diff --git a/src/librustdoc/html/markdown.rs b/src/librustdoc/html/markdown.rs
index db30836b43f..177fb1a9426 100644
--- a/src/librustdoc/html/markdown.rs
+++ b/src/librustdoc/html/markdown.rs
@@ -1166,6 +1166,7 @@ fn parse(
         let allow_error_code_check = allow_error_code_check.as_bool();
         let mut seen_rust_tags = false;
         let mut seen_other_tags = false;
+        let mut seen_custom_tag = false;
         let mut data = LangString::default();
         let mut ignores = vec![];
 
@@ -1195,6 +1196,9 @@ fn parse(
                     data.rust = true;
                     seen_rust_tags = true;
                 }
+                LangStringToken::LangToken("custom") => {
+                    seen_custom_tag = true;
+                }
                 LangStringToken::LangToken("test_harness") => {
                     data.test_harness = true;
                     seen_rust_tags = !seen_other_tags || seen_rust_tags;
@@ -1264,7 +1268,6 @@ fn parse(
                     data.unknown.push(x.to_owned());
                 }
                 LangStringToken::KeyValueAttribute(key, value) => {
-                    seen_other_tags = true;
                     if key == "class" {
                         data.added_classes.push(value.to_owned());
                     } else if let Some(extra) = extra {
@@ -1273,7 +1276,6 @@ fn parse(
                     }
                 }
                 LangStringToken::ClassAttribute(class) => {
-                    seen_other_tags = true;
                     data.added_classes.push(class.to_owned());
                 }
             }
@@ -1284,7 +1286,7 @@ fn parse(
             data.ignore = Ignore::Some(ignores);
         }
 
-        data.rust &= !seen_other_tags || seen_rust_tags;
+        data.rust &= !seen_custom_tag && (!seen_other_tags || seen_rust_tags);
 
         data
     }
diff --git a/src/librustdoc/html/markdown/tests.rs b/src/librustdoc/html/markdown/tests.rs
index 35b243d3d29..7d89cb0c4e6 100644
--- a/src/librustdoc/html/markdown/tests.rs
+++ b/src/librustdoc/html/markdown/tests.rs
@@ -64,6 +64,12 @@ fn t(lg: LangString) {
     t(LangString { original: "{rust}".into(), rust: true, ..Default::default() });
     t(LangString {
         original: "{.rust}".into(),
+        rust: true,
+        added_classes: vec!["rust".into()],
+        ..Default::default()
+    });
+    t(LangString {
+        original: "custom,{.rust}".into(),
         rust: false,
         added_classes: vec!["rust".into()],
         ..Default::default()
@@ -154,12 +160,24 @@ fn t(lg: LangString) {
     t(LangString {
         original: "{class=test}".into(),
         added_classes: vec!["test".into()],
+        rust: true,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "custom,{class=test}".into(),
+        added_classes: vec!["test".into()],
         rust: false,
         ..Default::default()
     });
     t(LangString {
         original: "{.test}".into(),
         added_classes: vec!["test".into()],
+        rust: true,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "custom,{.test}".into(),
+        added_classes: vec!["test".into()],
         rust: false,
         ..Default::default()
     });
@@ -172,12 +190,24 @@ fn t(lg: LangString) {
     t(LangString {
         original: "{class=test:with:colon .test1}".into(),
         added_classes: vec!["test:with:colon".into(), "test1".into()],
+        rust: true,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "custom,{class=test:with:colon .test1}".into(),
+        added_classes: vec!["test:with:colon".into(), "test1".into()],
         rust: false,
         ..Default::default()
     });
     t(LangString {
         original: "{class=first,class=second}".into(),
         added_classes: vec!["first".into(), "second".into()],
+        rust: true,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "custom,{class=first,class=second}".into(),
+        added_classes: vec!["first".into(), "second".into()],
         rust: false,
         ..Default::default()
     });
@@ -206,6 +236,12 @@ fn t(lg: LangString) {
     t(LangString {
         original: r#"{class="first"}"#.into(),
         added_classes: vec!["first".into()],
+        rust: true,
+        ..Default::default()
+    });
+    t(LangString {
+        original: r#"custom,{class="first"}"#.into(),
+        added_classes: vec!["first".into()],
         rust: false,
         ..Default::default()
     });