Add diffusion coefficient quantities and related units.

[crystal-growth]

Should resolve #321

I'm not sure completely about singular for "stokes" unit, it seems it's always used in plural form.

[iliekturtles]

Thanks for all these PRs! I'll try to work through them ASAP but it will likely take me some days.

[crystal-growth]

Thank you! Please take your time!

[iliekturtles]

I've started my review. What's the best way to suggest changes? I've done all three for this PR.

• As suggestions in the PR.
• As a patch in a comment.
• As a branch in my fork. https://github.com/crystal-growth/uom/compare/add_diffusion_coefficient...iliekturtles:uom:add_diffusion_coefficient?expand=1

diff --git a/src/si/diffusion_coefficient.rs b/src/si/diffusion_coefficient.rs
index 37ec6b4..959d4ac 100644
--- a/src/si/diffusion_coefficient.rs
+++ b/src/si/diffusion_coefficient.rs
@@ -13,20 +13,19 @@ quantity! {
         Z0,     // amount of substance
         Z0>;    // luminous intensity
     units {
-        @square_meter_per_second: prefix!(none); "m² · s⁻¹",
-            "square meter per second", "square meters per second";
-        @square_centimeter_per_second:  prefix!(centi) * prefix!(centi); "cm² · s⁻¹",
-            "square centimeter per second", "square centimeters per second";    
-        @square_millimeter_per_second:  prefix!(milli) * prefix!(milli); "mm² · s⁻¹",
-            "square millimeter per second", "square millimeters per second";                
-        @square_micrometer_per_second:  prefix!(micro) * prefix!(micro); "µm² · s⁻¹",
-            "square micrometer per second", "square micrometers per second"; 
-        @square_nanometer_per_second:  prefix!(nano) * prefix!(nano); "nm² · s⁻¹",
-            "square nanometer per second", "square nanometers per second";  
-        @stokes:  prefix!(centi) * prefix!(centi); "St",
-            "stokes", "stokes";    
-        @centistokes:  prefix!(centi) * prefix!(centi) * prefix!(centi); "cSt",
-            "stokes", "stokes";                                     
+        @square_meter_per_second: prefix!(none); "m┬▓/s", "square meter per second",
+            "square meters per second";
+        @square_centimeter_per_second: prefix!(centi) * prefix!(centi); "cm┬▓/s",
+            "square centimeter per second", "square centimeters per second";
+        @square_millimeter_per_second: prefix!(milli) * prefix!(milli); "mm┬▓/s",
+            "square millimeter per second", "square millimeters per second";
+        @square_micrometer_per_second: prefix!(micro) * prefix!(micro); "┬╡m┬▓/s",
+            "square micrometer per second", "square micrometers per second";
+        @square_nanometer_per_second: prefix!(nano) * prefix!(nano); "nm┬▓/s",
+            "square nanometer per second", "square nanometers per second";
+        @stokes: prefix!(centi) * prefix!(centi); "St", "Stokes", "Stokes";
+        @centistokes: prefix!(centi) * prefix!(centi) * prefix!(centi); "cSt", "centistokes",
+            "centistokes";
         }
 }
 
@@ -35,7 +34,7 @@ mod test {
     storage_types! {
         use crate::num::One;
         use crate::si::quantities::*;
-        use crate::si::time as t;      
+        use crate::si::time as t;
         use crate::si::area as area;
         use crate::si::diffusion_coefficient as dc;
 
@@ -49,15 +48,15 @@ mod test {
 
         #[test]
         fn check_units() {
-            test::<t::second, area::square_meter, dc::square_meter_per_second>();
-            test::<t::second, area::square_centimeter, dc::square_centimeter_per_second>();
-            test::<t::second, area::square_millimeter, dc::square_millimeter_per_second>();
-            test::<t::second, area::square_micrometer, dc::square_micrometer_per_second>();
-            test::<t::second, area::square_nanometer, dc::square_nanometer_per_second>();
-            test::<t::second, area::square_centimeter, dc::stokes>();
-            test::<t::second, area::square_millimeter, dc::centistokes>();
+            test::<area::square_meter, t::second, dc::square_meter_per_second>();
+            test::<area::square_centimeter, t::second, dc::square_centimeter_per_second>();
+            test::<area::square_millimeter, t::second, dc::square_millimeter_per_second>();
+            test::<area::square_micrometer, t::second, dc::square_micrometer_per_second>();
+            test::<area::square_nanometer, t::second, dc::square_nanometer_per_second>();
+            test::<area::square_centimeter, t::second, dc::stokes>();
+            test::<area::square_millimeter, t::second, dc::centistokes>();
 
-            fn test< T: t::Conversion<V>, A: area::Conversion<V>, DC: dc::Conversion<V>>() {
+            fn test<A: area::Conversion<V>, T: t::Conversion<V>, DC: dc::Conversion<V>>() {
                 Test::assert_approx_eq(&DiffusionCoefficient::new::<DC>(V::one()),
                     &(Area::new::<A>(V::one())
                         / Time::new::<T>(V::one())));

[iliekturtles]

One more thing to add is that if you rebase on top of the latest master the 1.43.0 tests will stop failing.

[crystal-growth]

I think that suggestions in PR will be the easiest option.
I've applied your suggestions and rebased this branch to new master.

[iliekturtles]

Thanks for this PR! I'll be trying to work through the other PRs this week.