rustdoc: Remove redundant Impl.span field

It can be computed on-demand in `Item::span()`.

src/librustdoc/clean/auto_trait.rs

@@ -115,7 +115,6 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
             visibility: Inherited,
             def_id: ItemId::Auto { trait_: trait_def_id, for_: item_def_id },
             kind: box ImplItem(Impl {
-                span: Span::dummy(),
                 unsafety: hir::Unsafety::Normal,
                 generics: new_generics,
                 trait_: Some(trait_ref.clean(self.cx)),

src/librustdoc/clean/blanket_impl.rs

@@ -106,7 +106,6 @@ impl<'a, 'tcx> BlanketImplFinder<'a, 'tcx> {
                     visibility: Inherited,
                     def_id: ItemId::Blanket { impl_id: impl_def_id, for_: item_def_id },
                     kind: box ImplItem(Impl {
-                        span: Span::new(self.cx.tcx.def_span(impl_def_id)),
                         unsafety: hir::Unsafety::Normal,
                         generics: (
                             self.cx.tcx.generics_of(impl_def_id),

src/librustdoc/clean/inline.rs

@@ -490,7 +490,6 @@ crate fn build_impl(
         did,
         None,
         clean::ImplItem(clean::Impl {
-            span: clean::types::rustc_span(did, cx.tcx),
             unsafety: hir::Unsafety::Normal,
             generics,
             trait_,

src/librustdoc/clean/mod.rs

@@ -1889,7 +1889,6 @@ fn clean_impl(impl_: &hir::Impl<'_>, hir_id: hir::HirId, cx: &mut DocContext<'_>
     });
     let mut make_item = |trait_: Option<Path>, for_: Type, items: Vec<Item>| {
         let kind = ImplItem(Impl {
-            span: types::rustc_span(tcx.hir().local_def_id(hir_id).to_def_id(), tcx),
             unsafety: impl_.unsafety,
             generics: impl_.generics.clean(cx),
             trait_,

src/librustdoc/clean/types.rs

@@ -391,12 +391,19 @@ impl Item {
             ItemKind::StrippedItem(k) => k,
             _ => &*self.kind,
         };
-        if let ItemKind::ModuleItem(Module { span, .. }) | ItemKind::ImplItem(Impl { span, .. }) =
+        match kind {
-            kind
+            ItemKind::ModuleItem(Module { span, .. }) => *span,
-        {
+            ItemKind::ImplItem(Impl { synthetic: true, .. }) => Span::dummy(),
-            *span
+            ItemKind::ImplItem(Impl { blanket_impl: Some(_), .. }) => {
-        } else {
+                if let ItemId::Blanket { impl_id, .. } = self.def_id {
-            self.def_id.as_def_id().map(|did| rustc_span(did, tcx)).unwrap_or_else(Span::dummy)
+                    rustc_span(impl_id, tcx)
+                } else {
+                    panic!("blanket impl item has non-blanket ID")
+                }
+            }
+            _ => {
+                self.def_id.as_def_id().map(|did| rustc_span(did, tcx)).unwrap_or_else(Span::dummy)
+            }
         }
     }
 
@@ -2165,7 +2172,6 @@ impl Constant {
 
 #[derive(Clone, Debug)]
 crate struct Impl {
-    crate span: Span,
     crate unsafety: hir::Unsafety,
     crate generics: Generics,
     crate trait_: Option<Path>,

src/librustdoc/json/conversions.rs

@@ -509,7 +509,6 @@ impl FromWithTcx<clean::Impl> for Impl {
             negative_polarity,
             synthetic,
             blanket_impl,
-            span: _span,
         } = impl_;
         // FIXME: should `trait_` be a clean::Path equivalent in JSON?
         let trait_ = trait_.map(|path| {