Rollup merge of #93845 - compiler-errors:in-band-lifetimes, r=cjgillot

Remove in band lifetimes

As discussed in t-lang backlog bonanza, the `in_band_lifetimes` FCP closed in favor for the feature not being stabilized. This PR removes `#![feature(in_band_lifetimes)]` in its entirety.

Let me know if this PR is too hasty, and if we should instead do something intermediate for deprecate the feature first.

r? `@scottmcm` (or feel free to reassign, just saw your last comment on #44524)
Closes #44524

compiler/rustc_ast_lowering/src/lib.rs

@@ -142,13 +142,9 @@ struct LoweringContext<'a, 'hir: 'a> {
     /// indicate whether or not we're in a place where new lifetimes will result
     /// in in-band lifetime definitions, such a function or an impl header,
     /// including implicit lifetimes from `impl_header_lifetime_elision`.
-    is_collecting_in_band_lifetimes: bool,
+    is_collecting_anonymous_lifetimes: bool,
 
     /// Currently in-scope lifetimes defined in impl headers, fn headers, or HRTB.
-    /// When `is_collecting_in_band_lifetimes` is true, each lifetime is checked
-    /// against this list to see if it is already in-scope, or if a definition
-    /// needs to be created for it.
-    ///
     /// We always store a `normalize_to_macros_2_0()` version of the param-name in this
     /// vector.
     in_scope_lifetimes: Vec<ParamName>,
@@ -379,7 +375,7 @@ pub fn lower_crate<'a, 'hir>(
         task_context: None,
         current_item: None,
         lifetimes_to_define: Vec::new(),
-        is_collecting_in_band_lifetimes: false,
+        is_collecting_anonymous_lifetimes: false,
         in_scope_lifetimes: Vec::new(),
         allow_try_trait: Some([sym::try_trait_v2][..].into()),
         allow_gen_future: Some([sym::gen_future][..].into()),
@@ -726,13 +722,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         &mut self,
         f: impl FnOnce(&mut Self) -> T,
     ) -> (Vec<(Span, ParamName)>, T) {
-        let was_collecting = std::mem::replace(&mut self.is_collecting_in_band_lifetimes, true);
+        let was_collecting = std::mem::replace(&mut self.is_collecting_anonymous_lifetimes, true);
         let len = self.lifetimes_to_define.len();
 
         let res = f(self);
 
         let lifetimes_to_define = self.lifetimes_to_define.split_off(len);
-        self.is_collecting_in_band_lifetimes = was_collecting;
+        self.is_collecting_anonymous_lifetimes = was_collecting;
         (lifetimes_to_define, res)
     }
 
@@ -749,7 +745,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         // that collisions are ok here and this shouldn't
         // really show up for end-user.
         let (str_name, kind) = match hir_name {
-            ParamName::Plain(ident) => (ident.name, hir::LifetimeParamKind::InBand),
+            ParamName::Plain(ident) => (ident.name, hir::LifetimeParamKind::Explicit),
             ParamName::Fresh(_) => (kw::UnderscoreLifetime, hir::LifetimeParamKind::Elided),
             ParamName::Error => (kw::UnderscoreLifetime, hir::LifetimeParamKind::Error),
         };
@@ -773,38 +769,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
-    /// When there is a reference to some lifetime `'a`, and in-band
-    /// lifetimes are enabled, then we want to push that lifetime into
-    /// the vector of names to define later. In that case, it will get
-    /// added to the appropriate generics.
-    fn maybe_collect_in_band_lifetime(&mut self, ident: Ident) {
-        if !self.is_collecting_in_band_lifetimes {
-            return;
-        }
-
-        if !self.sess.features_untracked().in_band_lifetimes {
-            return;
-        }
-
-        if self.in_scope_lifetimes.contains(&ParamName::Plain(ident.normalize_to_macros_2_0())) {
-            return;
-        }
-
-        let hir_name = ParamName::Plain(ident);
-
-        if self.lifetimes_to_define.iter().any(|(_, lt_name)| {
-            lt_name.normalize_to_macros_2_0() == hir_name.normalize_to_macros_2_0()
-        }) {
-            return;
-        }
-
-        self.lifetimes_to_define.push((ident.span, hir_name));
-    }
-
     /// When we have either an elided or `'_` lifetime in an impl
     /// header, we convert it to an in-band lifetime.
-    fn collect_fresh_in_band_lifetime(&mut self, span: Span) -> ParamName {
-        assert!(self.is_collecting_in_band_lifetimes);
+    fn collect_fresh_anonymous_lifetime(&mut self, span: Span) -> ParamName {
+        assert!(self.is_collecting_anonymous_lifetimes);
         let index = self.lifetimes_to_define.len() + self.in_scope_lifetimes.len();
         let hir_name = ParamName::Fresh(index);
         self.lifetimes_to_define.push((span, hir_name));
@@ -1946,7 +1914,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             }
             ident if ident.name == kw::UnderscoreLifetime => match self.anonymous_lifetime_mode {
                 AnonymousLifetimeMode::CreateParameter => {
-                    let fresh_name = self.collect_fresh_in_band_lifetime(span);
+                    let fresh_name = self.collect_fresh_anonymous_lifetime(span);
                     self.new_named_lifetime(l.id, span, hir::LifetimeName::Param(fresh_name))
                 }
 
@@ -1957,7 +1925,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 AnonymousLifetimeMode::ReportError => self.new_error_lifetime(Some(l.id), span),
             },
             ident => {
-                self.maybe_collect_in_band_lifetime(ident);
                 let param_name = ParamName::Plain(self.lower_ident(ident));
                 self.new_named_lifetime(l.id, span, hir::LifetimeName::Param(param_name))
             }
@@ -2001,8 +1968,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
         let (name, kind) = match param.kind {
             GenericParamKind::Lifetime => {
-                let was_collecting_in_band = self.is_collecting_in_band_lifetimes;
-                self.is_collecting_in_band_lifetimes = false;
+                let was_collecting_in_band = self.is_collecting_anonymous_lifetimes;
+                self.is_collecting_anonymous_lifetimes = false;
 
                 let lt = self
                     .with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
@@ -2025,7 +1992,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 let kind =
                     hir::GenericParamKind::Lifetime { kind: hir::LifetimeParamKind::Explicit };
 
-                self.is_collecting_in_band_lifetimes = was_collecting_in_band;
+                self.is_collecting_anonymous_lifetimes = was_collecting_in_band;
 
                 (param_name, kind)
             }
@@ -2384,7 +2351,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             // Hence `impl Foo for &u32` becomes `impl<'f> Foo for &'f u32` for some fresh
             // `'f`.
             AnonymousLifetimeMode::CreateParameter => {
-                let fresh_name = self.collect_fresh_in_band_lifetime(span);
+                let fresh_name = self.collect_fresh_anonymous_lifetime(span);
                 hir::Lifetime {
                     hir_id: self.next_id(),
                     span: self.lower_span(span),

compiler/rustc_error_codes/src/error_codes/E0687.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 In-band lifetimes cannot be used in `fn`/`Fn` syntax.
 
 Erroneous code examples:
 
-```compile_fail,E0687
+```ignore (feature got removed)
 #![feature(in_band_lifetimes)]
 
 fn foo(x: fn(&'a u32)) {} // error!

compiler/rustc_error_codes/src/error_codes/E0688.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 In-band lifetimes were mixed with explicit lifetime binders.
 
 Erroneous code example:
 
-```compile_fail,E0688
+```ignore (feature got removed)
 #![feature(in_band_lifetimes)]
 
 fn foo<'a>(x: &'a u32, y: &'b u32) {}   // error!

compiler/rustc_feature/src/active.rs

@@ -400,8 +400,6 @@ declare_features! (
     (active, if_let_guard, "1.47.0", Some(51114), None),
     /// Allows using imported `main` function
     (active, imported_main, "1.53.0", Some(28937), None),
-    /// Allows in-band quantification of lifetime bindings (e.g., `fn foo(x: &'a u8) -> &'a u8`).
-    (active, in_band_lifetimes, "1.23.0", Some(44524), None),
     /// Allows inferring `'static` outlives requirements (RFC 2093).
     (active, infer_static_outlives_requirements, "1.26.0", Some(54185), None),
     /// Allows associated types in inherent impls.

compiler/rustc_feature/src/removed.rs

@@ -104,6 +104,9 @@ declare_features! (
     (removed, impl_trait_in_bindings, "1.55.0", Some(63065), None,
      Some("the implementation was not maintainable, the feature may get reintroduced once the current refactorings are done")),
     (removed, import_shadowing, "1.0.0", None, None, None),
+    /// Allows in-band quantification of lifetime bindings (e.g., `fn foo(x: &'a u8) -> &'a u8`).
+    (removed, in_band_lifetimes, "1.23.0", Some(44524), None,
+     Some("removed due to unsolved ergonomic questions and added lifetime resolution complexity")),
     /// Lazily evaluate constants. This allows constants to depend on type parameters.
     (removed, lazy_normalization_consts, "1.46.0", Some(72219), None, Some("superseded by `generic_const_exprs`")),
     /// Allows using the `#[link_args]` attribute.

compiler/rustc_hir/src/hir.rs

@@ -471,11 +471,6 @@ pub enum LifetimeParamKind {
     // `fn foo<'a>(x: &'a u8) -> &'a u8 { x }`).
     Explicit,
 
-    // Indicates that the lifetime definition was synthetically added
-    // as a result of an in-band lifetime usage (e.g., in
-    // `fn foo(x: &'a u8) -> &'a u8 { x }`).
-    InBand,
-
     // Indication that the lifetime was elided (e.g., in both cases in
     // `fn foo(x: &u8) -> &'_ u8 { x }`).
     Elided,

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/find_anon_type.rs

@@ -125,7 +125,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
                     // Find the index of the named region that was part of the
                     // error. We will then search the function parameters for a bound
                     // region at the right depth with the same index
-                    (Some(rl::Region::EarlyBound(_, id, _)), ty::BrNamed(def_id, _)) => {
+                    (Some(rl::Region::EarlyBound(_, id)), ty::BrNamed(def_id, _)) => {
                         debug!("EarlyBound id={:?} def_id={:?}", id, def_id);
                         if id == def_id {
                             self.found_type = Some(arg);
@@ -137,7 +137,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
                     // error. We will then search the function parameters for a bound
                     // region at the right depth with the same index
                     (
-                        Some(rl::Region::LateBound(debruijn_index, _, id, _)),
+                        Some(rl::Region::LateBound(debruijn_index, _, id)),
                         ty::BrNamed(def_id, _),
                     ) => {
                         debug!(
@@ -155,8 +155,8 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
                         Some(
                             rl::Region::Static
                             | rl::Region::Free(_, _)
-                            | rl::Region::EarlyBound(_, _, _)
-                            | rl::Region::LateBound(_, _, _, _)
+                            | rl::Region::EarlyBound(_, _)
+                            | rl::Region::LateBound(_, _, _)
                             | rl::Region::LateBoundAnon(_, _, _),
                         )
                         | None,
@@ -221,15 +221,15 @@ impl<'tcx> Visitor<'tcx> for TyPathVisitor<'tcx> {
                 }
             }
 
-            (Some(rl::Region::EarlyBound(_, id, _)), ty::BrNamed(def_id, _)) => {
+            (Some(rl::Region::EarlyBound(_, id)), ty::BrNamed(def_id, _)) => {
                 debug!("EarlyBound id={:?} def_id={:?}", id, def_id);
                 if id == def_id {
                     self.found_it = true;
                     return; // we can stop visiting now
                 }
             }
 
-            (Some(rl::Region::LateBound(debruijn_index, _, id, _)), ty::BrNamed(def_id, _)) => {
+            (Some(rl::Region::LateBound(debruijn_index, _, id)), ty::BrNamed(def_id, _)) => {
                 debug!("FindNestedTypeVisitor::visit_ty: LateBound depth = {:?}", debruijn_index,);
                 debug!("id={:?}", id);
                 debug!("def_id={:?}", def_id);
@@ -242,8 +242,8 @@ impl<'tcx> Visitor<'tcx> for TyPathVisitor<'tcx> {
             (
                 Some(
                     rl::Region::Static
-                    | rl::Region::EarlyBound(_, _, _)
-                    | rl::Region::LateBound(_, _, _, _)
+                    | rl::Region::EarlyBound(_, _)
+                    | rl::Region::LateBound(_, _, _)
                     | rl::Region::LateBoundAnon(_, _, _)
                     | rl::Region::Free(_, _),
                 )

compiler/rustc_middle/src/middle/resolve_lifetime.rs

@@ -4,47 +4,14 @@ use crate::ty;
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::{GenericParam, ItemLocalId};
-use rustc_hir::{GenericParamKind, LifetimeParamKind};
+use rustc_hir::ItemLocalId;
 use rustc_macros::HashStable;
 
-/// The origin of a named lifetime definition.
-///
-/// This is used to prevent the usage of in-band lifetimes in `Fn`/`fn` syntax.
-#[derive(Copy, Clone, PartialEq, Eq, Hash, TyEncodable, TyDecodable, Debug, HashStable)]
-pub enum LifetimeDefOrigin {
-    // Explicit binders like `fn foo<'a>(x: &'a u8)` or elided like `impl Foo<&u32>`
-    ExplicitOrElided,
-    // In-band declarations like `fn foo(x: &'a u8)`
-    InBand,
-    // Some kind of erroneous origin
-    Error,
-}
-
-impl LifetimeDefOrigin {
-    pub fn from_param(param: &GenericParam<'_>) -> Self {
-        match param.kind {
-            GenericParamKind::Lifetime { kind } => match kind {
-                LifetimeParamKind::InBand => LifetimeDefOrigin::InBand,
-                LifetimeParamKind::Explicit => LifetimeDefOrigin::ExplicitOrElided,
-                LifetimeParamKind::Elided => LifetimeDefOrigin::ExplicitOrElided,
-                LifetimeParamKind::Error => LifetimeDefOrigin::Error,
-            },
-            _ => bug!("expected a lifetime param"),
-        }
-    }
-}
-
 #[derive(Clone, Copy, PartialEq, Eq, Hash, TyEncodable, TyDecodable, Debug, HashStable)]
 pub enum Region {
     Static,
-    EarlyBound(/* index */ u32, /* lifetime decl */ DefId, LifetimeDefOrigin),
-    LateBound(
-        ty::DebruijnIndex,
-        /* late-bound index */ u32,
-        /* lifetime decl */ DefId,
-        LifetimeDefOrigin,
-    ),
+    EarlyBound(/* index */ u32, /* lifetime decl */ DefId),
+    LateBound(ty::DebruijnIndex, /* late-bound index */ u32, /* lifetime decl */ DefId),
     LateBoundAnon(ty::DebruijnIndex, /* late-bound index */ u32, /* anon index */ u32),
     Free(DefId, /* lifetime decl */ DefId),
 }

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -1838,7 +1838,6 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
             lifetime_ref
         );
         err.span_label(lifetime_ref.span, "undeclared lifetime");
-        let mut suggests_in_band = false;
         let mut suggested_spans = vec![];
         for missing in &self.missing_named_lifetime_spots {
             match missing {
@@ -1854,7 +1853,6 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
                     }) {
                         (param.span.shrink_to_lo(), format!("{}, ", lifetime_ref))
                     } else {
-                        suggests_in_band = true;
                         (generics.span, format!("<{}>", lifetime_ref))
                     };
                     if suggested_spans.contains(&span) {
@@ -1889,15 +1887,6 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
                 _ => {}
             }
         }
-        if self.tcx.sess.is_nightly_build()
-            && !self.tcx.features().in_band_lifetimes
-            && suggests_in_band
-        {
-            err.help(
-                "if you want to experiment with in-band lifetime bindings, \
-                 add `#![feature(in_band_lifetimes)]` to the crate attributes",
-            );
-        }
         err.emit();
     }