fix Issue 16798 - Extend -Ipath switch to -Imodule=path so path heira…

…rchy doesn't have to match package heirarchy

src/dmodule.d

@@ -417,10 +417,42 @@ extern (C++) final class Module : Package
         if (packages && packages.dim)
         {
             OutBuffer buf;
+            OutBuffer dotmods;
+            auto ms = global.params.modFileAliasStrings;
+            const msdim = ms ? ms.dim : 0;
+
+            void checkModFileAlias(const(char)* p)
+            {
+                /* Check and replace the contents of buf[] with
+                 * an alias string from global.params.modFileAliasStrings[]
+                 */
+                dotmods.writestring(p);
+            Lmain:
+                for (size_t j = msdim; j--;)
+                {
+                    const m = (*ms)[j];
+                    const q = strchr(m, '=');
+                    assert(q);
+                    if (dotmods.offset <= q - m && memcmp(dotmods.peekString(), m, q - m) == 0)
+                    {
+                        buf.reset();
+                        auto qlen = strlen(q + 1);
+                        if (qlen && (q[qlen] == '/' || q[qlen] == '\\'))
+                            --qlen;             // remove trailing separator
+                        buf.writestring(q[1 .. qlen + 1]);
+                        break Lmain;            // last matching entry in ms[] wins
+                    }
+                }
+                dotmods.writeByte('.');
+            }
+
             for (size_t i = 0; i < packages.dim; i++)
             {
                 Identifier pid = (*packages)[i];
-                buf.writestring(pid.toChars());
+                const p = pid.toChars();
+                buf.writestring(p);
+                if (msdim)
+                    checkModFileAlias(p);
                 version (Windows)
                 {
                     buf.writeByte('\\');
@@ -431,6 +463,8 @@ extern (C++) final class Module : Package
                 }
             }
             buf.writestring(filename);
+            if (msdim)
+                checkModFileAlias(filename);
             buf.writeByte(0);
             filename = buf.extractData();
         }

src/globals.d

@@ -120,6 +120,7 @@ struct Param
     BOUNDSCHECK useArrayBounds;
 
     const(char)* argv0;                 // program name
+    Array!(const(char)*)* modFileAliasStrings; // array of char*'s of -I module filename alias strings
     Array!(const(char)*)* imppath;      // array of char*'s of where to look for import modules
     Array!(const(char)*)* fileImppath;  // array of char*'s of where to look for file import modules
     const(char)* objdir;                // .obj/.lib file output directory

src/globals.h

@@ -102,6 +102,7 @@ struct Param
     BOUNDSCHECK useArrayBounds;
 
     const char *argv0;    // program name
+    Array<const char *> *modFileAliasStrings; // array of char*'s of -I module filename alias strings
     Array<const char *> *imppath;     // array of char*'s of where to look for import modules
     Array<const char *> *fileImppath; // array of char*'s of where to look for file import modules
     const char *objdir;   // .obj/.lib file output directory

src/mars.d

@@ -149,6 +149,7 @@ Where:
   -map             generate linker .map file
   -mavx            use AVX instruction set" ~
   "%s" /* placeholder for mscrtlib */ ~ "
+  -mv=<package.module>=<filespec>  use <filespec> as source file for <package.module>
   -noboundscheck   no array bounds checking (deprecated, use -boundscheck=off)
   -O               optimize
   -o-              do not write object file
@@ -794,6 +795,17 @@ Language changes listed by -transition=id:
                     global.params.imppath = new Strings();
                 global.params.imppath.push(p + 2 + (p[2] == '='));
             }
+            else if (p[1] == 'm' && p[2] == 'v' && p[3] == '=')
+            {
+                if (p[4] && strchr(p + 5, '='))
+                {
+                    if (!global.params.modFileAliasStrings)
+                        global.params.modFileAliasStrings = new Strings();
+                    global.params.modFileAliasStrings.push(p + 4);
+                }
+                else
+                    goto Lerror;
+            }
             else if (p[1] == 'J')
             {
                 if (!global.params.fileImppath)

test/compilable/imports/imp16798.d

@@ -0,0 +1,4 @@
+
+module its.a.dessert.topping;
+
+pragma(msg, "it's a dessert topping");

test/compilable/imports/wax16798.d

@@ -0,0 +1,4 @@
+module its.a.floorwax.wax16798;
+
+pragma(msg, "it's a floor wax");
+

test/compilable/test16798.d

@@ -0,0 +1,13 @@
+/*
+REQUIRED_ARGS: -mv=its.a.dessert.topping=imports/imp16798.d -mv=its.a.floorwax=imports/
+PERMUTE_ARGS:
+TEST_OUTPUT:
+---
+it's a floor wax
+it's a dessert topping
+---
+*/
+
+import its.a.floorwax.wax16798;
+import its.a.dessert.topping;
+