diff --git a/clang-tools-extra/clangd/CMakeLists.txt b/clang-tools-extra/clangd/CMakeLists.txt
--- a/clang-tools-extra/clangd/CMakeLists.txt
+++ b/clang-tools-extra/clangd/CMakeLists.txt
@@ -37,6 +37,7 @@
   CompileCommands.cpp
   Compiler.cpp
   Config.cpp
+  ConfigCompile.cpp
   ConfigYAML.cpp
   Diagnostics.cpp
   DraftStore.cpp
diff --git a/clang-tools-extra/clangd/ConfigCompile.cpp b/clang-tools-extra/clangd/ConfigCompile.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/ConfigCompile.cpp
@@ -0,0 +1,140 @@
+//===--- ConfigCompile.cpp - Translating Fragments into Config ------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Fragments are applied to Configs in two steps:
+//
+// 1. (When the fragment is first loaded)
+//    FragmentCompiler::compile() traverses the Fragment and creates
+//    function objects that know how to apply the configuration.
+// 2. (Every time a config is required)
+//    CompiledFragment::apply() executes these functions to populate the Config.
+//
+// Work could be split between these steps in different ways. We try to
+// do as much work as possible in the first step. For example, regexes are
+// compiled in stage 1 and captured by the apply function. This is because:
+//
+//  - it's more efficient, as the work done in stage 1 must only be done once
+//  - problems can be reported in stage 1, in stage 2 we must silently recover
+//
+//===----------------------------------------------------------------------===//
+
+#include "Config.h"
+#include "ConfigProvider.h"
+#include "support/Logger.h"
+#include "support/Trace.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Regex.h"
+#include "llvm/Support/SMLoc.h"
+#include "llvm/Support/SourceMgr.h"
+
+namespace clang {
+namespace clangd {
+namespace config {
+namespace {
+
+// Wrapper around condition compile() functions to reduce arg-passing.
+struct FragmentCompiler {
+  std::vector<llvm::unique_function<bool(const Params &)>> &Conditions;
+  std::vector<llvm::unique_function<void(Config &)>> &Apply;
+  DiagnosticCallback Diagnostic;
+  llvm::SourceMgr *SourceMgr;
+
+  std::string RegexError = "";
+  llvm::Optional<llvm::Regex> compileRegex(const Located<std::string> &Text) {
+    std::string Anchored = "^(" + *Text + ")$";
+    llvm::Regex Result(Anchored);
+    if (!Result.isValid(RegexError)) {
+      diag(Error, "Invalid regex " + Anchored + ": " + RegexError, Text.Range);
+      return llvm::None;
+    }
+    return Result;
+  }
+
+  void compile(Fragment &&F) {
+    compile(std::move(F.Condition));
+    compile(std::move(F.CompileFlags));
+  }
+
+  void compile(Fragment::ConditionBlock &&F) {
+    if (F.HasUnrecognizedCondition)
+      Conditions.push_back([&](const Params &) { return false; });
+
+    auto PathMatch = std::make_unique<std::vector<llvm::Regex>>();
+    for (auto &Entry : F.PathMatch) {
+      if (auto RE = compileRegex(Entry))
+        PathMatch->push_back(std::move(*RE));
+    }
+    if (!PathMatch->empty()) {
+      Conditions.push_back([PathMatch(std::move(PathMatch))](const Params &P) {
+        if (P.Path.empty())
+          return false;
+        return llvm::any_of(*PathMatch, [&](const llvm::Regex &RE) {
+          return RE.match(P.Path);
+        });
+      });
+    }
+  }
+
+  void compile(Fragment::CompileFlagsBlock &&F) {
+    if (!F.Add.empty()) {
+      std::vector<std::string> Add;
+      for (auto &A : F.Add)
+        Add.push_back(std::move(*A));
+      Apply.push_back([Add(std::move(Add))](Config &C) {
+        C.CompileFlags.Edits.push_back([Add](std::vector<std::string> &Args) {
+          Args.insert(Args.end(), Add.begin(), Add.end());
+        });
+      });
+    }
+  }
+
+  constexpr static auto Error = llvm::SourceMgr::DK_Error;
+  void diag(llvm::SourceMgr::DiagKind Kind, llvm::StringRef Message,
+            llvm::SMRange Range) {
+    if (Range.isValid() && SourceMgr != nullptr)
+      Diagnostic(SourceMgr->GetMessage(Range.Start, Kind, Message, Range));
+    else
+      Diagnostic(llvm::SMDiagnostic("", Kind, Message));
+  }
+};
+
+} // namespace
+
+CompiledFragment::CompiledFragment(Fragment F, DiagnosticCallback D) {
+  llvm::StringRef SourceFile = "<unknown>";
+  std::pair<unsigned, unsigned> LineCol = {0, 0};
+  if (auto *SM = F.Source.Manager.get()) {
+    unsigned BufID = SM->getMainFileID();
+    LineCol = SM->getLineAndColumn(F.Source.Location, BufID);
+    SourceFile = SM->getBufferInfo(BufID).Buffer->getBufferIdentifier();
+  }
+  vlog("config::Fragment: compiling {0}:{1} -> {2}", SourceFile, LineCol.first,
+       this);
+  trace::Span Tracer("ConfigCompile");
+  SPAN_ATTACH(Tracer, "ConfigFile", SourceFile);
+
+  FragmentCompiler{Conditions, Apply, D, F.Source.Manager.get()}.compile(
+      std::move(F));
+}
+
+bool CompiledFragment::apply(const Params &P, Config &C) const {
+  for (auto &C : Conditions) {
+    if (!C(P)) {
+      dlog("config::Fragment {0}: condition not met", this);
+      return false;
+    }
+  }
+  dlog("config::Fragment {0}: applying {1} rules", this, Apply.size());
+  for (auto &A : Apply)
+    A(C);
+  return true;
+}
+
+} // namespace config
+} // namespace clangd
+} // namespace clang
diff --git a/clang-tools-extra/clangd/ConfigProvider.h b/clang-tools-extra/clangd/ConfigProvider.h
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/ConfigProvider.h
@@ -0,0 +1,63 @@
+//===--- ConfigProvider.h - Loading of user configuration --------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Various clangd features have configurable behaviour (or can be disabled).
+// The configuration system allows users to control this:
+//  - in a user config file, a project config file, via LSP, or via flags
+//  - specifying different settings for different files
+// This file defines the structures used for this, that produce a Config.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONFIG_PROVIDER_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONFIG_PROVIDER_H
+
+#include "ConfigFragment.h"
+#include "llvm/ADT/FunctionExtras.h"
+#include "llvm/Support/SMLoc.h"
+#include <string>
+#include <vector>
+
+namespace clang {
+namespace clangd {
+struct Config;
+namespace config {
+
+/// Describes the context used to evaluate configuration fragments.
+struct Params {
+  /// Absolute path to file we're targeting. Unix slashes.
+  /// Empty if not configuring a particular file.
+  llvm::StringRef Path;
+};
+
+/// A chunk of configuration that has been fully analyzed and is ready to apply.
+///
+/// Fragments are compiled by Providers when first loaded, and cached for reuse.
+/// Like a compiled program, this is good for performance and also encourages
+/// errors to be reported early and only once.
+class CompiledFragment {
+public:
+  /// Analyzes and consumes a fragment, possibly yielding more diagnostics.
+  /// This always produces a usable compiled fragment (errors are recovered).
+  explicit CompiledFragment(Fragment, DiagnosticCallback);
+
+  /// Updates the configuration to reflect settings from the fragment.
+  /// Returns true if the condition was met and the settings were used.
+  bool apply(const Params &, Config &) const;
+
+private:
+  // FIXME: remove mutable once unique_function is const-compatible.
+  mutable std::vector<llvm::unique_function<bool(const Params &)>> Conditions;
+  mutable std::vector<llvm::unique_function<void(Config &)>> Apply;
+};
+
+} // namespace config
+} // namespace clangd
+} // namespace clang
+
+#endif
diff --git a/clang-tools-extra/clangd/unittests/CMakeLists.txt b/clang-tools-extra/clangd/unittests/CMakeLists.txt
--- a/clang-tools-extra/clangd/unittests/CMakeLists.txt
+++ b/clang-tools-extra/clangd/unittests/CMakeLists.txt
@@ -41,6 +41,7 @@
   CollectMacrosTests.cpp
   CompileCommandsTests.cpp
   CompilerTests.cpp
+  ConfigCompileTests.cpp
   ConfigYAMLTests.cpp
   DexTests.cpp
   DiagnosticsTests.cpp
diff --git a/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp b/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp
@@ -0,0 +1,88 @@
+//===-- ConfigCompileTests.cpp --------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Config.h"
+#include "ConfigProvider.h"
+#include "ConfigTesting.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace clangd {
+namespace config {
+namespace {
+using ::testing::ElementsAre;
+using ::testing::IsEmpty;
+using ::testing::SizeIs;
+using ::testing::StartsWith;
+
+class ConfigCompileTests : public ::testing::Test {
+protected:
+  CapturedDiags Diags;
+  Config Conf;
+  Fragment Frag;
+  Params Parm;
+
+  bool compile() {
+    Conf = Config();
+    Diags.Diagnostics.clear();
+    CompiledFragment CF(Frag, Diags.callback());
+    return CF.apply(Parm, Conf);
+  }
+};
+
+TEST_F(ConfigCompileTests, Condition) {
+  // No condition.
+  Frag.CompileFlags.Add.emplace_back("X");
+  EXPECT_TRUE(compile()) << "Empty config";
+  EXPECT_THAT(Diags.Diagnostics, IsEmpty());
+  EXPECT_THAT(Conf.CompileFlags.Edits, SizeIs(1));
+
+  // Regex with no file.
+  Frag.Condition.PathMatch.emplace_back("fo*");
+  EXPECT_FALSE(compile());
+  EXPECT_THAT(Diags.Diagnostics, IsEmpty());
+  EXPECT_THAT(Conf.CompileFlags.Edits, SizeIs(0));
+
+  // Non-matching regex.
+  Parm.Path = "bar";
+  EXPECT_FALSE(compile());
+  EXPECT_THAT(Diags.Diagnostics, IsEmpty());
+
+  // Matching regex.
+  Frag.Condition.PathMatch.emplace_back("ba*r");
+  EXPECT_TRUE(compile());
+  EXPECT_THAT(Diags.Diagnostics, IsEmpty());
+
+  // Invalid regex.
+  Frag.Condition.PathMatch.emplace_back("**]@theu");
+  EXPECT_TRUE(compile());
+  EXPECT_THAT(Diags.Diagnostics, SizeIs(1));
+  EXPECT_THAT(Diags.Diagnostics.front().Message, StartsWith("Invalid regex"));
+  EXPECT_THAT(Conf.CompileFlags.Edits, SizeIs(1));
+  Frag.Condition.PathMatch.pop_back();
+
+  // Valid regex and unknown key.
+  Frag.Condition.HasUnrecognizedCondition = true;
+  EXPECT_FALSE(compile());
+  EXPECT_THAT(Diags.Diagnostics, IsEmpty());
+}
+
+TEST_F(ConfigCompileTests, CompileCommands) {
+  Frag.CompileFlags.Add.emplace_back("-foo");
+  std::vector<std::string> Argv = {"clang", "a.cc"};
+  EXPECT_TRUE(compile());
+  EXPECT_THAT(Conf.CompileFlags.Edits, SizeIs(1));
+  Conf.CompileFlags.Edits.front()(Argv);
+  EXPECT_THAT(Argv, ElementsAre("clang", "a.cc", "-foo"));
+}
+
+} // namespace
+} // namespace config
+} // namespace clangd
+} // namespace clang
diff --git a/clang-tools-extra/clangd/unittests/ConfigTesting.h b/clang-tools-extra/clangd/unittests/ConfigTesting.h
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/unittests/ConfigTesting.h
@@ -0,0 +1,77 @@
+//===-- ConfigTesting.h - Helpers for configuration tests -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_UNITTESTS_CONFIGTESTING_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_UNITTESTS_CONFIGTESTING_H
+
+#include "Protocol.h"
+#include "llvm/Support/ScopedPrinter.h"
+#include "llvm/Support/SourceMgr.h"
+#include "gmock/gmock.h"
+#include <functional>
+
+namespace clang {
+namespace clangd {
+namespace config {
+
+// Provides a DiagnosticsCallback that records diganostics.
+// Unlike just pushing them into a vector, underlying storage need not survive.
+struct CapturedDiags {
+  std::function<void(const llvm::SMDiagnostic &)> callback() {
+    return [this](const llvm::SMDiagnostic &D) {
+      Diagnostics.emplace_back();
+      Diag &Out = Diagnostics.back();
+      Out.Message = D.getMessage().str();
+      Out.Kind = D.getKind();
+      Out.Pos.line = D.getLineNo() - 1;
+      Out.Pos.character = D.getColumnNo(); // Zero-based - bug in SourceMgr?
+      if (!D.getRanges().empty()) {
+        const auto &R = D.getRanges().front();
+        Out.Range.emplace();
+        Out.Range->start.line = Out.Range->end.line = Out.Pos.line;
+        Out.Range->start.character = R.first;
+        Out.Range->end.character = R.second;
+      }
+    };
+  }
+  struct Diag {
+    std::string Message;
+    llvm::SourceMgr::DiagKind Kind;
+    Position Pos;
+    llvm::Optional<Range> Range;
+
+    friend void PrintTo(const Diag &D, std::ostream *OS) {
+      *OS << (D.Kind == llvm::SourceMgr::DK_Error ? "error: " : "warning: ")
+          << D.Message << "@" << llvm::to_string(D.Pos);
+    }
+  };
+  std::vector<Diag> Diagnostics;
+};
+
+MATCHER_P(DiagMessage, M, "") { return arg.Message == M; }
+MATCHER_P(DiagKind, K, "") { return arg.Kind == K; }
+MATCHER_P(DiagPos, P, "") { return arg.Pos == P; }
+MATCHER_P(DiagRange, R, "") { return arg.Range == R; }
+
+inline Position toPosition(llvm::SMLoc L, const llvm::SourceMgr &SM) {
+  auto LineCol = SM.getLineAndColumn(L);
+  Position P;
+  P.line = LineCol.first - 1;
+  P.character = LineCol.second - 1;
+  return P;
+}
+
+inline Range toRange(llvm::SMRange R, const llvm::SourceMgr &SM) {
+  return Range{toPosition(R.Start, SM), toPosition(R.End, SM)};
+}
+
+} // namespace config
+} // namespace clangd
+} // namespace clang
+
+#endif
diff --git a/clang-tools-extra/clangd/unittests/ConfigYAMLTests.cpp b/clang-tools-extra/clangd/unittests/ConfigYAMLTests.cpp
--- a/clang-tools-extra/clangd/unittests/ConfigYAMLTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ConfigYAMLTests.cpp
@@ -8,14 +8,13 @@
 
 #include "Annotations.h"
 #include "ConfigFragment.h"
-#include "Matchers.h"
+#include "ConfigTesting.h"
 #include "Protocol.h"
 #include "llvm/Support/SMLoc.h"
 #include "llvm/Support/ScopedPrinter.h"
 #include "llvm/Support/SourceMgr.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "gtest/internal/gtest-internal.h"
 
 namespace clang {
 namespace clangd {
@@ -36,55 +35,6 @@
   return false;
 }
 
-Position toPosition(llvm::SMLoc L, const llvm::SourceMgr &SM) {
-  auto LineCol = SM.getLineAndColumn(L);
-  Position P;
-  P.line = LineCol.first - 1;
-  P.character = LineCol.second - 1;
-  return P;
-}
-
-Range toRange(llvm::SMRange R, const llvm::SourceMgr &SM) {
-  return Range{toPosition(R.Start, SM), toPosition(R.End, SM)};
-}
-
-struct CapturedDiags {
-  std::function<void(const llvm::SMDiagnostic &)> callback() {
-    return [this](const llvm::SMDiagnostic &D) {
-      Diagnostics.emplace_back();
-      Diag &Out = Diagnostics.back();
-      Out.Message = D.getMessage().str();
-      Out.Kind = D.getKind();
-      Out.Pos.line = D.getLineNo() - 1;
-      Out.Pos.character = D.getColumnNo(); // Zero-based - bug in SourceMgr?
-      if (!D.getRanges().empty()) {
-        const auto &R = D.getRanges().front();
-        Out.Range.emplace();
-        Out.Range->start.line = Out.Range->end.line = Out.Pos.line;
-        Out.Range->start.character = R.first;
-        Out.Range->end.character = R.second;
-      }
-    };
-  }
-  struct Diag {
-    std::string Message;
-    llvm::SourceMgr::DiagKind Kind;
-    Position Pos;
-    llvm::Optional<Range> Range;
-
-    friend void PrintTo(const Diag &D, std::ostream *OS) {
-      *OS << (D.Kind == llvm::SourceMgr::DK_Error ? "error: " : "warning: ")
-          << D.Message << "@" << llvm::to_string(D.Pos);
-    }
-  };
-  std::vector<Diag> Diagnostics;
-};
-
-MATCHER_P(DiagMessage, M, "") { return arg.Message == M; }
-MATCHER_P(DiagKind, K, "") { return arg.Kind == K; }
-MATCHER_P(DiagPos, P, "") { return arg.Pos == P; }
-MATCHER_P(DiagRange, R, "") { return arg.Range == R; }
-
 TEST(ParseYAML, SyntacticForms) {
   CapturedDiags Diags;
   const char *YAML = R"yaml(