tidy: add `auto:` prefix to --extra-checks syntax

currently this just uses a very simple
extension-based heirustic.

9 files changed, 67 insertions, 20 deletions

diff --git a/src/bootstrap/src/core/config/flags.rs b/src/bootstrap/src/core/config/flags.rs
index bfc06f90d4f..19e0d3413bb 100644
--- a/src/bootstrap/src/core/config/flags.rs
+++ b/src/bootstrap/src/core/config/flags.rs
@@ -383,7 +383,10 @@ pub enum Subcommand {
         bless: bool,
         #[arg(long)]
         /// comma-separated list of other files types to check (accepts py, py:lint,
-        /// py:fmt, shell, shell:lint, cpp, cpp:fmt, spellcheck, spellcheck:fix)
+        /// py:fmt, shell, shell:lint, cpp, cpp:fmt, spellcheck)
+        ///
+        /// Any argument can be prefixed with "auto:" to only run if
+        /// relevant files are modified (eg. "auto:py").
         extra_checks: Option<String>,
         #[arg(long)]
         /// rerun tests even if the inputs are unchanged
diff --git a/src/etc/completions/x.fish b/src/etc/completions/x.fish
index 69bd525a312..3009dd85e93 100644
--- a/src/etc/completions/x.fish
+++ b/src/etc/completions/x.fish
@@ -293,7 +293,7 @@ complete -c x -n "__fish_x_using_subcommand doc" -l skip-std-check-if-no-downloa
 complete -c x -n "__fish_x_using_subcommand doc" -s h -l help -d 'Print help (see more with \'--help\')'
 complete -c x -n "__fish_x_using_subcommand test" -l test-args -d 'extra arguments to be passed for the test tool being used (e.g. libtest, compiletest or rustdoc)' -r
 complete -c x -n "__fish_x_using_subcommand test" -l compiletest-rustc-args -d 'extra options to pass the compiler when running compiletest tests' -r
-complete -c x -n "__fish_x_using_subcommand test" -l extra-checks -d 'comma-separated list of other files types to check (accepts py, py:lint, py:fmt, shell, shell:lint, cpp, cpp:fmt, spellcheck, spellcheck:fix)' -r
+complete -c x -n "__fish_x_using_subcommand test" -l extra-checks -d 'comma-separated list of other files types to check (accepts py, py:lint, py:fmt, shell, shell:lint, cpp, cpp:fmt, spellcheck)' -r
 complete -c x -n "__fish_x_using_subcommand test" -l compare-mode -d 'mode describing what file the actual ui output will be compared to' -r
 complete -c x -n "__fish_x_using_subcommand test" -l pass -d 'force {check,build,run}-pass tests to this mode' -r
 complete -c x -n "__fish_x_using_subcommand test" -l run -d 'whether to execute run-* tests' -r
diff --git a/src/etc/completions/x.ps1 b/src/etc/completions/x.ps1
index 7b142ba97ce..8002ec31592 100644
--- a/src/etc/completions/x.ps1
+++ b/src/etc/completions/x.ps1
@@ -339,7 +339,7 @@ Register-ArgumentCompleter -Native -CommandName 'x' -ScriptBlock {
         'x;test' {
             [CompletionResult]::new('--test-args', '--test-args', [CompletionResultType]::ParameterName, 'extra arguments to be passed for the test tool being used (e.g. libtest, compiletest or rustdoc)')
             [CompletionResult]::new('--compiletest-rustc-args', '--compiletest-rustc-args', [CompletionResultType]::ParameterName, 'extra options to pass the compiler when running compiletest tests')
-            [CompletionResult]::new('--extra-checks', '--extra-checks', [CompletionResultType]::ParameterName, 'comma-separated list of other files types to check (accepts py, py:lint, py:fmt, shell, shell:lint, cpp, cpp:fmt, spellcheck, spellcheck:fix)')
+            [CompletionResult]::new('--extra-checks', '--extra-checks', [CompletionResultType]::ParameterName, 'comma-separated list of other files types to check (accepts py, py:lint, py:fmt, shell, shell:lint, cpp, cpp:fmt, spellcheck)')
             [CompletionResult]::new('--compare-mode', '--compare-mode', [CompletionResultType]::ParameterName, 'mode describing what file the actual ui output will be compared to')
             [CompletionResult]::new('--pass', '--pass', [CompletionResultType]::ParameterName, 'force {check,build,run}-pass tests to this mode')
             [CompletionResult]::new('--run', '--run', [CompletionResultType]::ParameterName, 'whether to execute run-* tests')
diff --git a/src/etc/completions/x.py.fish b/src/etc/completions/x.py.fish
index e982cde634f..4766a5ee0c7 100644
--- a/src/etc/completions/x.py.fish
+++ b/src/etc/completions/x.py.fish
@@ -293,7 +293,7 @@ complete -c x.py -n "__fish_x.py_using_subcommand doc" -l skip-std-check-if-no-d
 complete -c x.py -n "__fish_x.py_using_subcommand doc" -s h -l help -d 'Print help (see more with \'--help\')'
 complete -c x.py -n "__fish_x.py_using_subcommand test" -l test-args -d 'extra arguments to be passed for the test tool being used (e.g. libtest, compiletest or rustdoc)' -r
 complete -c x.py -n "__fish_x.py_using_subcommand test" -l compiletest-rustc-args -d 'extra options to pass the compiler when running compiletest tests' -r
-complete -c x.py -n "__fish_x.py_using_subcommand test" -l extra-checks -d 'comma-separated list of other files types to check (accepts py, py:lint, py:fmt, shell, shell:lint, cpp, cpp:fmt, spellcheck, spellcheck:fix)' -r
+complete -c x.py -n "__fish_x.py_using_subcommand test" -l extra-checks -d 'comma-separated list of other files types to check (accepts py, py:lint, py:fmt, shell, shell:lint, cpp, cpp:fmt, spellcheck)' -r
 complete -c x.py -n "__fish_x.py_using_subcommand test" -l compare-mode -d 'mode describing what file the actual ui output will be compared to' -r
 complete -c x.py -n "__fish_x.py_using_subcommand test" -l pass -d 'force {check,build,run}-pass tests to this mode' -r
 complete -c x.py -n "__fish_x.py_using_subcommand test" -l run -d 'whether to execute run-* tests' -r
diff --git a/src/etc/completions/x.py.ps1 b/src/etc/completions/x.py.ps1
index f4b26fac0bc..1aff3c433b4 100644
--- a/src/etc/completions/x.py.ps1
+++ b/src/etc/completions/x.py.ps1
@@ -339,7 +339,7 @@ Register-ArgumentCompleter -Native -CommandName 'x.py' -ScriptBlock {
         'x.py;test' {
             [CompletionResult]::new('--test-args', '--test-args', [CompletionResultType]::ParameterName, 'extra arguments to be passed for the test tool being used (e.g. libtest, compiletest or rustdoc)')
             [CompletionResult]::new('--compiletest-rustc-args', '--compiletest-rustc-args', [CompletionResultType]::ParameterName, 'extra options to pass the compiler when running compiletest tests')
-            [CompletionResult]::new('--extra-checks', '--extra-checks', [CompletionResultType]::ParameterName, 'comma-separated list of other files types to check (accepts py, py:lint, py:fmt, shell, shell:lint, cpp, cpp:fmt, spellcheck, spellcheck:fix)')
+            [CompletionResult]::new('--extra-checks', '--extra-checks', [CompletionResultType]::ParameterName, 'comma-separated list of other files types to check (accepts py, py:lint, py:fmt, shell, shell:lint, cpp, cpp:fmt, spellcheck)')
             [CompletionResult]::new('--compare-mode', '--compare-mode', [CompletionResultType]::ParameterName, 'mode describing what file the actual ui output will be compared to')
             [CompletionResult]::new('--pass', '--pass', [CompletionResultType]::ParameterName, 'force {check,build,run}-pass tests to this mode')
             [CompletionResult]::new('--run', '--run', [CompletionResultType]::ParameterName, 'whether to execute run-* tests')
diff --git a/src/etc/completions/x.py.zsh b/src/etc/completions/x.py.zsh
index 8fc4f052252..77f995c1704 100644
--- a/src/etc/completions/x.py.zsh
+++ b/src/etc/completions/x.py.zsh
@@ -338,7 +338,7 @@ _arguments "${_arguments_options[@]}" : \
 _arguments "${_arguments_options[@]}" : \
 '*--test-args=[extra arguments to be passed for the test tool being used (e.g. libtest, compiletest or rustdoc)]:ARGS:_default' \
 '*--compiletest-rustc-args=[extra options to pass the compiler when running compiletest tests]:ARGS:_default' \
-'--extra-checks=[comma-separated list of other files types to check (accepts py, py\:lint, py\:fmt, shell, shell\:lint, cpp, cpp\:fmt, spellcheck, spellcheck\:fix)]:EXTRA_CHECKS:_default' \
+'--extra-checks=[comma-separated list of other files types to check (accepts py, py\:lint, py\:fmt, shell, shell\:lint, cpp, cpp\:fmt, spellcheck)]:EXTRA_CHECKS:_default' \
 '--compare-mode=[mode describing what file the actual ui output will be compared to]:COMPARE MODE:_default' \
 '--pass=[force {check,build,run}-pass tests to this mode]:check | build | run:_default' \
 '--run=[whether to execute run-* tests]:auto | always | never:_default' \
diff --git a/src/etc/completions/x.zsh b/src/etc/completions/x.zsh
index c495e8318ba..6c6d7b3f49f 100644
--- a/src/etc/completions/x.zsh
+++ b/src/etc/completions/x.zsh
@@ -338,7 +338,7 @@ _arguments "${_arguments_options[@]}" : \
 _arguments "${_arguments_options[@]}" : \
 '*--test-args=[extra arguments to be passed for the test tool being used (e.g. libtest, compiletest or rustdoc)]:ARGS:_default' \
 '*--compiletest-rustc-args=[extra options to pass the compiler when running compiletest tests]:ARGS:_default' \
-'--extra-checks=[comma-separated list of other files types to check (accepts py, py\:lint, py\:fmt, shell, shell\:lint, cpp, cpp\:fmt, spellcheck, spellcheck\:fix)]:EXTRA_CHECKS:_default' \
+'--extra-checks=[comma-separated list of other files types to check (accepts py, py\:lint, py\:fmt, shell, shell\:lint, cpp, cpp\:fmt, spellcheck)]:EXTRA_CHECKS:_default' \
 '--compare-mode=[mode describing what file the actual ui output will be compared to]:COMPARE MODE:_default' \
 '--pass=[force {check,build,run}-pass tests to this mode]:check | build | run:_default' \
 '--run=[whether to execute run-* tests]:auto | always | never:_default' \
diff --git a/src/tools/tidy/src/ext_tool_checks.rs b/src/tools/tidy/src/ext_tool_checks.rs
index c552cf8e400..be2f585a6d8 100644
--- a/src/tools/tidy/src/ext_tool_checks.rs
+++ b/src/tools/tidy/src/ext_tool_checks.rs
@@ -23,6 +23,8 @@ use std::process::Command;
 use std::str::FromStr;
 use std::{fmt, fs, io};
 
+use crate::CiInfo;
+
 const MIN_PY_REV: (u32, u32) = (3, 9);
 const MIN_PY_REV_STR: &str = "≥3.9";
 
@@ -37,15 +39,18 @@ const RUFF_CONFIG_PATH: &[&str] = &["src", "tools", "tidy", "config", "ruff.toml
 const RUFF_CACHE_PATH: &[&str] = &["cache", "ruff_cache"];
 const PIP_REQ_PATH: &[&str] = &["src", "tools", "tidy", "config", "requirements.txt"];
 
+const SPELLCHECK_DIRS: &[&str] = &["compiler", "library", "src/bootstrap", "src/librustdoc"];
+
 pub fn check(
     root_path: &Path,
     outdir: &Path,
+    ci_info: &CiInfo,
     bless: bool,
     extra_checks: Option<&str>,
     pos_args: &[String],
     bad: &mut bool,
 ) {
-    if let Err(e) = check_impl(root_path, outdir, bless, extra_checks, pos_args) {
+    if let Err(e) = check_impl(root_path, outdir, ci_info, bless, extra_checks, pos_args) {
         tidy_error!(bad, "{e}");
     }
 }
@@ -53,6 +58,7 @@ pub fn check(
 fn check_impl(
     root_path: &Path,
     outdir: &Path,
+    ci_info: &CiInfo,
     bless: bool,
     extra_checks: Option<&str>,
     pos_args: &[String],
@@ -73,7 +79,13 @@ fn check_impl(
                 (ExtraCheckArg::from_str(s), s)
             })
             .filter_map(|(res, src)| match res {
-                Ok(x) => Some(x),
+                Ok(arg) => {
+                    if arg.is_inactive_auto(ci_info) {
+                        None
+                    } else {
+                        Some(arg)
+                    }
+                }
                 Err(err) => {
                     eprintln!("warning: bad extra check argument {src:?}: {err:?}");
                     None
@@ -249,14 +261,9 @@ fn check_impl(
     if spellcheck {
         let config_path = root_path.join("typos.toml");
         // sync target files with .github/workflows/spellcheck.yml
-        let mut args = vec![
-            "-c",
-            config_path.as_os_str().to_str().unwrap(),
-            "./compiler",
-            "./library",
-            "./src/bootstrap",
-            "./src/librustdoc",
-        ];
+        let mut args = vec!["-c", config_path.as_os_str().to_str().unwrap()];
+
+        args.extend_from_slice(SPELLCHECK_DIRS);
 
         if bless {
             eprintln!("spellcheck files and fix");
@@ -663,9 +670,12 @@ enum ExtraCheckParseError {
     TooManyParts,
     /// Tried to parse the empty string
     Empty,
+    /// `auto` specified without lang part.
+    AutoRequiresLang,
 }
 
 struct ExtraCheckArg {
+    auto: bool,
     lang: ExtraCheckLang,
     /// None = run all extra checks for the given lang
     kind: Option<ExtraCheckKind>,
@@ -675,21 +685,47 @@ impl ExtraCheckArg {
     fn matches(&self, lang: ExtraCheckLang, kind: ExtraCheckKind) -> bool {
         self.lang == lang && self.kind.map(|k| k == kind).unwrap_or(true)
     }
+
+    /// Returns `true` if this is an auto arg and the relevant files are not modified.
+    fn is_inactive_auto(&self, ci_info: &CiInfo) -> bool {
+        if !self.auto {
+            return false;
+        }
+        let ext = match self.lang {
+            ExtraCheckLang::Py => ".py",
+            ExtraCheckLang::Cpp => ".cpp",
+            ExtraCheckLang::Shell => ".sh",
+            ExtraCheckLang::Spellcheck => {
+                return !crate::files_modified(ci_info, |s| {
+                    SPELLCHECK_DIRS.iter().any(|dir| Path::new(s).starts_with(dir))
+                });
+            }
+        };
+        !crate::files_modified(ci_info, |s| s.ends_with(ext))
+    }
 }
 
 impl FromStr for ExtraCheckArg {
     type Err = ExtraCheckParseError;
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let mut auto = false;
         let mut parts = s.split(':');
-        let Some(first) = parts.next() else {
+        let Some(mut first) = parts.next() else {
             return Err(ExtraCheckParseError::Empty);
         };
+        if first == "auto" {
+            let Some(part) = parts.next() else {
+                return Err(ExtraCheckParseError::AutoRequiresLang);
+            };
+            auto = true;
+            first = part;
+        }
         let second = parts.next();
         if parts.next().is_some() {
             return Err(ExtraCheckParseError::TooManyParts);
         }
-        Ok(Self { lang: first.parse()?, kind: second.map(|s| s.parse()).transpose()? })
+        Ok(Self { auto, lang: first.parse()?, kind: second.map(|s| s.parse()).transpose()? })
     }
 }
 
diff --git a/src/tools/tidy/src/main.rs b/src/tools/tidy/src/main.rs
index ef6ff5c9277..1eb5485f2b8 100644
--- a/src/tools/tidy/src/main.rs
+++ b/src/tools/tidy/src/main.rs
@@ -173,7 +173,15 @@ fn main() {
         };
         check!(unstable_book, &src_path, collected);
 
-        check!(ext_tool_checks, &root_path, &output_directory, bless, extra_checks, pos_args);
+        check!(
+            ext_tool_checks,
+            &root_path,
+            &output_directory,
+            &ci_info,
+            bless,
+            extra_checks,
+            pos_args
+        );
     });
 
     if bad.load(Ordering::Relaxed) {