WIP strip strings at compile time

src/libutil/include/nix/util/strings-inline.hh

@@ -114,4 +114,69 @@ std::string dropEmptyInitThenConcatStringsSep(const std::string_view sep, const
     return s;
 }
 
+/**
+ * Consteval helper for the "constexpr 2-step" pattern.
+ * Converts a std::string to a std::array<char, N> for stable compile-time storage.
+ *
+ * Usage:
+ *   static constexpr auto str = stripIndentationConsteval(...);
+ *   static constexpr auto arr = toArray<str.size()>(str);
+ *   static constexpr std::string_view view{arr.data(), arr.size()};
+ */
+template<size_t N>
+consteval std::string_view toArray(const std::string & s)
+{
+    static std::array<char, N> arr{};
+    for (size_t i = 0; i < N; ++i) {
+        arr[i] = s[i];
+    }
+    return {arr.data(), N};
+}
+
+constexpr std::string stripIndentationImpl(std::string_view s)
+{
+    size_t minIndent = 10000;
+    size_t curIndent = 0;
+    bool atStartOfLine = true;
+
+    for (auto & c : s) {
+        if (atStartOfLine && c == ' ')
+            curIndent++;
+        else if (c == '\n') {
+            if (atStartOfLine)
+                minIndent = std::max(minIndent, curIndent);
+            curIndent = 0;
+            atStartOfLine = true;
+        } else {
+            if (atStartOfLine) {
+                minIndent = std::min(minIndent, curIndent);
+                atStartOfLine = false;
+            }
+        }
+    }
+
+    std::string res;
+
+    size_t pos = 0;
+    while (pos < s.size()) {
+        auto eol = s.find('\n', pos);
+        if (eol == s.npos)
+            eol = s.size();
+        if (eol - pos > minIndent)
+            res.append(s.substr(pos + minIndent, eol - pos - minIndent));
+        res.push_back('\n');
+        pos = eol + 1;
+    }
+
+    return res;
+}
+
+template<std::string_view s>
+consteval std::string_view stripIndentationConsteval()
+{
+    constexpr auto stripped = stripIndentationImpl(s);
+    constexpr size_t n = stripped.size();
+    return toArray<n>(stripped);
+}
+
 } // namespace nix

src/libutil/include/nix/util/strings.hh

@@ -132,6 +132,13 @@ std::string optionalBracket(std::string_view prefix, const std::optional<T> & co
     return optionalBracket(prefix, std::string_view(*content), suffix);
 }
 
+/**
+ * Remove common leading whitespace from the lines in the string
+ * 's'. For example, if every line is indented by at least 3 spaces,
+ * then we remove 3 spaces from the start of every line.
+ */
+std::string stripIndentation(std::string_view s);
+
 /**
  * Hash implementation that can be used for zero-copy heterogenous lookup from
  * P1690R1[1] in unordered containers.

src/libutil/include/nix/util/util.hh

@@ -228,13 +228,6 @@ constexpr char treeLast[] = "└───";
 constexpr char treeLine[] = "│   ";
 constexpr char treeNull[] = "    ";
 
-/**
- * Remove common leading whitespace from the lines in the string
- * 's'. For example, if every line is indented by at least 3 spaces,
- * then we remove 3 spaces from the start of every line.
- */
-std::string stripIndentation(std::string_view s);
-
 /**
  * Get the prefix of 's' up to and excluding the next line break (LF
  * optionally preceded by CR), and the remainder following the line

src/libutil/strings.cc

@@ -152,4 +152,8 @@ std::string optionalBracket(std::string_view prefix, std::string_view content, s
     return result;
 }
 
+std::string stripIndentation(std::string_view s) {
+    return stripIndentationImpl(s);
+}
+
 } // namespace nix

src/libutil/util.cc

@@ -250,44 +250,6 @@ void ignoreExceptionExceptInterrupt(Verbosity lvl)
     }
 }
 
-std::string stripIndentation(std::string_view s)
-{
-    size_t minIndent = 10000;
-    size_t curIndent = 0;
-    bool atStartOfLine = true;
-
-    for (auto & c : s) {
-        if (atStartOfLine && c == ' ')
-            curIndent++;
-        else if (c == '\n') {
-            if (atStartOfLine)
-                minIndent = std::max(minIndent, curIndent);
-            curIndent = 0;
-            atStartOfLine = true;
-        } else {
-            if (atStartOfLine) {
-                minIndent = std::min(minIndent, curIndent);
-                atStartOfLine = false;
-            }
-        }
-    }
-
-    std::string res;
-
-    size_t pos = 0;
-    while (pos < s.size()) {
-        auto eol = s.find('\n', pos);
-        if (eol == s.npos)
-            eol = s.size();
-        if (eol - pos > minIndent)
-            res.append(s.substr(pos + minIndent, eol - pos - minIndent));
-        res.push_back('\n');
-        pos = eol + 1;
-    }
-
-    return res;
-}
-
 std::pair<std::string_view, std::string_view> getLine(std::string_view s)
 {
     auto newline = s.find('\n');