Commit fe98ddcfcfb6f185dbf4adeaf439d8a756da0273 | rust

author	bors <bors@rust-lang.org> 2026-01-17 16:15:18 UTC
committer	bors <bors@rust-lang.org> 2026-01-17 16:15:18 UTC
log	fe98ddcfcfb6f185dbf4adeaf439d8a756da0273
tree	f18aad59d84cb99132bb0545af5ac2e8b3b23e6f
parent	9f6cd6defbd7ef13f6777aa8e43b14d69f0a830a
parent	ac8e8505b7c12d826f970e8e8ec8c18b067c6dc8

Auto merge of #151232 - fmease:rustdoc-dont-eval-assoc-consts, r=yotamofek

rustdoc: Stop unconditionally evaluating the initializer of associated consts

See the descriptions of the added tests for details.

Fixes rust-lang/rust#131625.
Fixes [after beta-1.93 backport] rust-lang/rust#149635.
Fixes rust-lang/rust#150312.

Supersedes rust-lang/rust#150629 IINM.

CC @cuviper (https://github.com/rust-lang/rust/issues/149635#issuecomment-3761125727)
r? @GuillaumeGomez or @yotamofek (rust-lang/rust#150629)

9 files changed, 75 insertions(+), 14 deletions(-)

src/librustdoc/html/render/mod.rs+3-6

...	...	@@ -1050,14 +1050,11 @@ fn assoc_const(
1050	1050	ty = print_type(ty, cx),
1051	1051	)?;
1052	1052	if let AssocConstValue::TraitDefault(konst) \| AssocConstValue::Impl(konst) = value {
1053		// FIXME: `.value()` uses `clean::utils::format_integer_with_underscore_sep` under the
1054		// hood which adds noisy underscores and a type suffix to number literals.
1055		// This hurts readability in this context especially when more complex expressions
1056		// are involved and it doesn't add much of value.
1057		// Find a way to print constants here without all that jazz.
1058		let repr = konst.value(tcx).unwrap_or_else(\|\| konst.expr(tcx));
	1053	let repr = konst.expr(tcx);
1059	1054	if match value {
1060	1055	AssocConstValue::TraitDefault(_) => true, // always show
	1056	// FIXME: Comparing against the special string "_" denoting overly complex const exprs
	1057	// is rather hacky; `ConstKind::expr` should have a richer return type.
1061	1058	AssocConstValue::Impl(_) => repr != "_", // show if there is a meaningful value to show
1062	1059	AssocConstValue::None => unreachable!(),
1063	1060	} {

tests/rustdoc-html/anchors/anchors.no_const_anchor2.html+1-1

...	...	@@ -1 +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>
		\ No newline at end of file
	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> = 0</h4></section>
		\ No newline at end of file

tests/rustdoc-html/attributes.rs+1-1

...	...	@@ -59,7 +59,7 @@ pub enum Enum {
59	59	pub trait Trait {
60	60	//@ has 'foo/trait.Trait.html'
61	61	//@ has - '//[@id="associatedconstant.BAR"]/[@class="code-header"]/*[@class="code-attribute"]' '#[unsafe(link_section = "bar")]'
62		//@ has - '//[@id="associatedconstant.BAR"]/[@class="code-header"]' 'const BAR: u32 = 0u32'
	62	//@ has - '//[@id="associatedconstant.BAR"]/[@class="code-header"]' 'const BAR: u32 = 0'
63	63	#[unsafe(link_section = "bar")]
64	64	const BAR: u32 = 0;
65	65

tests/rustdoc-html/constant/assoc-const-has-projection-ty.rs created+31

...	...	@@ -0,0 +1,31 @@
	1	// Ensure that we properly print the value `1` as `1` in the initializer of associated constants
	2	// that have user type "projection".
	3	//
	4	// We once used to evaluate the initializer in rustdoc and use rustc's MIR pretty-printer to
	5	// render the resulting MIR const value. This pretty printer matches on the type to interpret
	6	// the data and falls back to a cryptic `"{transmute(0x$data): $ty}"` for types it can't handle.
	7	// Crucially, when constructing the MIR const we passed the unnormalized type of the initializer,
	8	// i.e., the projection `<Struct as Trait>::Ty` instead of the normalized `u32` which the
	9	// pretty printer obviously can't handle.
	10	//
	11	// Now we no longer evaluate it and use a custom printer for the const expr.
	12	//
	13	// issue: <https://github.com/rust-lang/rust/issues/150312>
	14
	15	#![crate_name = "it"]
	16
	17	pub trait Trait {
	18	type Ty;
	19
	20	const CT: Self::Ty;
	21	}
	22
	23	pub struct Struct;
	24
	25	impl Trait for Struct {
	26	type Ty = u32;
	27
	28	//@ has it/struct.Struct.html
	29	//@ has - '//*[@id="associatedconstant.CT"]' 'const CT: Self::Ty = 1'
	30	const CT: Self::Ty = 1;
	31	}

tests/rustdoc-html/constant/assoc-consts.rs+1-1

...	...	@@ -1,6 +1,6 @@
1	1	pub trait Foo {
2	2	//@ has assoc_consts/trait.Foo.html '//pre[@class="rust item-decl"]' \
3		// 'const FOO: usize = 13usize;'
	3	// 'const FOO: usize = _;'
4	4	//@ has - '//*[@id="associatedconstant.FOO"]' 'const FOO: usize'
5	5	const FOO: usize = 12 + 1;
6	6	//@ has - '//*[@id="associatedconstant.FOO_NO_DEFAULT"]' 'const FOO_NO_DEFAULT: bool'

tests/rustdoc-html/deref/deref-to-primitive.rs+1-1

...	...	@@ -3,7 +3,7 @@
3	3	//@ has 'foo/struct.Foo.html'
4	4	//@ has - '//*[@id="deref-methods-i32"]' 'Methods from Deref<Target = i32>'
5	5	//@ has - '//[@id="deref-methods-i32-1"]//[@id="associatedconstant.BITS"]/h4' \
6		// 'pub const BITS: u32 = 32u32'
	6	// 'pub const BITS: u32 = u32::BITS'
7	7	pub struct Foo(i32);
8	8
9	9	impl std::ops::Deref for Foo {

tests/rustdoc-html/display-hidden-items.rs+2-2

...	...	@@ -20,7 +20,7 @@ pub trait TraitHidden {}
20	20	//@ has 'foo/index.html' '//dt/a[@class="trait"]' 'Trait'
21	21	pub trait Trait {
22	22	//@ has 'foo/trait.Trait.html'
23		//@ has - '//[@id="associatedconstant.BAR"]/[@class="code-header"]' '#[doc(hidden)] const BAR: u32 = 0u32'
	23	//@ has - '//[@id="associatedconstant.BAR"]/[@class="code-header"]' '#[doc(hidden)] const BAR: u32 = 0'
24	24	#[doc(hidden)]
25	25	const BAR: u32 = 0;
26	26
...	...	@@ -44,7 +44,7 @@ impl Struct {
44	44	}
45	45
46	46	impl Trait for Struct {
47		//@ has - '//[@id="associatedconstant.BAR"]/[@class="code-header"]' '#[doc(hidden)] const BAR: u32 = 0u32'
	47	//@ has - '//[@id="associatedconstant.BAR"]/[@class="code-header"]' '#[doc(hidden)] const BAR: u32 = 0'
48	48	//@ has - '//[@id="method.foo"]/[@class="code-header"]' '#[doc(hidden)] fn foo()'
49	49	}
50	50	//@ has - '//[@id="impl-TraitHidden-for-Struct"]/[@class="code-header"]' 'impl TraitHidden for Struct'

tests/rustdoc-html/impl/impl-associated-items-order.rs+2-2

...	...	@@ -16,7 +16,7 @@ impl Bar {
16	16	// 'pub fn foo()'
17	17	pub fn foo() {}
18	18	//@ has - '//[@id="implementations-list"]//[@class="impl-items"]/section[1]/h4' \
19		// 'pub const X: u8 = 12u8'
	19	// 'pub const X: u8 = 12'
20	20	pub const X: u8 = 12;
21	21	//@ has - '//[@id="implementations-list"]//[@class="impl-items"]/section[2]/h4' \
22	22	// 'pub type Y = u8'
...	...	@@ -34,7 +34,7 @@ impl Foo for Bar {
34	34	// 'type Z = u8'
35	35	type Z = u8;
36	36	//@ has - '//[@id="trait-implementations-list"]//[@class="impl-items"]/section[1]/h4' \
37		// 'const W: u32 = 12u32'
	37	// 'const W: u32 = 12'
38	38	const W: u32 = 12;
39	39	//@ has - '//[@id="trait-implementations-list"]//[@class="impl-items"]/section[3]/h4' \
40	40	// 'fn yeay()'

tests/rustdoc-ui/diverging-assoc-consts.rs created+33

...	...	@@ -0,0 +1,33 @@
	1	// Ensure that we don't unconditionally evaluate the initializer of associated constants.
	2	//
	3	// We once used to evaluate them so we could display more kinds of expressions
	4	// (like `1 + 1` as `2`) given the fact that we generally only want to render
	5	// literals (otherwise we would risk dumping extremely large exprs or leaking
	6	// private struct fields).
	7	//
	8	// However, that deviated from rustc's behavior, made rustdoc accept less code
	9	// and was understandably surprising to users. So let's not.
	10	//
	11	// In the future we might provide users a mechanism to control this behavior.
	12	// E.g., via a new `#[doc(...)]` attribute.
	13	//
	14	// See also:
	15	// issue: <https://github.com/rust-lang/rust/issues/131625>
	16	// issue: <https://github.com/rust-lang/rust/issues/149635>
	17
	18	//@ check-pass
	19
	20	pub struct Type;
	21
	22	impl Type {
	23	pub const K0: () = panic!();
	24	pub const K1: std::convert::Infallible = loop {};
	25	}
	26
	27	pub trait Trait {
	28	const K2: i32 = panic!();
	29	}
	30
	31	impl Trait for Type {
	32	const K2: i32 = loop {};
	33	}