Look for typos when reporting an unknown nightly feature

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -16,7 +16,6 @@ use rustc_errors::{
     pluralize,
 };
 use rustc_session::errors::ExprParenthesesNeeded;
-use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::used_keywords;
 use rustc_span::{BytePos, DUMMY_SP, Ident, Span, SpanSnippetError, Symbol, kw, sym};
@@ -222,27 +221,24 @@ impl std::fmt::Display for UnaryFixity {
     style = "verbose"
 )]
 struct MisspelledKw {
+    // We use a String here because `Symbol::into_diag_arg` calls `Symbol::to_ident_string`, which
+    // prefix the keyword with a `r#` because it aims to print the symbol as an identifier.
     similar_kw: String,
     #[primary_span]
     span: Span,
     is_incorrect_case: bool,
 }
 
 /// Checks if the given `lookup` identifier is similar to any keyword symbol in `candidates`.
+///
+/// This is a specialized version of [`Symbol::find_similar`] that constructs an error when a
+/// candidate is found.
 fn find_similar_kw(lookup: Ident, candidates: &[Symbol]) -> Option<MisspelledKw> {
-    let lowercase = lookup.name.as_str().to_lowercase();
-    let lowercase_sym = Symbol::intern(&lowercase);
-    if candidates.contains(&lowercase_sym) {
-        Some(MisspelledKw { similar_kw: lowercase, span: lookup.span, is_incorrect_case: true })
-    } else if let Some(similar_sym) = find_best_match_for_name(candidates, lookup.name, None) {
-        Some(MisspelledKw {
-            similar_kw: similar_sym.to_string(),
-            span: lookup.span,
-            is_incorrect_case: false,
-        })
-    } else {
-        None
-    }
+    lookup.name.find_similar(candidates).map(|(similar_kw, is_incorrect_case)| MisspelledKw {
+        similar_kw: similar_kw.to_string(),
+        is_incorrect_case,
+        span: lookup.span,
+    })
 }
 
 struct MultiSugg {

compiler/rustc_passes/messages.ftl

@@ -421,6 +421,8 @@ passes_missing_panic_handler =
 passes_missing_stability_attr =
     {$descr} has missing stability attribute
 
+passes_misspelled_feature = there is a feature with a similar name: `{$actual_name}`
+
 passes_mixed_export_name_and_no_mangle = `{$no_mangle_attr}` attribute may not be used in combination with `{$export_name_attr}`
     .label = `{$no_mangle_attr}` is ignored
     .note = `{$export_name_attr}` takes precedence

compiler/rustc_passes/src/errors.rs

@@ -1183,6 +1183,21 @@ pub(crate) struct UnknownFeature {
     #[primary_span]
     pub span: Span,
     pub feature: Symbol,
+    #[subdiagnostic]
+    pub suggestion: Option<MisspelledFeature>,
+}
+
+#[derive(Subdiagnostic)]
+#[suggestion(
+    passes_misspelled_feature,
+    style = "verbose",
+    code = "{actual_name}",
+    applicability = "maybe-incorrect"
+)]
+pub(crate) struct MisspelledFeature {
+    #[primary_span]
+    pub span: Span,
+    pub actual_name: Symbol,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_passes/src/stability.rs

@@ -6,7 +6,7 @@ use std::num::NonZero;
 use rustc_ast_lowering::stability::extern_abi_stability;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::unord::{ExtendUnord, UnordMap, UnordSet};
-use rustc_feature::{EnabledLangFeature, EnabledLibFeature};
+use rustc_feature::{EnabledLangFeature, EnabledLibFeature, UNSTABLE_LANG_FEATURES};
 use rustc_hir::attrs::{AttributeKind, DeprecatedSince};
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{CRATE_DEF_ID, LOCAL_CRATE, LocalDefId, LocalModDefId};
@@ -1062,11 +1062,13 @@ pub fn check_unused_or_stable_features(tcx: TyCtxt<'_>) {
     // no unknown features, because the collection also does feature attribute validation.
     let local_defined_features = tcx.lib_features(LOCAL_CRATE);
     if !remaining_lib_features.is_empty() || !remaining_implications.is_empty() {
+        let crates = tcx.crates(());
+
         // Loading the implications of all crates is unavoidable to be able to emit the partial
         // stabilization diagnostic, but it can be avoided when there are no
         // `remaining_lib_features`.
         let mut all_implications = remaining_implications.clone();
-        for &cnum in tcx.crates(()) {
+        for &cnum in crates {
             all_implications
                 .extend_unord(tcx.stability_implications(cnum).items().map(|(k, v)| (*k, *v)));
         }
@@ -1079,7 +1081,7 @@ pub fn check_unused_or_stable_features(tcx: TyCtxt<'_>) {
             &all_implications,
         );
 
-        for &cnum in tcx.crates(()) {
+        for &cnum in crates {
             if remaining_lib_features.is_empty() && remaining_implications.is_empty() {
                 break;
             }
@@ -1091,10 +1093,26 @@ pub fn check_unused_or_stable_features(tcx: TyCtxt<'_>) {
                 &all_implications,
             );
         }
-    }
 
-    for (feature, span) in remaining_lib_features {
-        tcx.dcx().emit_err(errors::UnknownFeature { span, feature });
+        if !remaining_lib_features.is_empty() {
+            let lang_features =
+                UNSTABLE_LANG_FEATURES.iter().map(|feature| feature.name).collect::<Vec<_>>();
+            let lib_features = crates
+                .into_iter()
+                .flat_map(|&cnum| {
+                    tcx.lib_features(cnum).stability.keys().copied().into_sorted_stable_ord()
+                })
+                .collect::<Vec<_>>();
+
+            let valid_feature_names = [lang_features, lib_features].concat();
+
+            for (feature, span) in remaining_lib_features {
+                let suggestion = feature
+                    .find_similar(&valid_feature_names)
+                    .map(|(actual_name, _)| errors::MisspelledFeature { span, actual_name });
+                tcx.dcx().emit_err(errors::UnknownFeature { span, feature, suggestion });
+            }
+        }
     }
 
     for (&implied_by, &feature) in remaining_implications.to_sorted_stable_ord() {

compiler/rustc_span/src/symbol.rs

@@ -14,6 +14,7 @@ use rustc_data_structures::stable_hasher::{
 use rustc_data_structures::sync::Lock;
 use rustc_macros::{Decodable, Encodable, HashStable_Generic, symbols};
 
+use crate::edit_distance::find_best_match_for_name;
 use crate::{DUMMY_SP, Edition, Span, with_session_globals};
 
 #[cfg(test)]
@@ -2843,6 +2844,27 @@ impl Symbol {
         // Avoid creating an empty identifier, because that asserts in debug builds.
         if self == sym::empty { String::new() } else { Ident::with_dummy_span(self).to_string() }
     }
+
+    /// Checks if `self` is similar to any symbol in `candidates`.
+    ///
+    /// The returned boolean represents whether the candidate is the same symbol with a different
+    /// casing.
+    ///
+    /// All the candidates are assumed to be lowercase.
+    pub fn find_similar(
+        self,
+        candidates: &[Symbol],
+    ) -> Option<(Symbol, /* is incorrect case */ bool)> {
+        let lowercase = self.as_str().to_lowercase();
+        let lowercase_sym = Symbol::intern(&lowercase);
+        if candidates.contains(&lowercase_sym) {
+            Some((lowercase_sym, true))
+        } else if let Some(similar_sym) = find_best_match_for_name(candidates, self, None) {
+            Some((similar_sym, false))
+        } else {
+            None
+        }
+    }
 }
 
 impl fmt::Debug for Symbol {

tests/ui/feature-gates/unknown-feature.rs

@@ -1,3 +1,16 @@
-#![feature(unknown_rust_feature)] //~ ERROR unknown feature
+#![feature(
+    unknown_rust_feature,
+    //~^ ERROR unknown feature
+
+    // Typo for lang feature
+    associated_types_default,
+    //~^ ERROR unknown feature
+    //~| HELP there is a feature with a similar name
+
+    // Typo for lib feature
+    core_intrnisics,
+    //~^ ERROR unknown feature
+    //~| HELP there is a feature with a similar name
+)]
 
 fn main() {}

tests/ui/feature-gates/unknown-feature.stderr

@@ -1,9 +1,33 @@
 error[E0635]: unknown feature `unknown_rust_feature`
-  --> $DIR/unknown-feature.rs:1:12
+  --> $DIR/unknown-feature.rs:2:5
    |
-LL | #![feature(unknown_rust_feature)]
-   |            ^^^^^^^^^^^^^^^^^^^^
+LL |     unknown_rust_feature,
+   |     ^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 1 previous error
+error[E0635]: unknown feature `associated_types_default`
+  --> $DIR/unknown-feature.rs:6:5
+   |
+LL |     associated_types_default,
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: there is a feature with a similar name: `associated_type_defaults`
+   |
+LL -     associated_types_default,
+LL +     associated_type_defaults,
+   |
+
+error[E0635]: unknown feature `core_intrnisics`
+  --> $DIR/unknown-feature.rs:11:5
+   |
+LL |     core_intrnisics,
+   |     ^^^^^^^^^^^^^^^
+   |
+help: there is a feature with a similar name: `core_intrinsics`
+   |
+LL -     core_intrnisics,
+LL +     core_intrinsics,
+   |
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0635`.