Makefile: alias format to cargo fmt, add format-check command

This removes the previous `make format` command which, in essence,
runs some bash magic to achieve the exact same job that `cargo fmt`
does, if its run within workspace instead of a particular
crate. Besides this, many people's editors have a native rustfmt
integration anyways, which makes this script and its prompt for
unstaged files largely redundant.

Instead, we replace it with the more useful `make format-check` and
the associated `tools/check_format.sh`. This script does more than
`cargo fmt --check` does, for instance also ensuring that no Rust
source file contains tab characters (which rustfmt sometimes leaves
behind).

As part of `make prepush`, `check-format` is now more in line with the
other jobs, such as `cargo check` or Clippy: it'll complain, but not
touch any source code.

To retain compatibility, we introduce a `make format` and `make fmt`,
aliased to `cargo fmt` -- without prompting the user.

This also removes an optimization that uses Make's logic to check for
changed files in order to skip formatting. However, I doubt that this
will meaningfully impact users (`make format-check` completes in about
2sec and does not take a meaningful amount of time compared to the
other jobs executed as part of `prepush`).

Makefile

@@ -52,9 +52,12 @@ usage:
 	@echo "             doc: Builds Tock documentation for all boards"
 	@echo "           stack: Prints a basic stack frame analysis for all boards"
 	@echo "           clean: Clean all builds"
-	@echo "          format: Runs the rustfmt tool on all kernel sources"
+	@echo "    format-check: Checks for formatting errors in kernel sources"
 	@echo "            list: Lists available boards"
 	@echo
+	@echo "We also define the following aliases:"
+	@echo "          format: cargo fmt"
+	@echo
 	@echo "The make system also drives all continuous integration and testing:"
 	@echo "         $$(tput bold)prepush$$(tput sgr0): Fast checks to run before pushing changes upstream"
 	@echo "          ci-all: Run all continuous integration tests (possibly slow!)"
@@ -175,14 +178,15 @@ clean:
 	@echo "$$(tput bold)Clean ci-artifacts" && rm -rf tools/ci-artifacts
 
 .PHONY: fmt format
-fmt format: tools/.format_fresh
-	$(call banner,Formatting complete)
+fmt format:
+	$(call banner,Running \"cargo fmt\" -- for a complete format check run \"make format-check\")
+	cargo fmt
 
-# Get a list of all rust source files (everything fmt operates on)
-$(eval RUST_FILES_IN_TREE := $(shell (git ls-files | grep '\.rs$$') || find . -type f -name '*.rs'))
-tools/.format_fresh: $(RUST_FILES_IN_TREE)
-	@./tools/run_cargo_fmt.sh $(TOCK_FORMAT_MODE)
-	@touch tools/.format_fresh
+.PHONY: format-check
+format-check:
+	$(call banner,Formatting checker)
+	@./tools/check_format.sh
+	$(call banner,Check for formatting complete)
 
 .PHONY: list
 list:
@@ -215,7 +219,7 @@ ci-nosetup:
 # This is designed for developers, to be run often and before submitting code upstream.
 .PHONY: prepush
 prepush:\
-	format\
+	format-check\
 	ci-job-clippy\
 	ci-job-syntax\
 	licensecheck
@@ -350,9 +354,8 @@ ci-runner-netlify:\
 
 ### ci-runner-github-format jobs:
 .PHONY: ci-job-format
-ci-job-format: licensecheck
-	$(call banner,CI-Job: Format Check)
-	@NOWARNINGS=true TOCK_FORMAT_MODE=diff $(MAKE) format
+ci-job-format: licensecheck format-check
+	$(call banner,CI-Job: Format Check DONE)
 
 define ci_setup_markdown_toc
 	$(call banner,CI-Setup: Install markdown-toc)

tools/check_format.sh

@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+
+# Licensed under the Apache License, Version 2.0 or the MIT License.
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+# Copyright Tock Contributors 2023.
+#
+# Author: Leon Schuermann <[email protected]>
+# Author: Pat Pannuto <[email protected]>
+# Author: Brad Campbell <[email protected]>
+#
+set -e
+
+# Verify that we're running in the base directory
+if [ ! -x tools/check_format.sh ]; then
+	echo ERROR: $0 must be run from the tock repository root.
+	echo ""
+	exit 1
+fi
+
+set +e
+let FAIL=0
+set -e
+
+# Run `cargo fmt --check` in the workspace root, which will check all
+# crates in the workspace.
+echo "Running \`cargo fmt --check\`..."
+CARGO_FMT_CHECK_EXIT_CODE=0
+cargo fmt --check || CARGO_FMT_CHECK_EXIT_CODE=$?
+if [[ $CARGO_FMT_CHECK_EXIT_CODE -ne 0 ]]; then
+	let FAIL=FAIL+1
+else
+	echo "\`cargo fmt --check\` suceeded."
+fi
+printf "\n"
+
+# Check for tab characters in Rust source files that haven't been
+# removed by rustfmt
+echo "Checking for Rust source files with tab characters..."
+RUST_FILES_WITH_TABS="$(git grep --files-with-matches $'\t' -- '*.rs' || grep -lr --include '*.rs' $'\t' . || true)"
+if [ "$RUST_FILES_WITH_TABS" != "" ]; then
+	echo "ERROR: The following files contain tab characters, please use spaces instead:"
+	echo "$RUST_FILES_WITH_TABS" | sed 's/^/    -> /'
+	let FAIL=FAIL+1
+else
+	echo "No Rust source file containing tab characters found."
+fi
+
+if [[ $FAIL -ne 0 ]]; then
+	printf "\n"
+	echo "$(tput bold)Formatting errors.$(tput sgr0)"
+	echo "See above for details. You can try running \`cargo fmt\` to correct them."
+fi
+exit $FAIL