diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 1b38e32fb..4c66d08ee 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -22,6 +22,9 @@
 #include <dlfcn.h>
 #endif
 
+#include "config-impl.hh"
+
+
 namespace nix {
 
 
@@ -192,18 +195,18 @@ NLOHMANN_JSON_SERIALIZE_ENUM(SandboxMode, {
     {SandboxMode::smDisabled, false},
 });
 
-template<> void BaseSetting<SandboxMode>::set(const std::string & str, bool append)
+template<> SandboxMode BaseSetting<SandboxMode>::parse(const std::string & str) const
 {
-    if (str == "true") value = smEnabled;
-    else if (str == "relaxed") value = smRelaxed;
-    else if (str == "false") value = smDisabled;
+    if (str == "true") return smEnabled;
+    else if (str == "relaxed") return smRelaxed;
+    else if (str == "false") return smDisabled;
     else throw UsageError("option '%s' has invalid value '%s'", name, str);
 }
 
-template<> bool BaseSetting<SandboxMode>::isAppendable()
+template<> struct BaseSetting<SandboxMode>::trait
 {
-    return false;
-}
+    static constexpr bool appendable = false;
+};
 
 template<> std::string BaseSetting<SandboxMode>::to_string() const
 {
@@ -235,23 +238,23 @@ template<> void BaseSetting<SandboxMode>::convertToArg(Args & args, const std::s
     });
 }
 
-void MaxBuildJobsSetting::set(const std::string & str, bool append)
+unsigned int MaxBuildJobsSetting::parse(const std::string & str) const
 {
-    if (str == "auto") value = std::max(1U, std::thread::hardware_concurrency());
+    if (str == "auto") return std::max(1U, std::thread::hardware_concurrency());
     else {
         if (auto n = string2Int<decltype(value)>(str))
-            value = *n;
+            return *n;
         else
             throw UsageError("configuration setting '%s' should be 'auto' or an integer", name);
     }
 }
 
 
-void PluginFilesSetting::set(const std::string & str, bool append)
+Paths PluginFilesSetting::parse(const std::string & str) const
 {
     if (pluginsLoaded)
         throw UsageError("plugin-files set after plugins were loaded, you may need to move the flag before the subcommand");
-    BaseSetting<Paths>::set(str, append);
+    return BaseSetting<Paths>::parse(str);
 }
 
 
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index d6c5d437a..609cf53b8 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -26,7 +26,7 @@ struct MaxBuildJobsSetting : public BaseSetting<unsigned int>
         options->addSetting(this);
     }
 
-    void set(const std::string & str, bool append = false) override;
+    unsigned int parse(const std::string & str) const override;
 };
 
 struct PluginFilesSetting : public BaseSetting<Paths>
@@ -43,7 +43,7 @@ struct PluginFilesSetting : public BaseSetting<Paths>
         options->addSetting(this);
     }
 
-    void set(const std::string & str, bool append = false) override;
+    Paths parse(const std::string & str) const override;
 };
 
 const uint32_t maxIdsPerBuild =
diff --git a/src/libutil/config-impl.hh b/src/libutil/config-impl.hh
new file mode 100644
index 000000000..b6cae5ec3
--- /dev/null
+++ b/src/libutil/config-impl.hh
@@ -0,0 +1,71 @@
+#pragma once
+/**
+ * @file
+ *
+ * Template implementations (as opposed to mere declarations).
+ *
+ * One only needs to include this when one is declaring a
+ * `BaseClass<CustomType>` setting, or as derived class of such an
+ * instantiation.
+ */
+
+#include "config.hh"
+
+namespace nix {
+
+template<> struct BaseSetting<Strings>::trait
+{
+    static constexpr bool appendable = true;
+};
+template<> struct BaseSetting<StringSet>::trait
+{
+    static constexpr bool appendable = true;
+};
+template<> struct BaseSetting<StringMap>::trait
+{
+    static constexpr bool appendable = true;
+};
+template<> struct BaseSetting<std::set<ExperimentalFeature>>::trait
+{
+    static constexpr bool appendable = true;
+};
+
+template<typename T>
+struct BaseSetting<T>::trait
+{
+    static constexpr bool appendable = false;
+};
+
+template<typename T>
+bool BaseSetting<T>::isAppendable()
+{
+    return trait::appendable;
+}
+
+template<> void BaseSetting<Strings>::appendOrSet(Strings && newValue, bool append);
+template<> void BaseSetting<StringSet>::appendOrSet(StringSet && newValue, bool append);
+template<> void BaseSetting<StringMap>::appendOrSet(StringMap && newValue, bool append);
+template<> void BaseSetting<std::set<ExperimentalFeature>>::appendOrSet(std::set<ExperimentalFeature> && newValue, bool append);
+
+template<typename T>
+void BaseSetting<T>::appendOrSet(T && newValue, bool append)
+{
+    static_assert(!trait::appendable, "using default `appendOrSet` implementation with an appendable type");
+    assert(!append);
+    value = std::move(newValue);
+}
+
+template<typename T>
+void BaseSetting<T>::set(const std::string & str, bool append)
+{
+    if (experimentalFeatureSettings.isEnabled(experimentalFeature))
+        appendOrSet(parse(str), append);
+    else {
+        assert(experimentalFeature);
+        warn("Ignoring setting '%s' because experimental feature '%s' is not enabled",
+            name,
+            showExperimentalFeature(*experimentalFeature));
+    }
+}
+
+}
diff --git a/src/libutil/config.cc b/src/libutil/config.cc
index a42f3a849..085a884dc 100644
--- a/src/libutil/config.cc
+++ b/src/libutil/config.cc
@@ -3,6 +3,8 @@
 #include "abstract-setting-to-json.hh"
 #include "experimental-features.hh"
 
+#include "config-impl.hh"
+
 #include <nlohmann/json.hpp>
 
 namespace nix {
@@ -80,6 +82,8 @@ void Config::getSettings(std::map<std::string, SettingInfo> & res, bool overridd
 void AbstractConfig::applyConfig(const std::string & contents, const std::string & path) {
     unsigned int pos = 0;
 
+    std::vector<std::pair<std::string, std::string>> parsedContents;
+
     while (pos < contents.size()) {
         std::string line;
         while (pos < contents.size() && contents[pos] != '\n')
@@ -125,8 +129,21 @@ void AbstractConfig::applyConfig(const std::string & contents, const std::string
         auto i = tokens.begin();
         advance(i, 2);
 
-        set(name, concatStringsSep(" ", Strings(i, tokens.end()))); // FIXME: slow
+        parsedContents.push_back({
+            name,
+            concatStringsSep(" ", Strings(i, tokens.end())),
+        });
     };
+
+    // First apply experimental-feature related settings
+    for (auto & [name, value] : parsedContents)
+        if (name == "experimental-features" || name == "extra-experimental-features")
+            set(name, value);
+
+    // Then apply other settings
+    for (auto & [name, value] : parsedContents)
+        if (name != "experimental-features" && name != "extra-experimental-features")
+            set(name, value);
 }
 
 void AbstractConfig::applyConfigFile(const Path & path)
@@ -202,12 +219,6 @@ void AbstractSetting::convertToArg(Args & args, const std::string & category)
 {
 }
 
-template<typename T>
-bool BaseSetting<T>::isAppendable()
-{
-    return false;
-}
-
 template<typename T>
 void BaseSetting<T>::convertToArg(Args & args, const std::string & category)
 {
@@ -231,9 +242,9 @@ void BaseSetting<T>::convertToArg(Args & args, const std::string & category)
         });
 }
 
-template<> void BaseSetting<std::string>::set(const std::string & str, bool append)
+template<> std::string BaseSetting<std::string>::parse(const std::string & str) const
 {
-    value = str;
+    return str;
 }
 
 template<> std::string BaseSetting<std::string>::to_string() const
@@ -242,11 +253,11 @@ template<> std::string BaseSetting<std::string>::to_string() const
 }
 
 template<typename T>
-void BaseSetting<T>::set(const std::string & str, bool append)
+T BaseSetting<T>::parse(const std::string & str) const
 {
     static_assert(std::is_integral<T>::value, "Integer required.");
     if (auto n = string2Int<T>(str))
-        value = *n;
+        return *n;
     else
         throw UsageError("setting '%s' has invalid value '%s'", name, str);
 }
@@ -258,12 +269,12 @@ std::string BaseSetting<T>::to_string() const
     return std::to_string(value);
 }
 
-template<> void BaseSetting<bool>::set(const std::string & str, bool append)
+template<> bool BaseSetting<bool>::parse(const std::string & str) const
 {
     if (str == "true" || str == "yes" || str == "1")
-        value = true;
+        return true;
     else if (str == "false" || str == "no" || str == "0")
-        value = false;
+        return false;
     else
         throw UsageError("Boolean setting '%s' has invalid value '%s'", name, str);
 }
@@ -291,16 +302,15 @@ template<> void BaseSetting<bool>::convertToArg(Args & args, const std::string &
     });
 }
 
-template<> void BaseSetting<Strings>::set(const std::string & str, bool append)
+template<> Strings BaseSetting<Strings>::parse(const std::string & str) const
 {
-    auto ss = tokenizeString<Strings>(str);
-    if (!append) value.clear();
-    for (auto & s : ss) value.push_back(std::move(s));
+    return tokenizeString<Strings>(str);
 }
 
-template<> bool BaseSetting<Strings>::isAppendable()
+template<> void BaseSetting<Strings>::appendOrSet(Strings && newValue, bool append)
 {
-    return true;
+    if (!append) value.clear();
+    for (auto && s : std::move(newValue)) value.push_back(std::move(s));
 }
 
 template<> std::string BaseSetting<Strings>::to_string() const
@@ -308,16 +318,16 @@ template<> std::string BaseSetting<Strings>::to_string() const
     return concatStringsSep(" ", value);
 }
 
-template<> void BaseSetting<StringSet>::set(const std::string & str, bool append)
+template<> StringSet BaseSetting<StringSet>::parse(const std::string & str) const
 {
-    if (!append) value.clear();
-    for (auto & s : tokenizeString<StringSet>(str))
-        value.insert(s);
+    return tokenizeString<StringSet>(str);
 }
 
-template<> bool BaseSetting<StringSet>::isAppendable()
+template<> void BaseSetting<StringSet>::appendOrSet(StringSet && newValue, bool append)
 {
-    return true;
+    if (!append) value.clear();
+    for (auto && s : std::move(newValue))
+        value.insert(s);
 }
 
 template<> std::string BaseSetting<StringSet>::to_string() const
@@ -325,21 +335,24 @@ template<> std::string BaseSetting<StringSet>::to_string() const
     return concatStringsSep(" ", value);
 }
 
-template<> void BaseSetting<std::set<ExperimentalFeature>>::set(const std::string & str, bool append)
+template<> std::set<ExperimentalFeature> BaseSetting<std::set<ExperimentalFeature>>::parse(const std::string & str) const
 {
-    if (!append) value.clear();
+    std::set<ExperimentalFeature> res;
     for (auto & s : tokenizeString<StringSet>(str)) {
         auto thisXpFeature = parseExperimentalFeature(s);
         if (thisXpFeature)
-            value.insert(thisXpFeature.value());
+            res.insert(thisXpFeature.value());
         else
             warn("unknown experimental feature '%s'", s);
     }
+    return res;
 }
 
-template<> bool BaseSetting<std::set<ExperimentalFeature>>::isAppendable()
+template<> void BaseSetting<std::set<ExperimentalFeature>>::appendOrSet(std::set<ExperimentalFeature> && newValue, bool append)
 {
-    return true;
+    if (!append) value.clear();
+    for (auto && s : std::move(newValue))
+        value.insert(s);
 }
 
 template<> std::string BaseSetting<std::set<ExperimentalFeature>>::to_string() const
@@ -350,20 +363,23 @@ template<> std::string BaseSetting<std::set<ExperimentalFeature>>::to_string() c
     return concatStringsSep(" ", stringifiedXpFeatures);
 }
 
-template<> void BaseSetting<StringMap>::set(const std::string & str, bool append)
+template<> StringMap BaseSetting<StringMap>::parse(const std::string & str) const
 {
-    if (!append) value.clear();
+    StringMap res;
     for (auto & s : tokenizeString<Strings>(str)) {
         auto eq = s.find_first_of('=');
         if (std::string::npos != eq)
-            value.emplace(std::string(s, 0, eq), std::string(s, eq + 1));
+            res.emplace(std::string(s, 0, eq), std::string(s, eq + 1));
         // else ignored
     }
+    return res;
 }
 
-template<> bool BaseSetting<StringMap>::isAppendable()
+template<> void BaseSetting<StringMap>::appendOrSet(StringMap && newValue, bool append)
 {
-    return true;
+    if (!append) value.clear();
+    for (auto && [k, v] : std::move(newValue))
+        value.emplace(std::move(k), std::move(v));
 }
 
 template<> std::string BaseSetting<StringMap>::to_string() const
@@ -387,15 +403,15 @@ template class BaseSetting<StringSet>;
 template class BaseSetting<StringMap>;
 template class BaseSetting<std::set<ExperimentalFeature>>;
 
-void PathSetting::set(const std::string & str, bool append)
+Path PathSetting::parse(const std::string & str) const
 {
     if (str == "") {
         if (allowEmpty)
-            value = "";
+            return "";
         else
             throw UsageError("setting '%s' cannot be empty", name);
     } else
-        value = canonPath(str);
+        return canonPath(str);
 }
 
 bool GlobalConfig::set(const std::string & name, const std::string & value)
diff --git a/src/libutil/config.hh b/src/libutil/config.hh
index 162626791..2675baed7 100644
--- a/src/libutil/config.hh
+++ b/src/libutil/config.hh
@@ -215,8 +215,11 @@ protected:
 
     virtual void set(const std::string & value, bool append = false) = 0;
 
-    virtual bool isAppendable()
-    { return false; }
+    /**
+     * Whether the type is appendable; i.e. whether the `append`
+     * parameter to `set()` is allowed to be `true`.
+     */
+    virtual bool isAppendable() = 0;
 
     virtual std::string to_string() const = 0;
 
@@ -241,6 +244,23 @@ protected:
     const T defaultValue;
     const bool documentDefault;
 
+    /**
+     * Parse the string into a `T`.
+     *
+     * Used by `set()`.
+     */
+    virtual T parse(const std::string & str) const;
+
+    /**
+     * Append or overwrite `value` with `newValue`.
+     *
+     * Some types to do not support appending in which case `append`
+     * should never be passed. The default handles this case.
+     *
+     * @param append Whether to append or overwrite.
+     */
+    virtual void appendOrSet(T && newValue, bool append);
+
 public:
 
     BaseSetting(const T & def,
@@ -268,9 +288,25 @@ public:
     template<typename U>
     void setDefault(const U & v) { if (!overridden) value = v; }
 
-    void set(const std::string & str, bool append = false) override;
+    /**
+     * Require any experimental feature the setting depends on
+     *
+     * Uses `parse()` to get the value from `str`, and `appendOrSet()`
+     * to set it.
+     */
+    void set(const std::string & str, bool append = false) override final;
 
-    bool isAppendable() override;
+    /**
+     * C++ trick; This is template-specialized to compile-time indicate whether
+     * the type is appendable.
+     */
+    struct trait;
+
+    /**
+     * Always defined based on the C++ magic
+     * with `trait` above.
+     */
+    bool isAppendable() override final;
 
     virtual void override(const T & v)
     {
@@ -336,7 +372,7 @@ public:
         options->addSetting(this);
     }
 
-    void set(const std::string & str, bool append = false) override;
+    Path parse(const std::string & str) const override;
 
     Path operator +(const char * p) const { return value + p; }
 
diff --git a/src/libutil/tests/config.cc b/src/libutil/tests/config.cc
index f250e934e..886e70da5 100644
--- a/src/libutil/tests/config.cc
+++ b/src/libutil/tests/config.cc
@@ -82,6 +82,7 @@ namespace nix {
             TestSetting() : AbstractSetting("test", "test", {}) {}
             void set(const std::string & value, bool append) override {}
             std::string to_string() const override { return {}; }
+            bool isAppendable() override { return false; }
         };
 
         Config config;
@@ -90,6 +91,7 @@ namespace nix {
         ASSERT_FALSE(config.set("test", "value"));
         config.addSetting(&setting);
         ASSERT_TRUE(config.set("test", "value"));
+        ASSERT_FALSE(config.set("extra-test", "value"));
     }
 
     TEST(Config, withInitialValue) {
diff --git a/tests/experimental-features.sh b/tests/experimental-features.sh
index 73554da8c..607bf0a8e 100644
--- a/tests/experimental-features.sh
+++ b/tests/experimental-features.sh
@@ -23,20 +23,64 @@ source common.sh
 # # Medium case, the configuration effects --help
 # grep_both_ways store gc --help
 
-expect 1 nix --experimental-features 'nix-command' show-config --flake-registry 'https://no'
-nix --experimental-features 'nix-command flakes' show-config --flake-registry 'https://no'
+# Test settings that are gated on experimental features; the setting is ignored
+# with a warning if the experimental feature is not enabled. The order of the
+# `setting = value` lines in the configuration should not matter.
+
+# 'flakes' experimental-feature is disabled before, ignore and warn
+NIX_CONFIG='
+  experimental-features = nix-command
+  accept-flake-config = true
+' nix show-config accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
+grepQuiet "false" $TEST_ROOT/stdout
+grepQuiet "Ignoring setting 'accept-flake-config' because experimental feature 'flakes' is not enabled" $TEST_ROOT/stderr
+
+# 'flakes' experimental-feature is disabled after, ignore and warn
+NIX_CONFIG='
+  accept-flake-config = true
+  experimental-features = nix-command
+' nix show-config accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
+grepQuiet "false" $TEST_ROOT/stdout
+grepQuiet "Ignoring setting 'accept-flake-config' because experimental feature 'flakes' is not enabled" $TEST_ROOT/stderr
+
+# 'flakes' experimental-feature is enabled before, process
+NIX_CONFIG='
+  experimental-features = nix-command flakes
+  accept-flake-config = true
+' nix show-config accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
+grepQuiet "true" $TEST_ROOT/stdout
+grepQuietInverse "Ignoring setting 'accept-flake-config'" $TEST_ROOT/stderr
+
+# 'flakes' experimental-feature is enabled after, process
+NIX_CONFIG='
+  accept-flake-config = true
+  experimental-features = nix-command flakes
+' nix show-config accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
+grepQuiet "true" $TEST_ROOT/stdout
+grepQuietInverse "Ignoring setting 'accept-flake-config'" $TEST_ROOT/stderr
+
+function exit_code_both_ways {
+    expect 1 nix --experimental-features 'nix-command' "$@" 1>/dev/null
+    nix --experimental-features 'nix-command flakes' "$@" 1>/dev/null
+
+    # Also, the order should not matter
+    expect 1 nix "$@" --experimental-features 'nix-command' 1>/dev/null
+    nix "$@" --experimental-features 'nix-command flakes' 1>/dev/null
+}
+
+exit_code_both_ways show-config --flake-registry 'https://no'
 
 # Double check these are stable
-nix --experimental-features '' --help
-nix --experimental-features '' doctor --help
-nix --experimental-features '' repl --help
-nix --experimental-features '' upgrade-nix --help
+nix --experimental-features '' --help 1>/dev/null
+nix --experimental-features '' doctor --help 1>/dev/null
+nix --experimental-features '' repl --help 1>/dev/null
+nix --experimental-features '' upgrade-nix --help 1>/dev/null
 
 # These 3 arguments are currently given to all commands, which is wrong (as not
 # all care). To deal with fixing later, we simply make them require the
 # nix-command experimental features --- it so happens that the commands we wish
 # stabilizing to do not need them anyways.
 for arg in '--print-build-logs' '--offline' '--refresh'; do
-    nix --experimental-features 'nix-command' "$arg" --help
-    ! nix --experimental-features '' "$arg" --help
+    nix --experimental-features 'nix-command' "$arg" --help 1>/dev/null
+    expect 1 nix --experimental-features '' "$arg" --help 1>/dev/null
 done