rustdoc: do not use plain summary for trait impls

Fixes #38386.
Fixes #48332.
Fixes #49430.
Fixes #62741.
Fixes #73474.

src/test/rustdoc/trait-impl.rs

@@ -0,0 +1,46 @@
+pub trait Trait {
+    /// Some long docs here.
+    ///
+    /// These docs are long enough that a link will be added to the end.
+    fn a();
+
+    /// These docs contain a [reference link].
+    ///
+    /// [reference link]: https://example.com
+    fn b();
+
+    /// ```
+    /// This code block should not be in the output, but a Read more link should be generated
+    /// ```
+    fn c();
+
+    /// Escaped formatting a\*b\*c\* works
+    fn d();
+}
+
+pub struct Struct;
+
+impl Trait for Struct {
+    // @has trait_impl/struct.Struct.html '//*[@id="method.a"]/../div/p' 'Some long docs'
+    // @!has - '//*[@id="method.a"]/../div/p' 'link will be added'
+    // @has - '//*[@id="method.a"]/../div/p/a' 'Read more'
+    // @has - '//*[@id="method.a"]/../div/p/a/@href' 'trait.Trait.html'
+    fn a() {}
+
+    // @has trait_impl/struct.Struct.html '//*[@id="method.b"]/../div/p' 'These docs contain'
+    // @has - '//*[@id="method.b"]/../div/p/a' 'reference link'
+    // @has - '//*[@id="method.b"]/../div/p/a/@href' 'https://example.com'
+    // @has - '//*[@id="method.b"]/../div/p/a' 'Read more'
+    // @has - '//*[@id="method.b"]/../div/p/a/@href' 'trait.Trait.html'
+    fn b() {}
+
+    // @!has trait_impl/struct.Struct.html '//*[@id="method.c"]/../div/p' 'code block'
+    // @has - '//*[@id="method.c"]/../div/p/a' 'Read more'
+    // @has - '//*[@id="method.c"]/../div/p/a/@href' 'trait.Trait.html'
+    fn c() {}
+
+    // @has trait_impl/struct.Struct.html '//*[@id="method.d"]/../div/p' \
+    //   'Escaped formatting a*b*c* works'
+    // @!has trait_impl/struct.Struct.html '//*[@id="method.d"]/../div/p/em'
+    fn d() {}
+}