Concatenation of dict select expressions

[alandonovan]

(Re-reporting on behalf of @sitaktif; see bazelbuild/starlark#114.)

Starlark allows the concatenation of select statements that return lists, such as:

my_binary(
  name = "greatbin",
  deps = ["some", "deps"]
    + select({"//:special_cpu": ["//some:target_for_special_cpu"], "//conditions:default": []})
    + select({"//:special_os": ["//some:target_for_special_os"], "//conditions:default": []})
)

This is very handy when generating code, which commonly happens when writing bazel rules.

Unfortunately, one cannot concatenate selects that return dicts:

my_rule(
  name = "greatbin",
  extra_env = {"FROB_PATH": "somepath"}
    + select({"//:special_cpu": {"SPECIAL_CPUFLAGS": "myspecialcpu"}, "//conditions:default": {}})
    + select({"//:special_os": {"SPECIAL_OSFLAGS": "myspecialos"}, "//conditions:default": {}})
    + select({"//:debug_mode": {"DEBUG": "true"}, "//conditions:default": {}})
    + ...
)

as this results in the following error: type 'dict(label, string)' doesn't support select concatenation.

This error is hit both when concatenating a dict and a select(), and when concatenating two select()s.

Related to see bazelbuild/starlark#14.

[stepancheg]

There's inconsistency here:

{1: 2} + {3: 4}

doesn't work, but these three work:

{1: 2} + select({"//conditions:default": {3: 4})
select({"//conditions:default": {1: 2}) + {3: 4}
select({"//conditions:default": {1: 2}) + select({"//conditions:default": {3: 4})

while intuitively they look equivalent.

[sitaktif]

but these three work

{1: 2} + select({"//conditions:default": {3: 4})
select({"//conditions:default": {1: 2}) + {3: 4}
select({"//conditions:default": {1: 2}) + select({"//conditions:default": {3: 4})

@stepancheg if you look at #114, you will notice that this is incorrect: unless it changed very recently, the + operator is only supported for selects that return lists, not for ones that return dicts.

[fffonion]

With starlark's support of | operator of dicts (bazelbuild/starlark#14), it would be
awesome to add union operator support to select | dict and select | select.
Currently it's recognized as binary operator:

		env = nfpm_envs | {
Error: unsupported binary operation: select | dict

[fmeum]

This is available in Bazel 6: ebae486

@sgowroji Can be closed.

[sgowroji]

We are closing this FR with respect to the above commit. Please feel free to reach us for any further information. Thanks !