use cm::string_view for language extension lookups

Once the list of extensions is build the set is just a copy of the vector and
not modified anymore. Use a string_view for the members of the set, which saves
a small amount of memory. It also makes possible to use string_views as lookup
keys, so the callers do not need to create copies for the extensions anymore.

Source/cmAuxSourceDirectoryCommand.cxx

@@ -6,6 +6,8 @@
 #include <cstddef>
 #include <utility>
 
+#include <cm/string_view>
+
 #include "cmsys/Directory.hxx"
 
 #include "cmExecutionStatus.h"
@@ -50,11 +52,10 @@ bool cmAuxSourceDirectoryCommand(std::vector<std::string> const& args,
       // Split the filename into base and extension
       std::string::size_type dotpos = file.rfind('.');
       if (dotpos != std::string::npos) {
-        std::string ext = file.substr(dotpos + 1);
-        std::string base = file.substr(0, dotpos);
+        auto ext = cm::string_view(file).substr(dotpos + 1);
         // Process only source files
         auto cm = mf.GetCMakeInstance();
-        if (!base.empty() && cm->IsSourceExtension(ext)) {
+        if (dotpos > 0 && cm->IsSourceExtension(ext)) {
           std::string fullname = cmStrCat(templateDirectory, '/', file);
           // add the file as a class file so
           // depends can be done

Source/cmSourceFileLocation.cxx

@@ -4,6 +4,8 @@
 
 #include <cassert>
 
+#include <cm/string_view>
+
 #include "cmGlobalGenerator.h"
 #include "cmMakefile.h"
 #include "cmMessageType.h"
@@ -152,7 +154,7 @@ bool cmSourceFileLocation::MatchesAmbiguousExtension(
 
   // Only a fixed set of extensions will be tried to match a file on
   // disk.  One of these must match if loc refers to this source file.
-  std::string const& ext = this->Name.substr(loc.Name.size() + 1);
+  auto ext = cm::string_view(this->Name).substr(loc.Name.size() + 1);
   cmMakefile const* mf = this->Makefile;
   auto cm = mf->GetCMakeInstance();
   return cm->IsSourceExtension(ext) || cm->IsHeaderExtension(ext);

Source/cmake.cxx

@@ -1978,9 +1978,10 @@ std::string cmake::StripExtension(const std::string& file) const
 {
   auto dotpos = file.rfind('.');
   if (dotpos != std::string::npos) {
-    auto ext = file.substr(dotpos + 1);
 #if defined(_WIN32) || defined(__APPLE__)
-    ext = cmSystemTools::LowerCase(ext);
+    auto ext = cmSystemTools::LowerCase(file.substr(dotpos + 1));
+#else
+    auto ext = cm::string_view(file).substr(dotpos + 1);
 #endif
     if (this->IsSourceExtension(ext) || this->IsHeaderExtension(ext)) {
       return file.substr(0, dotpos);

Source/cmake.h

@@ -16,6 +16,8 @@
 #include <utility>
 #include <vector>
 
+#include <cm/string_view>
+
 #include "cmGeneratedFileStream.h"
 #include "cmInstalledFile.h"
 #include "cmListFileCache.h"
@@ -138,13 +140,13 @@ public:
 
   struct FileExtensions
   {
-    bool Test(std::string const& ext) const
+    bool Test(cm::string_view ext) const
     {
       return (this->unordered.find(ext) != this->unordered.end());
     }
 
     std::vector<std::string> ordered;
-    std::unordered_set<std::string> unordered;
+    std::unordered_set<cm::string_view> unordered;
   };
 
   using InstalledFilesMap = std::map<std::string, cmInstalledFile>;
@@ -266,7 +268,7 @@ public:
     return this->SourceFileExtensions.ordered;
   }
 
-  bool IsSourceExtension(const std::string& ext) const
+  bool IsSourceExtension(cm::string_view ext) const
   {
     return this->SourceFileExtensions.Test(ext);
   }
@@ -276,7 +278,7 @@ public:
     return this->HeaderFileExtensions.ordered;
   }
 
-  bool IsHeaderExtension(const std::string& ext) const
+  bool IsHeaderExtension(cm::string_view ext) const
   {
     return this->HeaderFileExtensions.Test(ext);
   }
@@ -286,7 +288,7 @@ public:
     return this->CudaFileExtensions.ordered;
   }
 
-  bool IsCudaExtension(const std::string& ext) const
+  bool IsCudaExtension(cm::string_view ext) const
   {
     return this->CudaFileExtensions.Test(ext);
   }
@@ -296,7 +298,7 @@ public:
     return this->FortranFileExtensions.ordered;
   }
 
-  bool IsFortranExtension(const std::string& ext) const
+  bool IsFortranExtension(cm::string_view ext) const
   {
     return this->FortranFileExtensions.Test(ext);
   }