We’ll use the LLVM project’s clang-tidy tool to find possible programming mistakes, and the clang-format tool to reformat the whitespace in our code.

Linting

We already have a .clang-tidy file in the repository. This is used by the clangd editor extension we installed earlier, and you may have noticed warning underlines like the following:

This is already great for any developers who have their editor setup this way. They can see warnings or errors based on the .clang-tidy configuration, and accept any auto-fixes it provides. However developers might choose to ignore the warnings. We can run clang-tidy with Bazel as well. Our approach doesn’t require any changes to BUILD files, as the cc_library targets already supply the “bare facts” about the code, and shouldn’t have to specify what to do with it. Look in the tools/lint folder. You’ll see a clang_tidy target in tools/lint/BUILD that runs the tool. In order to run it over existing targets, we use a Bazel feature called “aspects”. The clang_tidy aspect is declared in tools/lint/linters.bzl. The Aspect CLI’s lint task applies that aspect for you and prints the results directly, rather than leaving them in bazel-bin for you to hunt down:

% aspect lint :all
INFO: Analyzed target //src:magic (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //src:magic up-to-date (nothing to build)
Aspect //tools/lint:linters.bzl%clang_tidy of //src:magic up-to-date:
  bazel-bin/src/magic.AspectRulesLintClangTidy.out
  bazel-bin/src/magic.AspectRulesLintClangTidy.out.exit_code
  bazel-bin/src/magic.AspectRulesLintClangTidy.patch
INFO: Elapsed time: 0.113s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
Lint results for //src:magic:

6796 warnings generated.
warning: redefining builtin macro [clang-diagnostic-builtin-macro-redefined]
/private/var/folders/8n/7cfngjb106sgwb8mr1tytx700000gn/T/rules_lint_patcher-oheSnn/_main/src/magic.c:17:9: warning: the value returned by this function should be used [cert-err33-c]
        fprintf(stderr, "Failed to initialize libmagic\n");
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/private/var/folders/8n/7cfngjb106sgwb8mr1tytx700000gn/T/rules_lint_patcher-oheSnn/_main/src/magic.c:17:9: note: cast the expression to void to silence this warning
/private/var/folders/8n/7cfngjb106sgwb8mr1tytx700000gn/T/rules_lint_patcher-oheSnn/_main/src/magic.c:23:9: warning: the value returned by this function should be used [cert-err33-c]
        fprintf(stderr, "Cannot load magic database: %s\n", magic_error(magic));
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/private/var/folders/8n/7cfngjb106sgwb8mr1tytx700000gn/T/rules_lint_patcher-oheSnn/_main/src/magic.c:23:9: note: cast the expression to void to silence this warning
/private/var/folders/8n/7cfngjb106sgwb8mr1tytx700000gn/T/rules_lint_patcher-oheSnn/_main/src/magic.c:31:9: warning: the value returned by this function should be used [cert-err33-c]
        fprintf(stderr, "Error determining MIME type: %s\n", magic_error(magic));
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/private/var/folders/8n/7cfngjb106sgwb8mr1tytx700000gn/T/rules_lint_patcher-oheSnn/_main/src/magic.c:31:9: note: cast the expression to void to silence this warning
Suppressed 6790 warnings (6790 in non-user code).
Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.

With the tools/bazel wrapper installed, your team can keep typing bazel lint :all and Bazelisk routes it through the Aspect CLI.

aspect lint wraps aspect_rules_lint’s aspects. If you can’t use the Aspect CLI, you can apply the aspect by hand with vanilla Bazel — bazel build --aspects=//tools/lint:linters.bzl%clang_tidy //src:all — which adds clang-tidy build actions to the cc_library targets it visits (cached and remotable like any other action) and leaves the report at bazel-bin/src/magic.AspectRulesLintClangTidy.out. rules_lint also ships an example lint.sh that wraps that invocation. The CLI exists so you don’t have to.

​Linting

Linting