32 = u32::BITS'
pub struct Foo(i32);
impl std::ops::Deref for Foo {

4
tests/rustdoc-html/display-hidden-items.rs
View file

@ -20,7 +20,7 @@ pub trait TraitHidden {}
//@ has 'foo/index.html' '//dt/a[@class="trait"]' 'Trait'
pub trait Trait {
//@ has 'foo/trait.Trait.html'
//@ has - '//*[@id="associatedconstant.BAR"]/*[@class="code-header"]' '#[doc(hidden)] const BAR: u32 = 0u32'
//@ has - '//*[@id="associatedconstant.BAR"]/*[@class="code-header"]' '#[doc(hidden)] const BAR: u32 = 0'
#[doc(hidden)]
const BAR: u32 = 0;
@ -44,7 +44,7 @@ impl Struct {
}
impl Trait for Struct {
//@ has - '//*[@id="associatedconstant.BAR"]/*[@class="code-header"]' '#[doc(hidden)] const BAR: u32 = 0u32'
//@ has - '//*[@id="associatedconstant.BAR"]/*[@class="code-header"]' '#[doc(hidden)] const BAR: u32 = 0'
//@ has - '//*[@id="method.foo"]/*[@class="code-header"]' '#[doc(hidden)] fn foo()'
}
//@ has - '//*[@id="impl-TraitHidden-for-Struct"]/*[@class="code-header"]' 'impl TraitHidden for Struct'

4
tests/rustdoc-html/impl/impl-associated-items-order.rs
View file

@ -16,7 +16,7 @@ impl Bar {
// 'pub fn foo()'
pub fn foo() {}
//@ has - '//*[@id="implementations-list"]//*[@class="impl-items"]/section[1]/h4' \
// 'pub const X: u8 = 12u8'
// 'pub const X: u8 = 12'
pub const X: u8 = 12;
//@ has - '//*[@id="implementations-list"]//*[@class="impl-items"]/section[2]/h4' \
// 'pub type Y = u8'
@ -34,7 +34,7 @@ impl Foo for Bar {
// 'type Z = u8'
type Z = u8;
//@ has - '//*[@id="trait-implementations-list"]//*[@class="impl-items"]/section[1]/h4' \
// 'const W: u32 = 12u32'
// 'const W: u32 = 12'
const W: u32 = 12;
//@ has - '//*[@id="trait-implementations-list"]//*[@class="impl-items"]/section[3]/h4' \
// 'fn yeay()'

33
tests/rustdoc-ui/diverging-assoc-consts.rs Normal file
View file

@ -0,0 +1,33 @@
// Ensure that we don't unconditionally evaluate the initializer of associated constants.
//
// We once used to evaluate them so we could display more kinds of expressions
// (like `1 + 1` as `2`) given the fact that we generally only want to render
// literals (otherwise we would risk dumping extremely large exprs or leaking
// private struct fields).
//
// However, that deviated from rustc's behavior, made rustdoc accept less code
// and was understandably surprising to users. So let's not.
//
// In the future we *might* provide users a mechanism to control this behavior.
// E.g., via a new `#[doc(...)]` attribute.
//
// See also:
// issue: <https://github.com/rust-lang/rust/issues/131625>
// issue: <https://github.com/rust-lang/rust/issues/149635>
//@ check-pass
pub struct Type;
impl Type {
pub const K0: () = panic!();
pub const K1: std::convert::Infallible = loop {};
}
pub trait Trait {
const K2: i32 = panic!();
}
impl Trait for Type {
const K2: i32 = loop {};
}