util/string: use string_view in LineReader

diff --git a/src/bitcoin-cli.cpp b/src/bitcoin-cli.cpp
index 938ace35..a724f13d 100644
--- a/src/bitcoin-cli.cpp
+++ b/src/bitcoin-cli.cpp
@@ -104,7 +104,7 @@ void HTTPResponseHeaders::Read(util::LineReader& reader)
     // Headers https://httpwg.org/specs/rfc9110.html#rfc.section.6.3
     // A sequence of Field Lines https://httpwg.org/specs/rfc9110.html#rfc.section.5.2
     while (auto maybe_line = reader.ReadLine()) {
-        const std::string& line = *maybe_line;
+        const std::string_view line = *maybe_line;
 
         // An empty line indicates end of the headers section https://www.rfc-editor.org/rfc/rfc2616#section-4
         if (line.empty()) return;
@@ -965,7 +965,7 @@ HTTPResponse HTTPClient::ReadResponse()
         throw HTTPError{"Failed to read status line"};
     }
 
-    const std::string& status_str = *status_line;
+    const std::string_view status_str = *status_line;
     // Minimum status line is "HTTP/X.Y NNN" (e.g. "HTTP/1.1 200"), 12 characters.
     if (status_str.size() < 12 || !status_str.starts_with("HTTP/")) {
         throw HTTPError{"Invalid status line"};
@@ -976,7 +976,7 @@ HTTPResponse HTTPClient::ReadResponse()
         throw HTTPError{"Invalid status line format"};
     }
 
-    std::string status_code_str = status_str.substr(space1 + 1, 3);
+    const std::string_view status_code_str = status_str.substr(space1 + 1, 3);
     auto status_code = ToIntegral<int>(status_code_str);
     if (!status_code) {
         throw HTTPError{"Invalid status code"};
diff --git a/src/torcontrol.cpp b/src/torcontrol.cpp
index 5393ba7f..56cad019 100644
--- a/src/torcontrol.cpp
+++ b/src/torcontrol.cpp
@@ -191,7 +191,7 @@ bool TorControlConnection::ProcessBuffer()
         // Parse: <code><separator><data>
         // <status>(-|+| )<data>
         m_message.code = ToIntegral<int>(line->substr(0, 3)).value_or(0);
-        m_message.lines.push_back(line->substr(4));
+        m_message.lines.emplace_back(line->substr(4));
         char separator = (*line)[3]; // '-', '+', or ' '
 
         if (separator == ' ') {
diff --git a/src/util/string.cpp b/src/util/string.cpp
index d1ef7a18..42d1418a 100644
--- a/src/util/string.cpp
+++ b/src/util/string.cpp
@@ -20,7 +20,7 @@ void ReplaceAll(std::string& in_out, const std::string& search, const std::strin
 LineReader::LineReader(std::span<const std::byte> buffer, size_t max_line_length)
     : start(buffer.begin()), end(buffer.end()), max_line_length(max_line_length), it(buffer.begin()) {}
 
-std::optional<std::string> LineReader::ReadLine()
+std::optional<std::string_view> LineReader::ReadLine()
 {
     if (it == end) {
         return std::nullopt;
@@ -38,7 +38,7 @@ std::optional<std::string> LineReader::ReadLine()
         if (c == '\n') {
             const std::string_view untrimmed_line(reinterpret_cast<const char*>(std::to_address(line_start)), count);
             const std::string_view line = TrimStringView(untrimmed_line); // delete leading and trailing whitespace including \r and \n
-            return std::string(line);
+            return line;
         }
         // If the character we just consumed gives us a line length greater
         // than max_line_length, and we are not at the end of the line (or buffer) yet,
@@ -57,11 +57,11 @@ std::optional<std::string> LineReader::ReadLine()
 }
 
 // Ignores max_line_length but won't overflow
-std::string LineReader::ReadLength(size_t len)
+std::string_view LineReader::ReadLength(size_t len)
 {
-    if (len == 0) return "";
+    if (len == 0) return {};
     if (Remaining() < len) throw std::runtime_error("Not enough data in buffer");
-    std::string out(reinterpret_cast<const char*>(std::to_address(it)), len);
+    std::string_view out(reinterpret_cast<const char*>(std::to_address(it)), len);
     it += len;
     return out;
 }
diff --git a/src/util/string.h b/src/util/string.h
index da4ce5a3..797d5344 100644
--- a/src/util/string.h
+++ b/src/util/string.h
@@ -18,6 +18,8 @@
 #include <string_view>
 #include <vector>
 
+#include <attributes.h>
+
 namespace util {
 namespace detail {
 template <unsigned num_params>
@@ -280,7 +282,7 @@ struct LineReader {
      *          std::nullopt if end of buffer is reached without finding a \n.
      * @throws a std::runtime_error if max_line_length + 1 bytes are read without finding \n.
      */
-    std::optional<std::string> ReadLine();
+    std::optional<std::string_view> ReadLine() LIFETIMEBOUND;
 
     /**
      * Returns string from current iterator position of specified length
@@ -290,7 +292,7 @@ struct LineReader {
      * @returns a string of the expected length.
      * @throws a std::runtime_error if there is not enough data in the buffer.
      */
-    std::string ReadLength(size_t len);
+    std::string_view ReadLength(size_t len) LIFETIMEBOUND;
 
     /**
      * Returns remaining size of bytes in buffer