Add a tidy check prohibiting mod build

Because we have a .gitignore rule for `build/`, having a module directory named
"build" causes various problems for tools that try to skip ignored files.

Forbidding this in tidy is trickier than it looks, because tidy itself tries to
skip ignored files. So the rule instead looks in non-ignored files for module
declarations that would imply the existence of potentially-ignored module
directories.

This also has the side-effect of prohibiting `build.rs` files that aren't build
scripts.

Author: Zalathar

.gitignore

@@ -46,8 +46,11 @@ no_llvm_build
 /inst/
 /llvm/
 /mingw-build/
+# Ignore all "build" directories, in the root and in subdirectories.
+# This makes it a bad idea to have Rust modules named "build", so there is a
+# tidy rule prohibiting that. Try to avoid adding exceptions to this, because
+# some tools don't properly handle negative .gitignore rules.
 build/
-!/compiler/rustc_mir_build/src/build/
 /build-rust-analyzer/
 /dist/
 /unicode-downloads

src/tools/tidy/src/style.rs

@@ -343,7 +343,8 @@ pub fn check(path: &Path, bad: &mut bool) {
         let filename = file.file_name().unwrap().to_string_lossy();
 
         let is_style_file = filename.ends_with(".css");
-        let under_rustfmt = filename.ends_with(".rs") &&
+        let is_rust_file = filename.ends_with(".rs");
+        let under_rustfmt = is_rust_file &&
             // This list should ideally be sourced from rustfmt.toml but we don't want to add a toml
             // parser to tidy.
             !file.ancestors().any(|a| {
@@ -485,6 +486,21 @@ pub fn check(path: &Path, bad: &mut bool) {
                     err("Don't use magic numbers that spell things (consider 0x12345678)");
                 }
             }
+
+            // Forbid `mod build;` and `mod build {`, because they imply the
+            // possible existence of a source directory named "build" that would
+            // be ignored by our blanket .gitignore rule for `build/`.
+            // (Bear in mind that some tools don't support negated .gitignore rules.)
+            // This also has the side-effect of forbidding `build.rs` files that
+            // aren't build scripts.
+            if is_rust_file
+                && !trimmed.starts_with("//")
+                && static_regex!(r"\bmod\s+build\s*[;{]").is_match(trimmed)
+            {
+                err("modules named `build` can cause problems for git-adjacent tools; \
+                    use a different name (e.g. `builder`)");
+            }
+
             // for now we just check libcore
             if trimmed.contains("unsafe {")
                 && !trimmed.starts_with("//")