source_span_for_markdown_range: fix utf8 violation

it is non-trivial to reproduce this bug through rustdoc,
which uses this function less than clippy,
so the regression test was added as a unit test
instead of an integration test.

compiler/rustc_resolve/src/rustdoc/tests.rs

@@ -0,0 +1,50 @@
+use std::path::PathBuf;
+
+use rustc_span::source_map::{FilePathMapping, SourceMap};
+use rustc_span::symbol::sym;
+use rustc_span::{BytePos, Span};
+
+use super::{DocFragment, DocFragmentKind, source_span_for_markdown_range_inner};
+
+#[test]
+fn single_backtick() {
+    let sm = SourceMap::new(FilePathMapping::empty());
+    sm.new_source_file(PathBuf::from("foo.rs").into(), r#"#[doc = "`"] fn foo() {}"#.to_string());
+    let span = source_span_for_markdown_range_inner(
+        &sm,
+        "`",
+        &(0..1),
+        &[DocFragment {
+            span: Span::with_root_ctxt(BytePos(8), BytePos(11)),
+            item_id: None,
+            kind: DocFragmentKind::RawDoc,
+            doc: sym::empty, // unused placeholder
+            indent: 0,
+        }],
+    )
+    .unwrap();
+    assert_eq!(span.lo(), BytePos(9));
+    assert_eq!(span.hi(), BytePos(10));
+}
+
+#[test]
+fn utf8() {
+    // regression test for https://github.com/rust-lang/rust/issues/141665
+    let sm = SourceMap::new(FilePathMapping::empty());
+    sm.new_source_file(PathBuf::from("foo.rs").into(), r#"#[doc = "⚠"] fn foo() {}"#.to_string());
+    let span = source_span_for_markdown_range_inner(
+        &sm,
+        "⚠",
+        &(0..3),
+        &[DocFragment {
+            span: Span::with_root_ctxt(BytePos(8), BytePos(14)),
+            item_id: None,
+            kind: DocFragmentKind::RawDoc,
+            doc: sym::empty, // unused placeholder
+            indent: 0,
+        }],
+    )
+    .unwrap();
+    assert_eq!(span.lo(), BytePos(9));
+    assert_eq!(span.hi(), BytePos(12));
+}