dist: Make Windows installer uninstall first. Closes rust-lang#9563

This will remove existing files before installing new ones. Note
that I took some code with no license from stackoverflow, as
indicated in comments.

mk/dist.mk

@@ -118,7 +118,7 @@ PKG_EXE = dist/$(PKG_NAME)-install.exe
 %.ico: $(S)src/etc/pkg/%.ico
 	cp $< $@
 
-$(PKG_EXE): rust.iss modpath.iss LICENSE.txt rust-logo.ico \
+$(PKG_EXE): rust.iss modpath.iss upgrade.iss LICENSE.txt rust-logo.ico \
             $(CSREQ3_T_$(CFG_BUILD)_H_$(CFG_BUILD)) \
             dist-prepare-win
 	$(CFG_PYTHON) $(S)src/etc/copy-runtime-deps.py tmp/dist/win/bin

src/etc/pkg/modpath.iss

@@ -164,7 +164,7 @@ begin
 end;
 
 
-procedure CurStepChanged(CurStep: TSetupStep);
+procedure ModPathCurStepChanged(CurStep: TSetupStep);
 var
 	taskname:	String;
 begin

src/etc/pkg/rust.iss

@@ -49,4 +49,15 @@ begin
 	setArrayLength(Result, 1)
 	Result[0] := ExpandConstant('{app}\bin');
 end;
-#include "modpath.iss"
+
+#include "modpath.iss"
+#include "upgrade.iss"
+
+// Both modpath.iss and upgrade.iss want to overload CurStepChanged.
+// This version does the overload then delegates to each.
+
+procedure CurStepChanged(CurStep: TSetupStep);
+begin
+  UpgradeCurStepChanged(CurStep);
+  ModPathCurStepChanged(CurStep);
+end;

src/etc/pkg/upgrade.iss

@@ -0,0 +1,61 @@
+// The following code taken from https://stackoverflow.com/questions/2000296/innosetup-how-to-automatically-uninstall-previous-installed-version
+// It performs upgrades by running the uninstaller before the install
+
+/////////////////////////////////////////////////////////////////////
+function GetUninstallString(): String;
+var
+  sUnInstPath: String;
+  sUnInstallString: String;
+begin
+  sUnInstPath := ExpandConstant('Software\Microsoft\Windows\CurrentVersion\Uninstall\Rust_is1');
+  sUnInstallString := '';
+  if not RegQueryStringValue(HKLM, sUnInstPath, 'UninstallString', sUnInstallString) then
+    RegQueryStringValue(HKCU, sUnInstPath, 'UninstallString', sUnInstallString);
+  Result := sUnInstallString;
+end;
+
+
+/////////////////////////////////////////////////////////////////////
+function IsUpgrade(): Boolean;
+begin
+  Result := (GetUninstallString() <> '');
+end;
+
+
+/////////////////////////////////////////////////////////////////////
+function UnInstallOldVersion(): Integer;
+var
+  sUnInstallString: String;
+  iResultCode: Integer;
+begin
+// Return Values:
+// 1 - uninstall string is empty
+// 2 - error executing the UnInstallString
+// 3 - successfully executed the UnInstallString
+
+  // default return value
+  Result := 0;
+
+  // get the uninstall string of the old app
+  sUnInstallString := GetUninstallString();
+  if sUnInstallString <> '' then begin
+    sUnInstallString := RemoveQuotes(sUnInstallString);
+    if Exec(sUnInstallString, '/SILENT /NORESTART /SUPPRESSMSGBOXES','', SW_HIDE, ewWaitUntilTerminated, iResultCode) then
+      Result := 3
+    else
+      Result := 2;
+  end else
+    Result := 1;
+end;
+
+/////////////////////////////////////////////////////////////////////
+procedure UpgradeCurStepChanged(CurStep: TSetupStep);
+begin
+  if (CurStep=ssInstall) then
+  begin
+    if (IsUpgrade()) then
+    begin
+      UnInstallOldVersion();
+    end;
+  end;
+end;