rustdoc: Stop unconditionally evaluating the initializer of associated consts #151232

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged

rust-bors merged 1 commit into rust-lang:main from fmease:rustdoc-dont-eval-assoc-consts

Jan 17, 2026

src/librustdoc/html/render/mod.rs

-Original file line number
+Diff line change
@@ Expand Up / @@ -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!(),
                 } {
@@ Expand Down @@

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

Original file line number	Diff line number	Diff line change
		@@ -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>

tests/rustdoc-html/attributes.rs

-Original file line number
+Diff line change
@@ Expand Up / @@ -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;
@@ Expand Down @@

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

-Original file line number
+Diff line change
@@ -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;
+    }

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

-Original file line number
+Diff line change
@@ -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'
@@ Expand Down @@

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

-Original file line number
+Diff line change
@@ Expand Up / @@ -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 {
@@ Expand Down @@

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

            
                      Original file line number
                      Diff line number
                      Diff line change
                  
    @@ -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'

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

            
                      Original file line number
                      Diff line number
                      Diff line change
                  
    @@ -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()'

tests/rustdoc-ui/diverging-assoc-consts.rs

-Original file line number
+Diff line change
@@ -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 {};
+    }

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

rustdoc: Stop unconditionally evaluating the initializer of associated consts #151232

Uh oh!

Diff view

Diff view

There are no files selected for viewing

Uh oh!

Uh oh!