Label placement geometry generator code:

-- Name the (line) layer that "cuts" the layer
-- where the labels should be placed.
-- Also define the placement width for the labels.

with_variable('label_position_layer', 'layer_name',
with_variable('label_placement_width', 10, -- coordinate units

if( -- First, only apply to lines that actually are to be labled (improve speed)
intersects(
geometry1:=@geometry,
geometry2:=aggregate(
layer:=@label_position_layer,
aggregate:='collect',
expression:=$geometry
)
),
aggregate( -- The actual label placment
layer:=@label_position_layer,
aggregate:='collect',
expression:=intersection(
geometry1:=geometry(@parent),
geometry2:=buffer($geometry, @label_placement_width)
)
),
'') -- End of if-statement
))