Use Raster Layer as a Mask over a polygon in QGIS

Question

I have a raster layer, as seen on the attached image, that works as a background of my working area. The red line, that marks the limit between the two countries, is a .shp file that, naturally, goes beyond the area of interest, the aforementioned raster.

What I am trying to achieve is a way of displaying the red line only when it overlays the raster, not outside it.

Using Clip or Intersect inside the Vector - Geoprocessing Tools menu don't work here, as they only work Vector on Vector; I tried creating a Scratch layer with a polygon of the same shape as the raster. In this way, I can clip the border line, but it creates, naturally, a new line around the raster.

Is there a way to mask the red line as desired? Alternatively, is there a way to "cut" the polygon in two parts, by making two cuts (something similar as what the Scissors tool does in Illustrator) and just deleting the unneeded line?

Edit: This is the outcome using the method in this reply, which is the same outcome as using Clip or Intersect: https://i.sstatic.net/R11zD.jpg

The reason behind that, methinks, is that the border line marked in red is a closed polygon. See here:

With this update, any further solution comes to mind?

Answer 1

We can use the Scratch Layer polygon which has the same shape as the raster inside geometry generator for the line as follows:
My Scratch Layer here is called Extent. This layer can remain unrendered/invisible.

  intersection( $geometry, geometry( get_feature_by_id('Extent',1)))

Example:

Answer 2

The best way (fastest, no extra data or expressions, done purely in the graphical rendering step) would be to group the two layers, render them as group and let the raster mask the line. Here is an example:

Add both layers to a group and set the group to "Render Layers as Group":

Have the line layer on top of the raster layer and set its rendering mode to "Paint Inside Below":

Answer 3

Based on the answer by @Cushen, but with two improvements:

1. Avoid the problem of the outline along the raster's borders. It seems your vector layer is a polygon, not line. So you need the intersection of the boundary of the polygon with the raster, not of the polygon itself. use boundary() function for that.
2. Needs not auxiliary layer. You can get the extent of the raster layer automatically with the function layer_property().

Like this, you can do it all at once with geometry generator, using this expression (replace raster_layer with the name of your raster layer and be sure to set Geometry generator's Geomtry type to LineString):

intersection( 
    boundary($geometry),  
    layer_property ('raster_layer', 'extent')
)