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Describe a better approach of how to use {fmt} as a depency in a Bazel project (#2516)

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# Bazel support
Bazel is an open-source build tool.
More information about Bazel can be found [here](https://bazel.build/).
To get [Bazel](https://bazel.build/) working with {fmt} you can copy the files `BUILD.bazel`, `WORKSPACE.bazel`, `.bazelrc`, and `.bazelversion` from this folder (`support/bazel`) to the root folder of this project. This way {fmt} gets bazelized and can be used with Bazel (e.g. doing a `bazel build //...` on {fmt}).
## Using the fmt repository with Bazel
## Using {fmt} as a dependency
Even though the {fmt} repository does not contain a `WORKSPACE` file in its root directory,
there is an easy approach to use the {fmt} repository with Bazel out of the box.
This is demonstrated in the following example.
The following minimal example shows how to use {fmt} as a dependency within a Bazel project.
Add to your `WORKSPACE` file:
The following file structure is assumed:
```python
load("@bazel_tools//tools/build_defs/repo:git.bzl", "new_git_repository")
# Fetch all files from fmt including the BUILD file `support/bazel/BUILD.bazel`
new_git_repository(
name = "fmt_workaround",
branch = "master",
remote = "https://github.com/fmtlib/fmt/",
build_file_content = "# Empty build file on purpose"
)
# Now the BUILD file `support/bazel/BUILD.bazel` can be used:
new_git_repository(
name = "fmt",
branch = "master",
remote = "https://github.com/fmtlib/fmt/",
build_file = "@fmt_workaround//:support/bazel/BUILD.bazel"
)
```
example
├── BUILD.bazel
├── main.cpp
└── WORKSPACE.bazel
```
Create a `BUILD.bazel` file and add a dependency to {fmt}:
*main.cpp*:
```python
cc_binary( # Build a binary
name = "Demo", # Name of the binary
srcs = ["main.cpp"], # List of files - we only have main.cpp
deps = ["@fmt//:fmt"], # Depend on fmt
)
```
Make use of {fmt} in `main.cpp`:
```C++
```c++
#include "fmt/core.h"
int main() {
fmt::print("The answer is {}.\n", 42);
fmt::print("The answer is {}\n", 42);
}
```
The expected output of this example is `The answer is 42`.
## Bazelize fmt
First downloading a build file and then making use of it can be considered as a bit unclean, nevertheless, it works.
A cleaner Bazel solution would be to move the `WORKSPACE` and `BUILD` files to the root folder of the {fmt} Git repository.
In favor of keeping the {fmt} project directory clean, those files were not added to the project root directory.
If you do not like this, you can fork this repository and move the files `BUILD.bazel`, `WORKSPACE.bazel`, `.bazelrc`, and `.bazelversion` from this folder (`support/bazel`) to the root folder of this project.
This way {fmt} gets bazelized and can be used in your Bazel builds.
**Example**
Create a `WORKSPACE.bazel` file with the following content:
*WORKSPACE.bazel*:
```python
load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
# Fetch bazelized fmt
git_repository(
name = "fmt",
branch = "bazel-support", # A copy of master where BUILD.bazel, WORKSPACE.bazel, .bazelrc and .bazelversion are moved to root
remote = "https://github.com/<user_or_organisation>/fmt", # replace <user_or_organisation> by a valid account
branch = "master",
remote = "https://github.com/fmtlib/fmt",
patch_cmds = [
"mv support/bazel/.bazelrc .bazelrc",
"mv support/bazel/.bazelversion .bazelversion",
"mv support/bazel/BUILD.bazel BUILD.bazel",
"mv support/bazel/WORKSPACE.bazel WORKSPACE.bazel",
],
# Windows-related patch commands are only needed in the case MSYS2 is not installed.
# More details about the installation process of MSYS2 on Windows systems can be found here:
# https://docs.bazel.build/versions/main/install-windows.html#installing-compilers-and-language-runtimes
# Even if MSYS2 is installed the Windows related patch commands can still be used.
patch_cmds_win = [
"Move-Item -Path support/bazel/.bazelrc -Destination .bazelrc",
"Move-Item -Path support/bazel/.bazelversion -Destination .bazelversion",
"Move-Item -Path support/bazel/BUILD.bazel -Destination BUILD.bazel",
"Move-Item -Path support/bazel/WORKSPACE.bazel -Destination WORKSPACE.bazel",
],
)
```
Create a `BUILD.bazel` file and add a dependency to {fmt} (same as above).
In the *WORKSPACE* file, the {fmt} GitHub repository is fetched. Using the attribute `patch_cmds` the files `BUILD.bazel`, `WORKSPACE.bazel`, `.bazelrc`, and `.bazelversion` are moved to the root of the {fmt} repository. This way the {fmt} repository is recognized as a bazelized workspace.
*BUILD.bazel*:
```python
cc_binary(
name = "Demo",
srcs = ["main.cpp"],
deps = ["@fmt"],
)
```
The *BUILD* file defines a binary named `Demo` that has a dependency to {fmt}.
To execute the binary you can run `bazel run //:Demo`.
Make use of {fmt} in `main.cpp` (same as above).