Make impl Debug for rustdoc::clean::Item easier to read

Before:

```
Item { name: Some("Send"), attrs: Attributes { doc_strings: [DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:15:1: 15:60 (#0), parent_module: None, doc: " Types that can be transferred across thread boundaries.", kind: SugaredDoc, indent: 1 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:16:1: 16:4 (#0), parent_module: None, doc: "", kind: SugaredDoc, indent: 0 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:17:1: 17:78 (#0), parent_module: None, doc: " This trait is automatically implemented when the compiler determines it's", kind: SugaredDoc, indent: 1 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:18:1: 18:17 (#0), parent_module: None, doc: " appropriate.", kind: SugaredDoc, indent: 1 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:19:1: 19:4 (#0), parent_module: None, doc: "", kind: SugaredDoc, indent: 0 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:20:1: 20:70 (#0), parent_module: None, doc: " An example of a non-`Send` type is the reference-counting pointer", kind: SugaredDoc, indent: 1 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:21:1: 21:85 (#0), parent_module: None, doc: " [`rc::Rc`][`Rc`]. If two threads attempt to clone [`Rc`]s that point to the same", kind: SugaredDoc, indent: 1 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:22:1: 22:81 (#0), parent_module: None, doc: " reference-counted value, they might try to update the reference count at the", kind: SugaredDoc, indent: 1 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:23:1: 23:83 (#0), parent_module: None, doc: " same time, which is [undefined behavior][ub] because [`Rc`] doesn't use atomic", kind: SugaredDoc, indent: 1 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:24:1: 24:84 (#0), parent_module: None, doc: " operations. Its cousin [`sync::Arc`][arc] does use atomic operations (incurring", kind: SugaredDoc, indent: 1 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:25:1: 25:39 (#0), parent_module: None, doc: " some overhead) and thus is `Send`.", kind: SugaredDoc, indent: 1 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:26:1: 26:4 (#0), parent_module: None, doc: "", kind: SugaredDoc, indent: 0 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:27:1: 27:74 (#0), parent_module: None, doc: " See [the Nomicon](../../nomicon/send-and-sync.html) for more details.", kind: SugaredDoc, indent: 1 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:28:1: 28:4 (#0), parent_module: None, doc: "", kind: SugaredDoc, indent: 0 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:29:1: 29:40 (#0), parent_module: None, doc: " [`Rc`]: ../../std/rc/struct.Rc.html", kind: SugaredDoc, indent: 1 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:30:1: 30:42 (#0), parent_module: None, doc: " [arc]: ../../std/sync/struct.Arc.html", kind: SugaredDoc, indent: 1 }, DocFragment { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:31:1: 31:61 (#0), parent_module: None, doc: " [ub]: ../../reference/behavior-considered-undefined.html", kind: SugaredDoc, indent: 1 }], other_attrs: [Attribute { kind: Normal(AttrItem { path: Path { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:32:3: 32:9 (#0), segments: [PathSegment { ident: stable#0, id: NodeId(33577), args: None }], tokens: None }, args: Delimited(DelimSpan { open: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:32:9: 32:10 (#0), close: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:32:44: 32:45 (#0) }, Parenthesis, TokenStream([(Token(Token { kind: Ident("feature", false), span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:32:10: 32:17 (#0) }), Alone), (Token(Token { kind: Eq, span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:32:18: 32:19 (#0) }), Alone), (Token(Token { kind: Literal(Lit { kind: Str, symbol: "rust1", suffix: None }), span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:32:20: 32:27 (#0) }), Joint), (Token(Token { kind: Comma, span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:32:27: 32:28 (#0) }), Alone), (Token(Token { kind: Ident("since", false), span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:32:29: 32:34 (#0) }), Alone), (Token(Token { kind: Eq, span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:32:35: 32:36 (#0) }), Alone), (Token(Token { kind: Literal(Lit { kind: Str, symbol: "1.0.0", suffix: None }), span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:32:37: 32:44 (#0) }), Alone)])), tokens: None }, None), id: AttrId(48), style: Outer, span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:32:1: 32:46 (#0) }, Attribute { kind: Normal(AttrItem { path: Path { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:33:23: 33:44 (#0), segments: [PathSegment { ident: rustc_diagnostic_item#0, id: NodeId(33578), args: None }], tokens: None }, args: Eq(/rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:33:45: 33:46 (#0), Token { kind: Literal(Lit { kind: Str, symbol: "Send", suffix: None }), span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:33:47: 33:53 (#0) }), tokens: None }, None), id: AttrId(49), style: Outer, span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:33:23: 33:53 (#0) }, Attribute { kind: Normal(AttrItem { path: Path { span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:34:3: 34:25 (#0), segments: [PathSegment { ident: rustc_on_unimplemented#0, id: NodeId(33580), args: None }], tokens: None }, args: Delimited(DelimSpan { open: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:34:25: 34:26 (#0), close: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:37:1: 37:2 (#0) }, Parenthesis, TokenStream([(Token(Token { kind: Ident("message", false), span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:35:5: 35:12 (#0) }), Alone), (Token(Token { kind: Eq, span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:35:13: 35:14 (#0) }), Alone), (Token(Token { kind: Literal(Lit { kind: Str, symbol: "`{Self}` cannot be sent between threads safely", suffix: None }), span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:35:15: 35:63 (#0) }), Joint), (Token(Token { kind: Comma, span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:35:63: 35:64 (#0) }), Alone), (Token(Token { kind: Ident("label", false), span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:36:5: 36:10 (#0) }), Alone), (Token(Token { kind: Eq, span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:36:11: 36:12 (#0) }), Alone), (Token(Token { kind: Literal(Lit { kind: Str, symbol: "`{Self}` cannot be sent between threads safely", suffix: None }), span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:36:13: 36:61 (#0) }), Alone)])), tokens: None }, None), id: AttrId(50), style: Outer, span: /rustc/37b55c8a0cafdb60b9168da34f904acc70157df8/library/core/src/marker.rs:34:1: 37:3 (#0) }] }, visibility: Public, kind: TraitItem(Trait { unsafety: Unsafe, items: [], generics: Generics { params: [], where_predicates: [] }, bounds: [], is_auto: true }), def_id: DefId(DefId(2:3027 ~ core[7f4a]::marker::Send)), cfg: None }
```

After:

```
Item { name: Some("Send"), visibility: Public, def_id: DefId(DefId(2:3027 ~ core[7f4a]::marker::Send)), kind: Trait, docs: "Types that can be transferred across thread boundaries.\n\nThis trait is automatically implemented when the compiler determines it's\nappropriate.\n\nAn example of a non-`Send` type is the reference-counting pointer\n[`rc::Rc`][`Rc`]. If two threads attempt to clone [`Rc`]s that point to the same\nreference-counted value, they might try to update the reference count at the\nsame time, which is [undefined behavior][ub] because [`Rc`] doesn't use atomic\noperations. Its cousin [`sync::Arc`][arc] does use atomic operations (incurring\nsome overhead) and thus is `Send`.\n\nSee [the Nomicon](../../nomicon/send-and-sync.html) for more details.\n\n[`Rc`]: ../../std/rc/struct.Rc.html\n[arc]: ../../std/sync/struct.Arc.html\n[ub]: ../../reference/behavior-considered-undefined.html" }
```

Author: jyn514

src/librustdoc/clean/types.rs

@@ -1,3 +1,4 @@
+use core::fmt;
 use std::cell::RefCell;
 use std::default::Default;
 use std::hash::Hash;
@@ -351,7 +352,7 @@ crate enum ExternalLocation {
 /// Anything with a source location and set of attributes and, optionally, a
 /// name. That is, anything that can be documented. This doesn't correspond
 /// directly to the AST's concept of an item; it's a strict superset.
-#[derive(Clone, Debug)]
+#[derive(Clone)]
 crate struct Item {
     /// The name of this item.
     /// Optional because not every item has a name, e.g. impls.
@@ -366,6 +367,27 @@ crate struct Item {
     crate cfg: Option<Arc<Cfg>>,
 }
 
+impl fmt::Debug for Item {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let alternate = f.alternate();
+        // hand-picked fields that don't bloat the logs too much
+        let mut fmt = f.debug_struct("Item");
+        fmt.field("name", &self.name)
+            .field("visibility", &self.visibility)
+            .field("def_id", &self.def_id);
+        // allow printing the full item if someone really wants to
+        if alternate {
+            fmt.field("attrs", &self.attrs).field("kind", &self.kind).field("cfg", &self.cfg);
+        } else {
+            fmt.field("kind", &self.type_());
+            if let Some(docs) = self.doc_value() {
+                fmt.field("docs", &docs);
+            }
+        }
+        fmt.finish()
+    }
+}
+
 // `Item` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 rustc_data_structures::static_assert_size!(Item, 56);