Make StringRef's operator std::string explicit

This way it is explicit when there is a `StringRef` -> `std::string`
conversion and makes it easier to look for allocations that could
be avoided.

Doing this has already removed one allocation per registered test
case, as there was a completely pointless `StringRef` -> `std::string`
conversion when parsing tags of a test case.

projects/SelfTest/Baselines/compact.sw.approved.txt

@@ -1043,8 +1043,6 @@ String.tests.cpp:<line number>: passed: stdStr == "a stringref" for: "a stringre
 String.tests.cpp:<line number>: passed: stdStr.size() == sr.size() for: 11 == 11
 String.tests.cpp:<line number>: passed: stdStr == "a stringref" for: "a stringref" == "a stringref"
 String.tests.cpp:<line number>: passed: stdStr.size() == sr.size() for: 11 == 11
-String.tests.cpp:<line number>: passed: stdStr == "a stringref" for: "a stringref" == "a stringref"
-String.tests.cpp:<line number>: passed: stdStr.size() == sr.size() for: 11 == 11
 ToStringChrono.tests.cpp:<line number>: passed: minute == seconds for: 1 m == 60 s
 ToStringChrono.tests.cpp:<line number>: passed: hour != seconds for: 1 h != 60 s
 ToStringChrono.tests.cpp:<line number>: passed: micro != milli for: 1 us != 1 ms

projects/SelfTest/Baselines/console.std.approved.txt

@@ -1381,5 +1381,5 @@ due to unexpected exception with message:
 
 ===============================================================================
 test cases:  300 |  226 passed |  70 failed |  4 failed as expected
-assertions: 1559 | 1407 passed | 131 failed | 21 failed as expected
+assertions: 1557 | 1405 passed | 131 failed | 21 failed as expected
 

projects/SelfTest/Baselines/console.sw.approved.txt

@@ -7783,24 +7783,6 @@ String.tests.cpp:<line number>: PASSED:
 with expansion:
   17 == 17
 
--------------------------------------------------------------------------------
-StringRef
-  to std::string
-  implicitly constructed
--------------------------------------------------------------------------------
-String.tests.cpp:<line number>
-...............................................................................
-
-String.tests.cpp:<line number>: PASSED:
-  REQUIRE( stdStr == "a stringref" )
-with expansion:
-  "a stringref" == "a stringref"
-
-String.tests.cpp:<line number>: PASSED:
-  REQUIRE( stdStr.size() == sr.size() )
-with expansion:
-  11 == 11
-
 -------------------------------------------------------------------------------
 StringRef
   to std::string
@@ -12462,5 +12444,5 @@ Misc.tests.cpp:<line number>: PASSED:
 
 ===============================================================================
 test cases:  300 |  210 passed |  86 failed |  4 failed as expected
-assertions: 1576 | 1407 passed | 148 failed | 21 failed as expected
+assertions: 1574 | 1405 passed | 148 failed | 21 failed as expected
 

projects/SelfTest/Baselines/junit.sw.approved.txt

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <testsuitesloose text artifact
 >
-  <testsuite name="<exe-name>" errors="17" failures="132" tests="1577" hostname="tbd" time="{duration}" timestamp="{iso8601-timestamp}">
+  <testsuite name="<exe-name>" errors="17" failures="132" tests="1575" hostname="tbd" time="{duration}" timestamp="{iso8601-timestamp}">
     <properties>
       <property name="filters" value="~[!nonportable]~[!benchmark]~[approvals]"/>
       <property name="random-seed" value="1"/>
@@ -726,7 +726,6 @@ Matchers.tests.cpp:<line number>
     <testcase classname="<exe-name>.global" name="StringRef/from std::string/implicitly constructed" time="{duration}"/>
     <testcase classname="<exe-name>.global" name="StringRef/from std::string/explicitly constructed" time="{duration}"/>
     <testcase classname="<exe-name>.global" name="StringRef/from std::string/assigned" time="{duration}"/>
-    <testcase classname="<exe-name>.global" name="StringRef/to std::string/implicitly constructed" time="{duration}"/>
     <testcase classname="<exe-name>.global" name="StringRef/to std::string/explicitly constructed" time="{duration}"/>
     <testcase classname="<exe-name>.global" name="StringRef/to std::string/assigned" time="{duration}"/>
     <testcase classname="<exe-name>.global" name="Stringifying std::chrono::duration helpers" time="{duration}"/>

projects/SelfTest/Baselines/xml.sw.approved.txt

@@ -9601,28 +9601,6 @@ Message from section two
         </Section>
         <OverallResults successes="2" failures="0" expectedFailures="0"/>
       </Section>
-      <Section name="to std::string" filename="projects/<exe-name>/IntrospectiveTests/String.tests.cpp" >
-        <Section name="implicitly constructed" filename="projects/<exe-name>/IntrospectiveTests/String.tests.cpp" >
-          <Expression success="true" type="REQUIRE" filename="projects/<exe-name>/IntrospectiveTests/String.tests.cpp" >
-            <Original>
-              stdStr == "a stringref"
-            </Original>
-            <Expanded>
-              "a stringref" == "a stringref"
-            </Expanded>
-          </Expression>
-          <Expression success="true" type="REQUIRE" filename="projects/<exe-name>/IntrospectiveTests/String.tests.cpp" >
-            <Original>
-              stdStr.size() == sr.size()
-            </Original>
-            <Expanded>
-              11 == 11
-            </Expanded>
-          </Expression>
-          <OverallResults successes="2" failures="0" expectedFailures="0"/>
-        </Section>
-        <OverallResults successes="2" failures="0" expectedFailures="0"/>
-      </Section>
       <Section name="to std::string" filename="projects/<exe-name>/IntrospectiveTests/String.tests.cpp" >
         <Section name="explicitly constructed" filename="projects/<exe-name>/IntrospectiveTests/String.tests.cpp" >
           <Expression success="true" type="REQUIRE" filename="projects/<exe-name>/IntrospectiveTests/String.tests.cpp" >
@@ -14834,7 +14812,7 @@ loose text artifact
       </Section>
       <OverallResult success="true"/>
     </TestCase>
-    <OverallResults successes="1407" failures="149" expectedFailures="21"/>
+    <OverallResults successes="1405" failures="149" expectedFailures="21"/>
   </Group>
-  <OverallResults successes="1407" failures="148" expectedFailures="21"/>
+  <OverallResults successes="1405" failures="148" expectedFailures="21"/>
 </Catch>

projects/SelfTest/IntrospectiveTests/String.tests.cpp

@@ -142,11 +142,6 @@ TEST_CASE( "StringRef", "[Strings][StringRef]" ) {
     SECTION( "to std::string" ) {
         StringRef sr = "a stringref";
 
-        SECTION( "implicitly constructed" ) {
-            std::string stdStr = sr;
-            REQUIRE( stdStr == "a stringref" );
-            REQUIRE( stdStr.size() == sr.size() );
-        }
         SECTION( "explicitly constructed" ) {
             std::string stdStr( sr );
             REQUIRE( stdStr == "a stringref" );
@@ -154,7 +149,7 @@ TEST_CASE( "StringRef", "[Strings][StringRef]" ) {
         }
         SECTION( "assigned" ) {
             std::string stdStr;
-            stdStr = sr;
+            stdStr = static_cast<std::string>(sr);
             REQUIRE( stdStr == "a stringref" );
             REQUIRE( stdStr.size() == sr.size() );
         }