Auto merge of rust-lang#96296 - cjgillot:remove-label-lt-shadow, r=petrochenkov

Remove label/lifetime shadowing warnings

This PR removes some pre-1.0 shadowing warnings for labels and lifetimes.

The current behaviour of the compiler is to warn
* labels that shadow unrelated labels in the same function --> removed
```rust
'a: loop {}
'a: loop {} // STOP WARNING
```

* labels that shadow enclosing labels --> kept, but only if shadowing is hygienic
```rust
'a: loop {
  'a: loop {} // KEEP WARNING
}
```

* labels that shadow lifetime --> removed
```rust
fn foo<'a>() {
  'a: loop {} // STOP WARNING
}
```

* lifetimes that shadow labels --> removed
```rust
'a: loop {
  let b = Box::new(|x: &i8| *x) as Box<dyn for <'a> Fn(&'a i8) -> i8>; // STOP WARNING
}
```

* lifetimes that shadow lifetimes --> kept
```rust
fn foo<'a>() {
  let b = Box::new(|x: &i8| *x) as Box<dyn for <'a> Fn(&'a i8) -> i8>; // KEEP WARNING
}
```

Closes rust-lang#31745.

-----

From `@petrochenkov` in rust-lang#95781 (comment)
> I think we should remove these silly checks entirely.
> They were introduced long time ago in case some new language features appear and require this space.
> Now we have another mechanism for such language changes - editions, and if "lifetimes in expressions" or something like that needs to be introduced it could be introduced as an edition change.
> However, there was no plans to introduce anything like for years, so it's unlikely that even the edition mechanism will be necessary.

r? rust-lang/lang

Author: bors

compiler/rustc_ast_lowering/src/lib.rs

@@ -1848,14 +1848,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_generic_param(&mut self, param: &GenericParam) -> hir::GenericParam<'hir> {
         let (name, kind) = match param.kind {
             GenericParamKind::Lifetime => {
-                let param_name = if param.ident.name == kw::StaticLifetime
-                    || param.ident.name == kw::UnderscoreLifetime
-                {
-                    ParamName::Error
-                } else {
-                    let ident = self.lower_ident(param.ident);
-                    ParamName::Plain(ident)
-                };
+                // AST resolution emitted an error on those parameters, so we lower them using
+                // `ParamName::Error`.
+                let param_name =
+                    if let Some(LifetimeRes::Error) = self.resolver.get_lifetime_res(param.id) {
+                        ParamName::Error
+                    } else {
+                        let ident = self.lower_ident(param.ident);
+                        ParamName::Plain(ident)
+                    };
                 let kind =
                     hir::GenericParamKind::Lifetime { kind: hir::LifetimeParamKind::Explicit };
 
@@ -1880,10 +1881,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 )
             }
         };
-        let name = match name {
-            hir::ParamName::Plain(ident) => hir::ParamName::Plain(self.lower_ident(ident)),
-            name => name,
-        };
 
         let hir_id = self.lower_node_id(param.id);
         self.lower_attrs(hir_id, &param.attrs);

compiler/rustc_error_codes/src/error_codes/E0263.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 A lifetime was declared more than once in the same scope.
 
 Erroneous code example:
 
-```compile_fail,E0263
+```compile_fail,E0403
 fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str, z: &'a str) { // error!
 }
 ```

compiler/rustc_resolve/src/late.rs

@@ -172,6 +172,23 @@ impl RibKind<'_> {
             AssocItemRibKind | ItemRibKind(_) | ForwardGenericParamBanRibKind => true,
         }
     }
+
+    /// This rib forbids referring to labels defined in upwards ribs.
+    fn is_label_barrier(self) -> bool {
+        match self {
+            NormalRibKind | MacroDefinition(..) => false,
+
+            AssocItemRibKind
+            | ClosureOrAsyncRibKind
+            | FnItemRibKind
+            | ItemRibKind(..)
+            | ConstantItemRibKind(..)
+            | ModuleRibKind(..)
+            | ForwardGenericParamBanRibKind
+            | ConstParamTyRibKind
+            | InlineAsmSymRibKind => true,
+        }
+    }
 }
 
 /// A single local scope.
@@ -732,7 +749,7 @@ impl<'a: 'ast, 'ast> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast> {
         // Create a value rib for the function.
         self.with_rib(ValueNS, rib_kind, |this| {
             // Create a label rib for the function.
-            this.with_label_rib(rib_kind, |this| {
+            this.with_label_rib(FnItemRibKind, |this| {
                 let async_node_id = fn_kind.header().and_then(|h| h.asyncness.opt_return_id());
 
                 if let FnKind::Fn(_, _, _, _, generics, _) = fn_kind {
@@ -1531,13 +1548,9 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
 
     /// Searches the current set of local scopes for labels. Returns the `NodeId` of the resolved
     /// label and reports an error if the label is not found or is unreachable.
-    fn resolve_label(&mut self, mut label: Ident) -> Option<NodeId> {
+    fn resolve_label(&mut self, mut label: Ident) -> Result<(NodeId, Span), ResolutionError<'a>> {
         let mut suggestion = None;
 
-        // Preserve the original span so that errors contain "in this macro invocation"
-        // information.
-        let original_span = label.span;
-
         for i in (0..self.label_ribs.len()).rev() {
             let rib = &self.label_ribs[i];
 
@@ -1553,18 +1566,13 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
             if let Some((ident, id)) = rib.bindings.get_key_value(&ident) {
                 let definition_span = ident.span;
                 return if self.is_label_valid_from_rib(i) {
-                    Some(*id)
+                    Ok((*id, definition_span))
                 } else {
-                    self.report_error(
-                        original_span,
-                        ResolutionError::UnreachableLabel {
-                            name: label.name,
-                            definition_span,
-                            suggestion,
-                        },
-                    );
-
-                    None
+                    Err(ResolutionError::UnreachableLabel {
+                        name: label.name,
+                        definition_span,
+                        suggestion,
+                    })
                 };
             }
 
@@ -1573,34 +1581,16 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
             suggestion = suggestion.or_else(|| self.suggestion_for_label_in_rib(i, label));
         }
 
-        self.report_error(
-            original_span,
-            ResolutionError::UndeclaredLabel { name: label.name, suggestion },
-        );
-        None
+        Err(ResolutionError::UndeclaredLabel { name: label.name, suggestion })
     }
 
     /// Determine whether or not a label from the `rib_index`th label rib is reachable.
     fn is_label_valid_from_rib(&self, rib_index: usize) -> bool {
         let ribs = &self.label_ribs[rib_index + 1..];
 
         for rib in ribs {
-            match rib.kind {
-                NormalRibKind | MacroDefinition(..) => {
-                    // Nothing to do. Continue.
-                }
-
-                AssocItemRibKind
-                | ClosureOrAsyncRibKind
-                | FnItemRibKind
-                | ItemRibKind(..)
-                | ConstantItemRibKind(..)
-                | ModuleRibKind(..)
-                | ForwardGenericParamBanRibKind
-                | ConstParamTyRibKind
-                | InlineAsmSymRibKind => {
-                    return false;
-                }
+            if rib.kind.is_label_barrier() {
+                return false;
             }
         }
 
@@ -1895,6 +1885,8 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
         let mut function_value_rib = Rib::new(kind);
         let mut function_lifetime_rib = LifetimeRib::new(lifetime_kind);
         let mut seen_bindings = FxHashMap::default();
+        // Store all seen lifetimes names from outer scopes.
+        let mut seen_lifetimes = FxHashSet::default();
 
         // We also can't shadow bindings from the parent item
         if let AssocItemRibKind = kind {
@@ -1910,16 +1902,36 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
             add_bindings_for_ns(TypeNS);
         }
 
+        // Forbid shadowing lifetime bindings
+        for rib in self.lifetime_ribs.iter().rev() {
+            seen_lifetimes.extend(rib.bindings.iter().map(|(ident, _)| *ident));
+            if let LifetimeRibKind::Item = rib.kind {
+                break;
+            }
+        }
+
         for param in params {
             let ident = param.ident.normalize_to_macros_2_0();
             debug!("with_generic_param_rib: {}", param.id);
 
+            if let GenericParamKind::Lifetime = param.kind
+                && let Some(&original) = seen_lifetimes.get(&ident)
+            {
+                diagnostics::signal_lifetime_shadowing(self.r.session, original, param.ident);
+                // Record lifetime res, so lowering knows there is something fishy.
+                self.record_lifetime_res(param.id, LifetimeRes::Error);
+                continue;
+            }
+
             match seen_bindings.entry(ident) {
                 Entry::Occupied(entry) => {
                     let span = *entry.get();
                     let err = ResolutionError::NameAlreadyUsedInParameterList(ident.name, span);
-                    if !matches!(param.kind, GenericParamKind::Lifetime) {
-                        self.report_error(param.ident.span, err);
+                    self.report_error(param.ident.span, err);
+                    if let GenericParamKind::Lifetime = param.kind {
+                        // Record lifetime res, so lowering knows there is something fishy.
+                        self.record_lifetime_res(param.id, LifetimeRes::Error);
+                        continue;
                     }
                 }
                 Entry::Vacant(entry) => {
@@ -1936,6 +1948,8 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                 )
                 .span_label(param.ident.span, "`'_` is a reserved lifetime name")
                 .emit();
+                // Record lifetime res, so lowering knows there is something fishy.
+                self.record_lifetime_res(param.id, LifetimeRes::Error);
                 continue;
             }
 
@@ -1949,6 +1963,8 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                 )
                 .span_label(param.ident.span, "'static is a reserved lifetime name")
                 .emit();
+                // Record lifetime res, so lowering knows there is something fishy.
+                self.record_lifetime_res(param.id, LifetimeRes::Error);
                 continue;
             }
 
@@ -3114,6 +3130,11 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
             if label.ident.as_str().as_bytes()[1] != b'_' {
                 self.diagnostic_metadata.unused_labels.insert(id, label.ident.span);
             }
+
+            if let Ok((_, orig_span)) = self.resolve_label(label.ident) {
+                diagnostics::signal_label_shadowing(self.r.session, orig_span, label.ident)
+            }
+
             self.with_label_rib(NormalRibKind, |this| {
                 let ident = label.ident.normalize_to_macro_rules();
                 this.label_ribs.last_mut().unwrap().bindings.insert(ident, id);
@@ -3219,10 +3240,15 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
             }
 
             ExprKind::Break(Some(label), _) | ExprKind::Continue(Some(label)) => {
-                if let Some(node_id) = self.resolve_label(label.ident) {
-                    // Since this res is a label, it is never read.
-                    self.r.label_res_map.insert(expr.id, node_id);
-                    self.diagnostic_metadata.unused_labels.remove(&node_id);
+                match self.resolve_label(label.ident) {
+                    Ok((node_id, _)) => {
+                        // Since this res is a label, it is never read.
+                        self.r.label_res_map.insert(expr.id, node_id);
+                        self.diagnostic_metadata.unused_labels.remove(&node_id);
+                    }
+                    Err(error) => {
+                        self.report_error(label.ident.span, error);
+                    }
                 }
 
                 // visit `break` argument if any

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -25,6 +25,7 @@ use rustc_hir::def_id::{DefId, CRATE_DEF_ID, LOCAL_CRATE};
 use rustc_hir::PrimTy;
 use rustc_session::lint;
 use rustc_session::parse::feature_err;
+use rustc_session::Session;
 use rustc_span::edition::Edition;
 use rustc_span::hygiene::MacroKind;
 use rustc_span::lev_distance::find_best_match_for_name;
@@ -2036,6 +2037,34 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
     }
 }
 
+/// Report lifetime/lifetime shadowing as an error.
+pub fn signal_lifetime_shadowing(sess: &Session, orig: Ident, shadower: Ident) {
+    let mut err = struct_span_err!(
+        sess,
+        shadower.span,
+        E0496,
+        "lifetime name `{}` shadows a lifetime name that is already in scope",
+        orig.name,
+    );
+    err.span_label(orig.span, "first declared here");
+    err.span_label(shadower.span, format!("lifetime `{}` already in scope", orig.name));
+    err.emit();
+}
+
+/// Shadowing involving a label is only a warning for historical reasons.
+//FIXME: make this a proper lint.
+pub fn signal_label_shadowing(sess: &Session, orig: Span, shadower: Ident) {
+    let name = shadower.name;
+    let shadower = shadower.span;
+    let mut err = sess.struct_span_warn(
+        shadower,
+        &format!("label name `{}` shadows a label name that is already in scope", name),
+    );
+    err.span_label(orig, "first declared here");
+    err.span_label(shadower, format!("label `{}` already in scope", name));
+    err.emit();
+}
+
 impl<'tcx> LifetimeContext<'_, 'tcx> {
     pub(crate) fn report_missing_lifetime_specifiers(
         &self,