Consider the following module: --- import std.stdio; class Foo { void bar() { writeln("Hello world!"); } } --- While it includes a top-level import, that import isn't used in any of its declarations. dmd -H produces: --- // D import file generated from 'ditest.d' import std.stdio; class Foo { void bar(); } --- The unneeded import is retained. Thanks to Peaker on IRC for pointing this out -- apparently they have a project that takes many minutes to compile, but, aside from excessive import costs, incremental builds can finish within a couple seconds. As an aside, it would be relatively straightforward to map all imports to static imports in .di files, which would achieve significant savings if we made static imports lazy.
This is probably a WONTFIX since the header generation is done immediately after the parsing phase so at that point no symbol resolution is done to see which symbols come from which imports and the safe thing to do is to just put them there; a workaround for this is to use scoped imports : import std.stdio; class Foo { void bar() { import std.stdio : writeln; writeln("Hello world!"); } }