(In reply to comment #0)
> 2. Modify a static library your program uses.

I assume the library is linked in via pragma(lib, "libname")?

Ultimately, for this to work correctly all the time, rdmd would need to know where the linker will look for library files. This isn't really realistic as it depends on the linker implementation (e.g. it might consult its own configuration files or environment variables). It's possible to add some heuristics to rdmd, though, e.g. to search /usr/lib, %LIB% / $LIBRARY_PATH, and the current directory.

Commit pushed to master at https://github.com/D-Programming-Language/tools

https://github.com/D-Programming-Language/tools/commit/d7d17858bd8525e1481d931b5a0fb3cf2a3aad76
rdmd: Add some heuristics for detecting updated libraries (issue 6431)

I'm marking this as fixed, as a set of heuristics has been implemented in Git master.

If they don't adequately cover your use case, please open another more detailed bug report.

reopening as i just bumped on this issue
rdmd build 20171126
DMD64 D Compiler v2.077.0

build_lib
build_main
edit bar/foo.d
build_lib
build_main => doesn't rebuild

./setup.sh:
## rdmd doesn't rebuild if a dependent library changes
build_lib(){
        dmd -of=libfoo.a -lib bar/foo.d
}

build_main(){
        exe=./main
        rdmd --build-only -of$exe -Llibfoo.a --exclude=bar main.d
        $exe
}


./main.d:
import bar.foo;
void main(){ fun(); }


./bar/foo.d:
module bar.foo;
import std.stdio;
void fun(){writeln("ok4");}

also:
* rdmd --makedepend doesn't mention the dependent libs (libfoo.a)
* dmd -deps neither
* dmd -v neither

also:

when adding `pragma(lib, "foo");` in main.d:
(and s/-Llibfoo.a/-lfoo -L./ to make the build succeed), the dependency on lib foo will appear in both --makedepend, -deps, -v, and the rebuild will happen.

without `pragma(lib, "foo");` in main.d:
the dependency is not caught even if command line mentions -Llibfoo.a or -lfoo -L.

Is that desired behavior? requiring pragma(lib) seems less flexible

ok the existing behavior is definitely buggy: when the dependent library is in a directory not equal to ".", a change to that library will not trigger recompilation:

```
./setup.sh:

## buggy behavior (correct behavior only with build_dir=.)
build_dir=temp

build_lib(){
        mkdir -p temp
        dmd -of=$build_dir/libfoo.a -lib bar/foo.d
}

build_main(){
        exe=./main
        rdmd -v --build-only -of$exe -L-lfoo -L-L$build_dir --exclude=bar main.d
        $exe
}

build_all(){
        build_lib
        build_main
        touch $build_dir/libfoo.a
        build_main
}



./main.d:
pragma(msg, "compiling...");
import bar.foo;
pragma(lib, "foo");
void main(){ fun(); }


./bar/foo.d:
module bar.foo;
void fun(){
        import std.stdio;
        writeln("ok1");
}
```