[C++20][Modules][HU 2/5] Support searching Header Units in user or system search paths.

This is support for the user-facing options to create importable header units
from headers in the user or system search paths (or to be given an absolute path).

This means that an incomplete header path will be passed by the driver and the
lookup carried out using the search paths present when the front end is run.

To support this, we introduce file fypes for c++-{user,system,header-unit}-header.
These terms are the same as the ones used by GCC, to minimise the differences for
tooling (and users).

The preprocessor checks for headers before issuing a warning for
"#pragma once" in a header build.  We ensure that the importable header units
are recognised as headers in order to avoid such warnings.

Differential Revision: https://reviews.llvm.org/D121096

1 files changed, 55 insertions, 7 deletions

diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index be7828b..5bb480a 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -2567,6 +2567,20 @@ static void GenerateFrontendArgs(const FrontendOptions &Opts,
     StringRef Preprocessed = Opts.DashX.isPreprocessed() ? "-cpp-output" : "";
     StringRef ModuleMap =
         Opts.DashX.getFormat() == InputKind::ModuleMap ? "-module-map" : "";
+    StringRef HeaderUnit = "";
+    switch (Opts.DashX.getHeaderUnitKind()) {
+    case InputKind::HeaderUnit_None:
+      break;
+    case InputKind::HeaderUnit_User:
+      HeaderUnit = "-user";
+      break;
+    case InputKind::HeaderUnit_System:
+      HeaderUnit = "-system";
+      break;
+    case InputKind::HeaderUnit_Abs:
+      HeaderUnit = "-header-unit";
+      break;
+    }
     StringRef Header = IsHeader ? "-header" : "";
 
     StringRef Lang;
@@ -2611,7 +2625,8 @@ static void GenerateFrontendArgs(const FrontendOptions &Opts,
       break;
     }
 
-    GenerateArg(Args, OPT_x, Lang + Header + ModuleMap + Preprocessed, SA);
+    GenerateArg(Args, OPT_x,
+                Lang + HeaderUnit + Header + ModuleMap + Preprocessed, SA);
   }
 
   // OPT_INPUT has a unique class, generate it directly.
@@ -2756,13 +2771,32 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
   if (const Arg *A = Args.getLastArg(OPT_x)) {
     StringRef XValue = A->getValue();
 
-    // Parse suffixes: '<lang>(-header|[-module-map][-cpp-output])'.
+    // Parse suffixes:
+    // '<lang>(-[{header-unit,user,system}-]header|[-module-map][-cpp-output])'.
     // FIXME: Supporting '<lang>-header-cpp-output' would be useful.
     bool Preprocessed = XValue.consume_back("-cpp-output");
     bool ModuleMap = XValue.consume_back("-module-map");
-    IsHeaderFile = !Preprocessed && !ModuleMap &&
-                   XValue != "precompiled-header" &&
-                   XValue.consume_back("-header");
+    // Detect and consume the header indicator.
+    bool IsHeader =
+        XValue != "precompiled-header" && XValue.consume_back("-header");
+
+    // If we have c++-{user,system}-header, that indicates a header unit input
+    // likewise, if the user put -fmodule-header together with a header with an
+    // absolute path (header-unit-header).
+    InputKind::HeaderUnitKind HUK = InputKind::HeaderUnit_None;
+    if (IsHeader || Preprocessed) {
+      if (XValue.consume_back("-header-unit"))
+        HUK = InputKind::HeaderUnit_Abs;
+      else if (XValue.consume_back("-system"))
+        HUK = InputKind::HeaderUnit_System;
+      else if (XValue.consume_back("-user"))
+        HUK = InputKind::HeaderUnit_User;
+    }
+
+    // The value set by this processing is an un-preprocessed source which is
+    // not intended to be a module map or header unit.
+    IsHeaderFile = IsHeader && !Preprocessed && !ModuleMap &&
+                   HUK == InputKind::HeaderUnit_None;
 
     // Principal languages.
     DashX = llvm::StringSwitch<InputKind>(XValue)
@@ -2779,14 +2813,16 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
 
     // "objc[++]-cpp-output" is an acceptable synonym for
     // "objective-c[++]-cpp-output".
-    if (DashX.isUnknown() && Preprocessed && !IsHeaderFile && !ModuleMap)
+    if (DashX.isUnknown() && Preprocessed && !IsHeaderFile && !ModuleMap &&
+        HUK == InputKind::HeaderUnit_None)
       DashX = llvm::StringSwitch<InputKind>(XValue)
                   .Case("objc", Language::ObjC)
                   .Case("objc++", Language::ObjCXX)
                   .Default(Language::Unknown);
 
     // Some special cases cannot be combined with suffixes.
-    if (DashX.isUnknown() && !Preprocessed && !ModuleMap && !IsHeaderFile)
+    if (DashX.isUnknown() && !Preprocessed && !IsHeaderFile && !ModuleMap &&
+        HUK == InputKind::HeaderUnit_None)
       DashX = llvm::StringSwitch<InputKind>(XValue)
                   .Case("cpp-output", InputKind(Language::C).getPreprocessed())
                   .Case("assembler-with-cpp", Language::Asm)
@@ -2801,6 +2837,12 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
 
     if (Preprocessed)
       DashX = DashX.getPreprocessed();
+    // A regular header is considered mutually exclusive with a header unit.
+    if (HUK != InputKind::HeaderUnit_None) {
+      DashX = DashX.withHeaderUnit(HUK);
+      IsHeaderFile = true;
+    } else if (IsHeaderFile)
+      DashX = DashX.getHeader();
     if (ModuleMap)
       DashX = DashX.withFormat(InputKind::ModuleMap);
   }
@@ -2810,6 +2852,11 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
   Opts.Inputs.clear();
   if (Inputs.empty())
     Inputs.push_back("-");
+
+  if (DashX.getHeaderUnitKind() != InputKind::HeaderUnit_None &&
+      Inputs.size() > 1)
+    Diags.Report(diag::err_drv_header_unit_extra_inputs) << Inputs[1];
+
   for (unsigned i = 0, e = Inputs.size(); i != e; ++i) {
     InputKind IK = DashX;
     if (IK.isUnknown()) {
@@ -3857,6 +3904,7 @@ bool CompilerInvocation::ParseLangArgs(LangOptions &Opts, ArgList &Args,
   }
   if (Opts.FastRelaxedMath)
     Opts.setDefaultFPContractMode(LangOptions::FPM_Fast);
+
   llvm::sort(Opts.ModuleFeatures);
 
   // -mrtd option