Rollup merge of #85957 - BoxyUwU:rustdoc-const-generic-defaults, r=oli-obk

Display defaults on const params- rustdoc

previously rustdoc would render this struct declaration:
`pub struct Foo<const N: usize = 10>;`
as:
`pub struct Foo<const N: usize>;`
this PR changes it to render correctly

src/librustdoc/clean/auto_trait.rs

@@ -664,7 +664,10 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
                     }
                 }
                 GenericParamDefKind::Lifetime => {}
-                GenericParamDefKind::Const { .. } => {}
+                GenericParamDefKind::Const { ref mut default, .. } => {
+                    // We never want something like `impl<const N: usize = 10>`
+                    default.take();
+                }
             }
         }
 

src/librustdoc/clean/mod.rs

@@ -445,11 +445,15 @@ impl Clean<GenericParamDef> for ty::GenericParamDef {
                     },
                 )
             }
-            ty::GenericParamDefKind::Const { .. } => (
+            ty::GenericParamDefKind::Const { has_default, .. } => (
                 self.name,
                 GenericParamDefKind::Const {
                     did: self.def_id,
                     ty: cx.tcx.type_of(self.def_id).clean(cx),
+                    default: match has_default {
+                        true => Some(cx.tcx.const_param_default(self.def_id).to_string()),
+                        false => None,
+                    },
                 },
             ),
         };
@@ -487,12 +491,15 @@ impl Clean<GenericParamDef> for hir::GenericParam<'_> {
                     synthetic,
                 },
             ),
-            hir::GenericParamKind::Const { ref ty, default: _ } => (
+            hir::GenericParamKind::Const { ref ty, default } => (
                 self.name.ident().name,
                 GenericParamDefKind::Const {
                     did: cx.tcx.hir().local_def_id(self.hir_id).to_def_id(),
                     ty: ty.clean(cx),
-                    // FIXME(const_generics_defaults): add `default` field here for docs
+                    default: default.map(|ct| {
+                        let def_id = cx.tcx.hir().local_def_id(ct.hir_id);
+                        ty::Const::from_anon_const(cx.tcx, def_id).to_string()
+                    }),
                 },
             ),
         };

src/librustdoc/clean/types.rs

@@ -1220,6 +1220,7 @@ crate enum GenericParamDefKind {
     Const {
         did: DefId,
         ty: Type,
+        default: Option<String>,
     },
 }
 

src/librustdoc/html/format.rs

@@ -177,12 +177,22 @@ impl clean::GenericParamDef {
 
                 Ok(())
             }
-            clean::GenericParamDefKind::Const { ref ty, .. } => {
+            clean::GenericParamDefKind::Const { ref ty, ref default, .. } => {
                 if f.alternate() {
-                    write!(f, "const {}: {:#}", self.name, ty.print(cx))
+                    write!(f, "const {}: {:#}", self.name, ty.print(cx))?;
                 } else {
-                    write!(f, "const {}: {}", self.name, ty.print(cx))
+                    write!(f, "const {}: {}", self.name, ty.print(cx))?;
                 }
+
+                if let Some(default) = default {
+                    if f.alternate() {
+                        write!(f, " = {:#}", default)?;
+                    } else {
+                        write!(f, " = {}", default)?;
+                    }
+                }
+
+                Ok(())
             }
         })
     }

src/librustdoc/json/conversions.rs

@@ -317,7 +317,9 @@ impl FromWithTcx<clean::GenericParamDefKind> for GenericParamDefKind {
                 bounds: bounds.into_iter().map(|x| x.into_tcx(tcx)).collect(),
                 default: default.map(|x| x.into_tcx(tcx)),
             },
-            Const { did: _, ty } => GenericParamDefKind::Const(ty.into_tcx(tcx)),
+            Const { did: _, ty, default } => {
+                GenericParamDefKind::Const { ty: ty.into_tcx(tcx), default }
+            }
         }
     }
 }

src/librustdoc/json/mod.rs

@@ -234,7 +234,7 @@ impl<'tcx> FormatRenderer<'tcx> for JsonRenderer<'tcx> {
                     )
                 })
                 .collect(),
-            format_version: 5,
+            format_version: 6,
         };
         let mut p = self.out_path.clone();
         p.push(output.index.get(&output.root).unwrap().name.clone().unwrap());

src/rustdoc-json-types/lib.rs

@@ -324,7 +324,7 @@ pub struct GenericParamDef {
 pub enum GenericParamDefKind {
     Lifetime,
     Type { bounds: Vec<GenericBound>, default: Option<Type> },
-    Const(Type),
+    Const { ty: Type, default: Option<String> },
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]

src/test/rustdoc/const-generics/const-generic-defaults.rs

@@ -0,0 +1,6 @@
+#![crate_name = "foo"]
+#![feature(const_generics_defaults)]
+
+// @has foo/struct.Foo.html '//pre[@class="rust struct"]' \
+//      'pub struct Foo<const M: usize = 10_usize, const N: usize = M, T = i32>(_);'
+pub struct Foo<const M: usize = 10, const N: usize = M, T = i32>(T);