Auto merge of #100456 - Dylan-DPC:rollup-fn17z9f, r=Dylan-DPC

Rollup of 9 pull requests

Successful merges:

 - #100022 (Optimize thread ID generation)
 - #100030 (cleanup code w/ pointers in std a little)
 - #100229 (add -Zextra-const-ub-checks to enable more UB checking in const-eval)
 - #100247 (Generalize trait object generic param check to aliases.)
 - #100255 (Adding more verbose documentation for `std::fmt::Write`)
 - #100366 (errors: don't fail on broken primary translations)
 - #100396 (Suggest const and static for global variable)
 - #100409 (rustdoc: don't generate DOM element for operator)
 - #100443 (Add two let else regression tests)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -236,6 +236,16 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
 
     const PANIC_ON_ALLOC_FAIL: bool = false; // will be raised as a proper error
 
+    #[inline(always)]
+    fn enforce_alignment(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        ecx.tcx.sess.opts.unstable_opts.extra_const_ub_checks
+    }
+
+    #[inline(always)]
+    fn enforce_validity(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        ecx.tcx.sess.opts.unstable_opts.extra_const_ub_checks
+    }
+
     fn load_mir(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         instance: ty::InstanceDef<'tcx>,

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -436,24 +436,12 @@ pub macro compile_time_machine(<$mir: lifetime, $tcx: lifetime>) {
     type AllocExtra = ();
     type FrameExtra = ();
 
-    #[inline(always)]
-    fn enforce_alignment(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
-        // We do not check for alignment to avoid having to carry an `Align`
-        // in `ConstValue::ByRef`.
-        false
-    }
-
     #[inline(always)]
     fn force_int_for_alignment_check(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
         // We do not support `force_int`.
         false
     }
 
-    #[inline(always)]
-    fn enforce_validity(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
-        false // for now, we don't enforce validity
-    }
-
     #[inline(always)]
     fn enforce_number_init(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
         true

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -1005,6 +1005,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// It will error if the bits at the destination do not match the ones described by the layout.
     #[inline(always)]
     pub fn validate_operand(&self, op: &OpTy<'tcx, M::Provenance>) -> InterpResult<'tcx> {
+        // Note that we *could* actually be in CTFE here with `-Zextra-const-ub-checks`, but it's
+        // still correct to not use `ctfe_mode`: that mode is for validation of the final constant
+        // value, it rules out things like `UnsafeCell` in awkward places. It also can make checking
+        // recurse through references which, for now, we don't want here, either.
         self.validate_operand_internal(op, vec![], None, None)
     }
 }

compiler/rustc_errors/src/emitter.rs

@@ -273,40 +273,58 @@ pub trait Emitter {
             DiagnosticMessage::FluentIdentifier(identifier, attr) => (identifier, attr),
         };
 
-        let bundle = match self.fluent_bundle() {
-            Some(bundle) if bundle.has_message(&identifier) => bundle,
-            _ => self.fallback_fluent_bundle(),
-        };
+        let translate_with_bundle = |bundle: &'a FluentBundle| -> Option<(Cow<'_, str>, Vec<_>)> {
+            let message = bundle.get_message(&identifier)?;
+            let value = match attr {
+                Some(attr) => message.get_attribute(attr)?.value(),
+                None => message.value()?,
+            };
+            debug!(?message, ?value);
 
-        let message = bundle.get_message(&identifier).expect("missing diagnostic in fluent bundle");
-        let value = match attr {
-            Some(attr) => {
-                if let Some(attr) = message.get_attribute(attr) {
-                    attr.value()
-                } else {
-                    panic!("missing attribute `{attr}` in fluent message `{identifier}`")
-                }
-            }
-            None => {
-                if let Some(value) = message.value() {
-                    value
-                } else {
-                    panic!("missing value in fluent message `{identifier}`")
-                }
-            }
+            let mut errs = vec![];
+            let translated = bundle.format_pattern(value, Some(&args), &mut errs);
+            debug!(?translated, ?errs);
+            Some((translated, errs))
         };
 
-        let mut err = vec![];
-        let translated = bundle.format_pattern(value, Some(&args), &mut err);
-        trace!(?translated, ?err);
-        debug_assert!(
-            err.is_empty(),
-            "identifier: {:?}, args: {:?}, errors: {:?}",
-            identifier,
-            args,
-            err
-        );
-        translated
+        self.fluent_bundle()
+            .and_then(|bundle| translate_with_bundle(bundle))
+            // If `translate_with_bundle` returns `None` with the primary bundle, this is likely
+            // just that the primary bundle doesn't contain the message being translated, so
+            // proceed to the fallback bundle.
+            //
+            // However, when errors are produced from translation, then that means the translation
+            // is broken (e.g. `{$foo}` exists in a translation but `foo` isn't provided).
+            //
+            // In debug builds, assert so that compiler devs can spot the broken translation and
+            // fix it..
+            .inspect(|(_, errs)| {
+                debug_assert!(
+                    errs.is_empty(),
+                    "identifier: {:?}, attr: {:?}, args: {:?}, errors: {:?}",
+                    identifier,
+                    attr,
+                    args,
+                    errs
+                );
+            })
+            // ..otherwise, for end users, an error about this wouldn't be useful or actionable, so
+            // just hide it and try with the fallback bundle.
+            .filter(|(_, errs)| errs.is_empty())
+            .or_else(|| translate_with_bundle(self.fallback_fluent_bundle()))
+            .map(|(translated, errs)| {
+                // Always bail out for errors with the fallback bundle.
+                assert!(
+                    errs.is_empty(),
+                    "identifier: {:?}, attr: {:?}, args: {:?}, errors: {:?}",
+                    identifier,
+                    attr,
+                    args,
+                    errs
+                );
+                translated
+            })
+            .expect("failed to find message in primary or fallback fluent bundles")
     }
 
     /// Formats the substitutions of the primary_span

compiler/rustc_errors/src/lib.rs

@@ -6,9 +6,10 @@
 #![feature(drain_filter)]
 #![feature(if_let_guard)]
 #![cfg_attr(bootstrap, feature(let_chains))]
+#![feature(adt_const_params)]
 #![feature(let_else)]
 #![feature(never_type)]
-#![feature(adt_const_params)]
+#![feature(result_option_inspect)]
 #![feature(rustc_attrs)]
 #![allow(incomplete_features)]
 #![allow(rustc::potential_query_instability)]

compiler/rustc_mir_transform/src/const_prop.rs

@@ -183,6 +183,18 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx>
 
     type MemoryKind = !;
 
+    #[inline(always)]
+    fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        // We do not check for alignment to avoid having to carry an `Align`
+        // in `ConstValue::ByRef`.
+        false
+    }
+
+    #[inline(always)]
+    fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        false // for now, we don't enforce validity
+    }
+
     fn load_mir(
         _ecx: &InterpCx<'mir, 'tcx, Self>,
         _instance: ty::InstanceDef<'tcx>,

compiler/rustc_parse/src/parser/item.rs

@@ -68,7 +68,12 @@ impl<'a> Parser<'a> {
             if !self.maybe_consume_incorrect_semicolon(&items) {
                 let msg = &format!("expected item, found {token_str}");
                 let mut err = self.struct_span_err(self.token.span, msg);
-                err.span_label(self.token.span, "expected item");
+                let label = if self.is_kw_followed_by_ident(kw::Let) {
+                    "consider using `const` or `static` instead of `let` for global variables"
+                } else {
+                    "expected item"
+                };
+                err.span_label(self.token.span, label);
                 return Err(err);
             }
         }

compiler/rustc_session/src/options.rs

@@ -1310,6 +1310,8 @@ options! {
         "emit the bc module with thin LTO info (default: yes)"),
     export_executable_symbols: bool = (false, parse_bool, [TRACKED],
         "export symbols from executables, as if they were dynamic libraries"),
+    extra_const_ub_checks: bool = (false, parse_bool, [TRACKED],
+        "turns on more checks to detect const UB, which can be slow (default: no)"),
     #[cfg_attr(not(bootstrap), rustc_lint_opt_deny_field_access("use `Session::fewer_names` instead of this field"))]
     fewer_names: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "reduce memory use by retaining fewer names within compilation artifacts (LLVM-IR) \