add new unstable flag for minimal recursion

Author: Kivooeo

compiler/rustc_interface/src/limits.rs

@@ -11,13 +11,13 @@
 use rustc_hir::limit::Limit;
 use rustc_hir::{Attribute, find_attr};
 use rustc_middle::query::Providers;
-use rustc_session::Limits;
+use rustc_session::{Limits, Session};
 
 pub(crate) fn provide(providers: &mut Providers) {
     providers.limits = |tcx, ()| {
         let attrs = tcx.hir_krate_attrs();
         Limits {
-            recursion_limit: get_recursion_limit(tcx.hir_krate_attrs()),
+            recursion_limit: get_recursion_limit(tcx.hir_krate_attrs(), tcx.sess),
             move_size_limit: find_attr!(attrs, MoveSizeLimit { limit, .. } => *limit)
                 .unwrap_or(Limit::new(tcx.sess.opts.unstable_opts.move_size_limit.unwrap_or(0))),
             type_length_limit: find_attr!(attrs, TypeLengthLimit { limit, .. } => *limit)
@@ -30,6 +30,13 @@ pub(crate) fn provide(providers: &mut Providers) {
 }
 
 // This one is separate because it must be read prior to macro expansion.
-pub(crate) fn get_recursion_limit(attrs: &[Attribute]) -> Limit {
-    find_attr!(attrs, RecursionLimit { limit, .. } => *limit).unwrap_or(Limit::new(128))
+pub(crate) fn get_recursion_limit(attrs: &[Attribute], sess: &Session) -> Limit {
+    let limit_from_crate =
+        find_attr!(attrs, RecursionLimit { limit, .. } => limit.0).unwrap_or(128);
+    Limit::new(
+        sess.opts
+            .unstable_opts
+            .min_recursion_limit
+            .map_or(limit_from_crate, |min| min.max(limit_from_crate)),
+    )
 }

compiler/rustc_interface/src/passes.rs

@@ -1429,5 +1429,5 @@ fn get_recursion_limit(krate_attrs: &[ast::Attribute], sess: &Session) -> Limit
         // So, no lints here to avoid duplicates.
         ShouldEmit::EarlyFatal { also_emit_lints: false },
     );
-    crate::limits::get_recursion_limit(attr.as_slice())
+    crate::limits::get_recursion_limit(attr.as_slice(), sess)
 }

compiler/rustc_interface/src/tests.rs

@@ -822,6 +822,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(maximal_hir_to_mir_coverage, true);
     tracked!(merge_functions, Some(MergeFunctions::Disabled));
     tracked!(min_function_alignment, Some(Align::EIGHT));
+    tracked!(min_recursion_limit, Some(256));
     tracked!(mir_emit_retag, true);
     tracked!(mir_enable_passes, vec![("DestProp".to_string(), false)]);
     tracked!(mir_opt_level, Some(4));

compiler/rustc_session/src/options.rs

@@ -2456,6 +2456,8 @@ options! {
         "the directory metrics emitted by rustc are dumped into (implicitly enables default set of metrics)"),
     min_function_alignment: Option<Align> = (None, parse_align, [TRACKED],
         "align all functions to at least this many bytes. Must be a power of 2"),
+    min_recursion_limit: Option<usize> = (None, parse_opt_number, [TRACKED],
+        "set a minimum recursion limit (final limit = max(this, recursion_limit_from_crate))"),
     mir_emit_retag: bool = (false, parse_bool, [TRACKED],
         "emit Retagging MIR statements, interpreted e.g., by miri; implies -Zmir-opt-level=0 \
         (default: no)"),

src/doc/embedded-book

@@ -0,0 +1,12 @@
+# `min-recursion-limit`
+
+This flag sets a minimum recursion limit for the compiler. The final recursion limit is calculated as `max(min_recursion_limit, recursion_limit_from_crate)`. This cannot ever lower the recursion limit. Unless the current crate has an explicitly low `recursion_limit` attribute, any value less than the current default does not have an effect.
+
+The recursion limit affects (among other things):
+
+- macro expansion
+- the trait solver
+- const evaluation
+- query depth
+
+This flag is particularly useful when using the next trait solver (`-Z next-solver`), which may require a higher recursion limit for crates that were compiled successfully with the old solver.

src/doc/nomicon

@@ -0,0 +1,20 @@
+//@ compile-flags: -Z min-recursion-limit=0
+//@ check-pass
+
+// Checks that `min-recursion-limit` cannot lower the default recursion limit
+
+macro_rules! count {
+    () => {};
+    ($_:tt $($rest:tt)*) => { count!($($rest)*) };
+}
+
+fn main() {
+    // 100
+    count!(
+        a a a a a a a a a a a a a a a a a a a a
+        a a a a a a a a a a a a a a a a a a a a
+        a a a a a a a a a a a a a a a a a a a a
+        a a a a a a a a a a a a a a a a a a a a
+        a a a a a a a a a a a a a a a a a a a a
+    );
+}