Looks like that package was removed from Debian. I wouldn't be surprised if it was due to not following Debian policy.
Here are the first 100 entries:
$ apt-cache show librust-winapi-dev |grep ^Provides: |sed "s/Provides: "// |sed "s/, /\\n/g" |sed "s/ .*//g" |sort -V |head -100
librust-winapi+accctrl-dev
librust-winapi+aclapi-dev
librust-winapi+activation-dev
librust-winapi+adhoc-dev
librust-winapi+appmgmt-dev
librust-winapi+audioclient-dev
librust-winapi+audiosessiontypes-dev
librust-winapi+avrt-dev
librust-winapi+basetsd-dev
librust-winapi+bcrypt-dev
librust-winapi+bits1-5-dev
librust-winapi+bits2-0-dev
librust-winapi+bits2-5-dev
librust-winapi+bits3-0-dev
librust-winapi+bits4-0-dev
librust-winapi+bits5-0-dev
librust-winapi+bits10-1-dev
librust-winapi+bitscfg-dev
librust-winapi+bitsmsg-dev
librust-winapi+bits-dev
librust-winapi+bluetoothapis-dev
librust-winapi+bluetoothleapis-dev
librust-winapi+bthdef-dev
librust-winapi+bthioctl-dev
librust-winapi+bthledef-dev
librust-winapi+bthsdpdef-dev
librust-winapi+bugcodes-dev
librust-winapi+cderr-dev
librust-winapi+cfgmgr32-dev
librust-winapi+cfg-dev
librust-winapi+cguid-dev
librust-winapi+combaseapi-dev
librust-winapi+coml2api-dev
librust-winapi+commapi-dev
librust-winapi+commctrl-dev
librust-winapi+commdlg-dev
librust-winapi+commoncontrols-dev
librust-winapi+consoleapi-dev
librust-winapi+corecrt-dev
librust-winapi+corsym-dev
librust-winapi+d2d1effectauthor-dev
librust-winapi+d2d1effects-1-dev
librust-winapi+d2d1effects-2-dev
librust-winapi+d2d1effects-dev
librust-winapi+d2d1svg-dev
librust-winapi+d2d1-1-dev
librust-winapi+d2d1-2-dev
librust-winapi+d2d1-3-dev
librust-winapi+d2d1-dev
librust-winapi+d2dbasetypes-dev
librust-winapi+d3d9caps-dev
librust-winapi+d3d9types-dev
librust-winapi+d3d9-dev
librust-winapi+d3d10effect-dev
librust-winapi+d3d10misc-dev
librust-winapi+d3d10sdklayers-dev
librust-winapi+d3d10shader-dev
librust-winapi+d3d10-1shader-dev
librust-winapi+d3d10-1-dev
librust-winapi+d3d10-dev
librust-winapi+d3d11on12-dev
librust-winapi+d3d11sdklayers-dev
librust-winapi+d3d11shader-dev
librust-winapi+d3d11tokenizedprogramformat-dev
librust-winapi+d3d11-1-dev
librust-winapi+d3d11-2-dev
librust-winapi+d3d11-3-dev
librust-winapi+d3d11-4-dev
librust-winapi+d3d11-dev
librust-winapi+d3d12sdklayers-dev
librust-winapi+d3d12shader-dev
librust-winapi+d3d12-dev
librust-winapi+d3dcommon-dev
librust-winapi+d3dcompiler-dev
librust-winapi+d3dcsx-dev
librust-winapi+d3dkmdt-dev
librust-winapi+d3dkmthk-dev
librust-winapi+d3dukmdt-dev
librust-winapi+d3dx10core-dev
librust-winapi+d3dx10math-dev
librust-winapi+d3dx10mesh-dev
librust-winapi+d3d-dev
librust-winapi+datetimeapi-dev
librust-winapi+davclnt-dev
librust-winapi+dbghelp-dev
librust-winapi+dbt-dev
librust-winapi+dcommon-dev
librust-winapi+dcompanimation-dev
librust-winapi+dcomptypes-dev
librust-winapi+dcomp-dev
librust-winapi+dde-dev
librust-winapi+ddrawint-dev
librust-winapi+ddrawi-dev
librust-winapi+ddraw-dev
librust-winapi+debugapi-dev
librust-winapi+debug-dev
librust-winapi+default-dev
librust-winapi+devguid-dev
librust-winapi+devicetopology-dev
librust-winapi+devpkey-dev
I haven't reviewed the relevant policies lately, but I suspect those should have been separate packages, not thousands of entries in a single package's Provides: field.