D and Rust are on the opposite sides at dealing with memory safety. Rust ensures safety by constantly making you think about memory with its highly sophisticated compile-time checks. D, on the other hand, offers you to either employ a GC and forget about (almost) all memory-safety concerns or a block scoped opt-out with cowboy-style manual memory management.

D retains object-oriented programming but also allows functional programming, while Rust seems to be specifically designed for functional programming and does not allow OOP in the conventional sense.

I've been working with D for a couple of months now and I noticed that it's almost a no-brainer to port C/C++ code to D because it mostly builds on the same semantics. With Rust, porting a piece of code may often require rethinking the whole thing from scratch.

> Are there technical reasons that Rust took off and D didn't?

As someone who considered it back then when it actually stood a chance to become the next big thing, from what I remember, the whole ecosystem was just too confusing and simply didn't look stable and reliable enough to build upon long-term. A few examples:

* The compiler situation: The official compiler was not yet FOSS and other compilers were not available or at least not usable. Switch to FOSS happened way too late and GCC support took too long to mature.

* This whole D version 1 vs version 2 thingy

* This whole Phobos vs Tango standard library thingy

* This whole GC vs no-GC thingy

This is not a judgement on D itself or its governance. I always thought it's a very nice language and the project simply lacked man-power and commercial backing to overcome the magical barrier of wide adoption. There was some excitement when Facebook picked it up, but unfortunately, it seems it didn't really stick.

I think 3 things

1. D had a split similar to python 2 vs 3 early on with having the garbage collector or not (and therefor effectively 2 standard libraries), but unlike python it didn't already have a massive community that was willing to suffer through it.

2. It didn't really have any big backing. Rust having Mozilla backing it for integration with Firefox makes a pretty big difference.

3. D wasn't different enough, it felt much more "this is c++ done better" than it's own language, but unlike c++ where it's mostly a superset of c you couldn't do "c with classes" style migrations

> Are there technical reasons that Rust took off and D didn't?

My (somewhat outdated) experience is that D feels like a better and more elegant C++. Rust certainly has been influenced by C and C++, but it also took a lot of inspiration from the ML-family of languages and it has a much stronger type system as a consequence.

One feature of D that i really wish other languages would adopt (not sure about Rust but i also think it lacks it, though if it has it to a similar extent as D it might be the reason i check it again more seriously) is the metaprogramming and compile-time code evaluation features it has (IIRC you can use most of the language during compile time as it runs in a bytecode VM), down to even having functions that generate source code which is then treated as part of the compilation process.

Of course you can make codegen as part of your build process with any language, but that can be kludgy (and often limited to a single project).

More like the companies that jumped into D versus Rust, D only had Facebook and Remedy Games toy a bit with it.

Many of us believe on automatic memory management for systems programming, having used quite a few in such scenarios, so that is already one thing that D does better than Rust.

There is the GC phobia, mostly by folks that don't get not all GCs were born alike, and just like you need to pick and chose your malloc()/free() implementation depending on the scenario, there are many ways to implement a GC, and having a GC doesn't preclude having value types, stack and global memory segment allocation.

D has compile time reflection, and compile time metaprogramming is much easier to use than Rust macros, and it does compile time execution as well.

And the compile times! It is like using Turbo Pascal, Delphi,... even thought the language is like C++ in capabilities. Yet another proof complexity doesn't imply slow compile natives in a native systems language.

For me, C# and Swift replace the tasks at work were I in the past could have reached for D instead, mostly due to who is behind those languages, and I don't want to be that guy that leaves and is the one that knew the stack.

> Are there technical reasons that Rust took off and D didn't?

Yes. D tried to jump on the "systems programming with garbage collection" dead horse, with predictable results.

(People who want that sort of stupidity already have Go and Java, they don't need D.)