Replace MSVCR120.dll fallback with the Universal CRT on Windows.

changelog/dmd.ucrt-runtime.dd

@@ -0,0 +1,22 @@
+UCRT is now used for the C runtime on Windows
+
+DMD on Windows now uses the Universal CRT (UCRT) for its C runtime in all cases.
+
+For installations with Visual Studio 2015 (or later) or the Windows 10 SDK, the default
+C runtime is unchanged: it remains $(TT libcmt), which statically links the UCRT.
+
+When no such Visual C installation is found, DMD now falls back on the UCRT-based
+libraries bundled in the MinGW folder shipped with DMD ($(TT ucrtbase.lib) together with
+$(TT vcruntime140.lib)), instead of the old $(TT msvcrt120) runtime which did not support
+modern format specifiers such as `%zd`. This fallback is fully supported.
+
+If no UCRT-capable toolchain is detected and no $(TT -mscrtlib) switch is given, DMD now
+reports a clear error instead of silently producing a non-working link.
+
+Detection of Visual Studio versions older than 2015 (2008 - 2013) is deprecated, because
+those versions predate the UCRT. Falling back on such an installation emits a deprecation
+warning.
+
+The $(TT -mscrtlib) switch is unchanged and can still be used to select any C runtime,
+including $(TT msvcrt) (dynamic), the debug variants $(TT libcmtd) / $(TT msvcrtd), or a
+legacy $(TT msvcrtNNN) runtime.

compiler/src/dmd/cli.d

@@ -725,13 +725,13 @@ dmd -cov -unittest myprog.d
             "If building MS-COFF object files when targeting Windows, embed a reference to
             the given C runtime library $(I libname) into the object file containing `main`,
             `DllMain` or `WinMain` for automatic linking. The default is $(TT libcmt)
-            (release version with static linkage), the other usual alternatives are
-            $(TT libcmtd), $(TT msvcrt) and $(TT msvcrtd).
-            If no Visual C installation is detected, a wrapper for the redistributable
-            VC2010 dynamic runtime library and mingw based platform import libraries will
-            be linked instead using the LLD linker provided by the LLVM project.
-            The detection can be skipped explicitly if $(TT msvcrt120) is specified as
-            $(I libname).
+            (release version with static linkage), which uses the Universal CRT (UCRT)
+            shipped with Visual Studio 2015 (or later) and the Windows 10 SDK. The other
+            usual alternatives are $(TT libcmtd), $(TT msvcrt) and $(TT msvcrtd).
+            If no Universal CRT capable Visual C installation is detected, the UCRT-based
+            libraries bundled with DMD (the mingw $(TT ucrtbase.lib) and
+            $(TT vcruntime140.lib)) are linked instead using the LLD linker provided by the
+            LLVM project.
             If $(I libname) is empty, no C runtime library is automatically linked in.",
             TargetOS.Windows,
         ),

compiler/src/dmd/link.d

@@ -286,9 +286,13 @@ public int runLINK(bool verbose, ErrorSink eSink)
             }
 
             VSOptions vsopt;
-            // if a runtime library (msvcrtNNN.lib) from the mingw folder is selected explicitly, do not detect VS and use lld
-            if (driverParams.mscrtlib.length <= 6 ||
-                driverParams.mscrtlib[0..6] != "msvcrt" || !isdigit(driverParams.mscrtlib[6]))
+            // If a MinGW-folder runtime is selected — the UCRT-based `ucrtbase`, or a
+            // legacy `msvcrtNNN` such as `msvcrt120` — do not detect Visual Studio; use
+            // lld-link with the MinGW libraries instead.
+            const isMingwRuntime = driverParams.mscrtlib == "ucrtbase" ||
+                (driverParams.mscrtlib.length > 6 &&
+                 driverParams.mscrtlib[0 .. 6] == "msvcrt" && isdigit(driverParams.mscrtlib[6]));
+            if (!isMingwRuntime)
                 vsopt.initialize();
 
             const(char)* linkcmd = getenv(target.isX86_64 ? "LINKCMD64" : "LINKCMD");
@@ -301,11 +305,15 @@ public int runLINK(bool verbose, ErrorSink eSink)
             {
                 // object files not SAFESEH compliant, but LLD is more picky than MS link
                 cmdbuf.writestring(" /SAFESEH:NO");
-                // if we are using LLD as a fallback, don't link to any VS libs even if
-                // we detected a VS installation and they are present
-                vsopt.uninitialize();
+                // lld-link is used here as a generic linker fallback; keep any detected
+                // VS/UCRT library paths so the Universal CRT can still be linked.
             }
 
+            // The UCRT-based MinGW fallback runtime links ucrtbase.lib via the object file's
+            // /DEFAULTLIB directive; it also needs the VC runtime library.
+            if (driverParams.mscrtlib == "ucrtbase")
+                cmdbuf.writestring(" vcruntime140.lib");
+
             if (const(char)* lflags = vsopt.linkOptions(target.isX86_64))
             {
                 cmdbuf.writeByte(' ');

compiler/src/dmd/main.d

@@ -1194,7 +1194,23 @@ void reconcileLinkRunLib(ref Param params, size_t numSrcFiles, const char[] obj_
             {
                 VSOptions vsopt;
                 vsopt.initialize();
-                driverParams.mscrtlib = vsopt.defaultRuntimeLibrary(target.isX86_64).toDString;
+                if (const rtlib = vsopt.defaultRuntimeLibrary(target.isX86_64))
+                    driverParams.mscrtlib = rtlib.toDString;
+                else
+                {
+                    // No UCRT-capable Visual C installation (VS2015+ or the Windows SDK
+                    // with the Universal CRT) and no MinGW fallback libraries were found.
+                    if (driverParams.link)
+                        eSink.error(Loc.initial, "no compatible C runtime found; install Visual Studio 2015 or later, or the Windows SDK with the Universal CRT, or specify the runtime with `-mscrtlib`");
+                    // still embed a name in the object file so it can be linked elsewhere
+                    driverParams.mscrtlib = "libcmt";
+                }
+
+                // @@@ Deprecated v2.117
+                // Deprecated in 2.113
+                // Remove this when the feature is removed from the language
+                if (vsopt.usedDeprecatedVSVersion)
+                    eSink.deprecation(Loc.initial, "Visual Studio versions prior to 2015 are deprecated because they lack the Universal CRT (UCRT); install Visual Studio 2015 or later");
             }
             else
             {