Here’s the link to the latest weekly snapshot of darktable 3.7, https://drive.google.com/file/d/1hBhGFRDtSWTwNwBNrCZMij_FqA7RCH3E/view?usp=sharing. We are in feature freeze right now, so only bug fixes are being added. Please test and report any problems.
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This is such a laudable effort. Thank you.
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Thanks @wpferguson for your effort, this new version looks very promising 
I think the strings to identify the new Canon RF lenses aren’t correct in lensfun’s mil_canon.xml:
line 563 is: RF24-105mm F4 L IS USM
line 563 should be: Canon RF 24-105mm F4 L IS USM
line 583 is: RF70-200mm F2.8 L IS USM
line 583 should be: Canon RF 70-200mm F2.8 L IS USM
Where can I report bugs concerning lensfun ?
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You can also use .exiv2 file to give lensfun the name it wants.
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Thanks @MStraeten and @paperdigits for your information.
I dug a little deeper and analyzed the exif-Data using exiftool.
exiftool delivers the following lens relevant information (CR3 file):
Lens Info : 24-105mm f/0
Lens Serial Number : bla
Lens Type : Canon RF 50mm F1.2L USM or other Canon RF Lens
RF Lens Type : Canon RF 24-105mm F4L IS USM
Lens Model : RF24-105mm F4 L IS USM
Shutter Release Without Lens : Disable
Retract Lens On Power Off : Enable
USM Lens Electronic MF : Disable in AF Mode
Lens Drive When AF Impossible : Continue Focus Search
Lens Focal Length : 105
Lens : 24.0 - 105.0 mm
Lens ID : Canon RF 24-105mm F4L IS USM
Lens : 24.0 - 105.0 mm (35 mm equivalent: 24.0 - 104.8 mm)
The interesting ones in this context are:
Lens Model : RF24-105mm F4 L IS USM
Lens ID : Canon RF 24-105mm F4L IS USM
The string darktable shows in its exif information window is:
lens: Canon RF 24-105mm F4 L IS USM
This seems to be the string behind “Lens ID” in the exif-Data which is different to “Lens Model”
In the lensfun database we find:
[model]RF24-105mm F4 L IS USM[/model] → So, the “Lens Model” string should be used.
Ok, so far, so good.
Now let’s check the same file converted to a DNG
Exiftools shows the exact same strings for “Lens Model” and “Lens ID”
But the string darktable shows in its exif information window is:
lens: RF24-105mm F4 L IS USM
Obviously this is the string behind “Lens Model” in the exif-Data, and as expected, the lens is found correctly via lensfun 
.
Does this mean we have a bug in darktable for lens detection in CR3-files ?
P.S.: For my tests I used the 020A0005.CR3 file of the dpreview CR5 review sample gallery.
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A little off-topic, but maybe a kind of explanation:
I compared files of a 6DMII:
Lens relevant info by exiftools is e.g.:
Lens Type : Canon EF 85mm f/1.8 USM or Sigma Lens
Lens Drive No AF : Focus search on
Retract Lens On Power Off : Enable
Lens Info : 85mm f/0
Lens Model : EF85mm f/1.8 USM
Lens Serial Number : bla
Lens : 85.0 mm
Lens ID : Canon EF 85mm f/1.8 USM
Lens : 85.0 mm (35 mm equivalent: 129.1 mm)
In the lensfun database this lens is identified by:
[model]Canon EF 85mm f/1.8L USM[/model]
Which seems to be the string behind “Lens ID”, not “Lens Model”
The darktable information window shows:
lens: Canon EF 85mm f/1.8 USM
And as expected, the lens is found correctly via lensfun.
The slr-canon.xml contains 146 lenses. 144 begin with "Canon " and most likely point to the “Lens ID” not to the “Lens Model”. Maybe a change of the paradigm took place and they now use the correct exif-entry which lead to the DNG-CR3 mixup in darktable?
Better late than never 
For Windows users who face the same problem, the following addition to the link @MStraeten posted may be of use:
- under Windows, the .exiv2 file is located directly in the Windows user directory and named exiv2.ini
- for the changes in .exiv2 to take effect a refresh of the exif-data of the affected images should be executed. (lighttable->selected image[s]->metadata->refresh exif) note: image ratings are lost and must be updated manually.