Add test for rustdoc showing underscore as assoc const value

Author: dtolnay

tests/rustdoc/assoc-consts-underscore.rs

@@ -0,0 +1,27 @@
+pub struct Struct {
+    _private: (),
+}
+
+pub trait Trait {
+    //@ has assoc_consts_underscore/trait.Trait.html '//pre[@class="rust item-decl"]' \
+    //      'const REQUIRED: Struct;'
+    //@ !has - '//*[@id="associatedconstant.REQUIRED"]' 'const REQUIRED: Struct = _'
+    //@ has - '//*[@id="associatedconstant.REQUIRED"]' 'const REQUIRED: Struct'
+    const REQUIRED: Struct;
+    //@ has - '//pre[@class="rust item-decl"]' 'const OPTIONAL: Struct = _;'
+    //@ has - '//*[@id="associatedconstant.OPTIONAL"]' 'const OPTIONAL: Struct = _'
+    const OPTIONAL: Struct = Struct { _private: () };
+}
+
+impl Trait for Struct {
+    //@ has assoc_consts_underscore/struct.Struct.html '//*[@id="associatedconstant.REQUIRED"]' \
+    //      'const REQUIRED: Struct = _'
+    const REQUIRED: Struct = Struct { _private: () };
+    //@ has - '//*[@id="associatedconstant.OPTIONAL"]' 'const OPTIONAL: Struct = _'
+    const OPTIONAL: Struct = Struct { _private: () };
+}
+
+impl Struct {
+    //@ has - '//*[@id="associatedconstant.INHERENT"]' 'const INHERENT: Struct = _'
+    pub const INHERENT: Struct = Struct { _private: () };
+}