Keep track of patterns that could have introduced a binding, but didn't

When we recover from a pattern parse error, or a pattern uses `..`, we keep track of that and affect resolution error for missing bindings that could have been provided by that pattern. We differentiate between `..` and parse recovery. We silence resolution errors likely caused by the pattern parse error.

```
error[E0425]: cannot find value `title` in this scope
  --> $DIR/struct-pattern-with-missing-fields-resolve-error.rs:19:30
   |
LL |         println!("[{}]({})", title, url);
   |                              ^^^^^ not found in this scope
   |
note: `Website` has a field `title` which could have been included in this pattern, but it wasn't
  --> $DIR/struct-pattern-with-missing-fields-resolve-error.rs:17:12
   |
LL | / struct Website {
LL | |     url: String,
LL | |     title: Option<String> ,
   | |     ----- defined here
LL | | }
   | |_-
...
LL |       if let Website { url, .. } = website {
   |              ^^^^^^^^^^^^^^^^^^^ this pattern doesn't include `title`, which is available in `Website`
```

Fix #74863.

Author: estebank

compiler/rustc_ast/src/ast.rs

@@ -859,6 +859,8 @@ pub enum PatKind {
 pub enum PatFieldsRest {
     /// `module::StructName { field, ..}`
     Rest,
+    /// `module::StructName { field, syntax error }`
+    Recovered(ErrorGuaranteed),
     /// `module::StructName { field }`
     None,
 }

compiler/rustc_ast_lowering/src/pat.rs

@@ -92,7 +92,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                 span: self.lower_span(f.span),
                             }
                         }));
-                        break hir::PatKind::Struct(qpath, fs, *etc == ast::PatFieldsRest::Rest);
+                        break hir::PatKind::Struct(
+                            qpath,
+                            fs,
+                            matches!(
+                                etc,
+                                ast::PatFieldsRest::Rest | ast::PatFieldsRest::Recovered(_)
+                            ),
+                        );
                     }
                     PatKind::Tuple(pats) => {
                         let (pats, ddpos) = self.lower_pat_tuple(pats, "tuple");

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1653,11 +1653,14 @@ impl<'a> State<'a> {
                     },
                     |f| f.pat.span,
                 );
-                if *etc == ast::PatFieldsRest::Rest {
+                if let ast::PatFieldsRest::Rest | ast::PatFieldsRest::Recovered(_) = etc {
                     if !fields.is_empty() {
                         self.word_space(",");
                     }
                     self.word("..");
+                    if let ast::PatFieldsRest::Recovered(_) = etc {
+                        self.word("/* recovered parse error */");
+                    }
                 }
                 if !empty {
                     self.space();

compiler/rustc_parse/src/parser/pat.rs

@@ -1371,10 +1371,10 @@ impl<'a> Parser<'a> {
         self.bump();
         let (fields, etc) = self.parse_pat_fields().unwrap_or_else(|mut e| {
             e.span_label(path.span, "while parsing the fields for this pattern");
-            e.emit();
+            let guar = e.emit();
             self.recover_stmt();
             // When recovering, pretend we had `Foo { .. }`, to avoid cascading errors.
-            (ThinVec::new(), PatFieldsRest::Rest)
+            (ThinVec::new(), PatFieldsRest::Recovered(guar))
         });
         self.bump();
         Ok(PatKind::Struct(qself, path, fields, etc))

compiler/rustc_resolve/src/late.rs

@@ -264,12 +264,17 @@ impl RibKind<'_> {
 #[derive(Debug)]
 pub(crate) struct Rib<'ra, R = Res> {
     pub bindings: IdentMap<R>,
+    pub patterns_with_skipped_bindings: FxHashMap<DefId, Vec<(Span, bool /* recovered error */)>>,
     pub kind: RibKind<'ra>,
 }
 
 impl<'ra, R> Rib<'ra, R> {
     fn new(kind: RibKind<'ra>) -> Rib<'ra, R> {
-        Rib { bindings: Default::default(), kind }
+        Rib {
+            bindings: Default::default(),
+            patterns_with_skipped_bindings: Default::default(),
+            kind,
+        }
     }
 }
 
@@ -3753,6 +3758,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
     /// When a whole or-pattern has been dealt with, the thing happens.
     ///
     /// See the implementation and `fresh_binding` for more details.
+    #[tracing::instrument(skip(self, bindings), level = "debug")]
     fn resolve_pattern_inner(
         &mut self,
         pat: &Pat,
@@ -3761,7 +3767,6 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
     ) {
         // Visit all direct subpatterns of this pattern.
         pat.walk(&mut |pat| {
-            debug!("resolve_pattern pat={:?} node={:?}", pat, pat.kind);
             match pat.kind {
                 PatKind::Ident(bmode, ident, ref sub) => {
                     // First try to resolve the identifier as some existing entity,
@@ -3787,8 +3792,9 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                 PatKind::Path(ref qself, ref path) => {
                     self.smart_resolve_path(pat.id, qself, path, PathSource::Pat);
                 }
-                PatKind::Struct(ref qself, ref path, ..) => {
+                PatKind::Struct(ref qself, ref path, ref _fields, ref rest) => {
                     self.smart_resolve_path(pat.id, qself, path, PathSource::Struct);
+                    self.record_patterns_with_skipped_bindings(pat, rest);
                 }
                 PatKind::Or(ref ps) => {
                     // Add a new set of bindings to the stack. `Or` here records that when a
@@ -3821,6 +3827,27 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
         });
     }
 
+    fn record_patterns_with_skipped_bindings(&mut self, pat: &Pat, rest: &ast::PatFieldsRest) {
+        match rest {
+            ast::PatFieldsRest::Rest | ast::PatFieldsRest::Recovered(_) => {
+                // Record that the pattern doesn't introduce all the bindings it could.
+                if let Some(partial_res) = self.r.partial_res_map.get(&pat.id)
+                    && let Some(res) = partial_res.full_res()
+                    && let Some(def_id) = res.opt_def_id()
+                {
+                    self.ribs[ValueNS]
+                        .last_mut()
+                        .unwrap()
+                        .patterns_with_skipped_bindings
+                        .entry(def_id)
+                        .or_default()
+                        .push((pat.span, matches!(rest, ast::PatFieldsRest::Recovered(_))));
+                }
+            }
+            ast::PatFieldsRest::None => {}
+        }
+    }
+
     fn fresh_binding(
         &mut self,
         ident: Ident,

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -430,6 +430,7 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
         let mut err = self.r.dcx().struct_span_err(base_error.span, base_error.msg.clone());
         err.code(code);
 
+        self.detect_missing_binding_available_from_pattern(&mut err, path, following_seg);
         self.suggest_at_operator_in_slice_pat_with_range(&mut err, path);
         self.suggest_swapping_misplaced_self_ty_and_trait(&mut err, source, res, base_error.span);
 
@@ -1120,6 +1121,65 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
         true
     }
 
+    fn detect_missing_binding_available_from_pattern(
+        &mut self,
+        err: &mut Diag<'_>,
+        path: &[Segment],
+        following_seg: Option<&Segment>,
+    ) {
+        let [segment] = path else { return };
+        let None = following_seg else { return };
+        'outer: for rib in self.ribs[ValueNS].iter().rev() {
+            for (def_id, spans) in &rib.patterns_with_skipped_bindings {
+                if let Some(fields) = self.r.field_idents(*def_id) {
+                    for field in fields {
+                        if field.name == segment.ident.name {
+                            if spans.iter().all(|(_, was_recovered)| *was_recovered) {
+                                // This resolution error will likely be fixed by fixing a
+                                // syntax error in a pattern, so it is irrelevant to the user.
+                                let multispan: MultiSpan =
+                                    spans.iter().map(|(s, _)| *s).collect::<Vec<_>>().into();
+                                err.span_note(
+                                    multispan,
+                                    "this pattern had a recovered parse error which likely \
+                                        lost the expected fields",
+                                );
+                                err.downgrade_to_delayed_bug();
+                            }
+                            let mut multispan: MultiSpan = spans
+                                .iter()
+                                .filter(|(_, was_recovered)| !was_recovered)
+                                .map(|(sp, _)| *sp)
+                                .collect::<Vec<_>>()
+                                .into();
+                            let def_span = self.r.def_span(*def_id);
+                            let ty = self.r.tcx.item_name(*def_id);
+                            multispan.push_span_label(def_span, String::new());
+                            multispan.push_span_label(field.span, "defined here".to_string());
+                            for (span, _) in spans.iter().filter(|(_, r)| !r) {
+                                multispan.push_span_label(
+                                    *span,
+                                    format!(
+                                        "this pattern doesn't include `{field}`, which is \
+                                         available in `{ty}`",
+                                    ),
+                                );
+                            }
+                            err.span_note(
+                                multispan,
+                                format!(
+                                    "`{ty}` has a field `{field}` which could have been included \
+                                     in this pattern, but it wasn't",
+                                ),
+                            );
+                            break 'outer;
+                        }
+                    }
+                }
+            }
+        }
+    }
+
     fn suggest_at_operator_in_slice_pat_with_range(
         &mut self,
         err: &mut Diag<'_>,

tests/ui/pattern/struct-pattern-with-missing-fields-resolve-error.rs

@@ -0,0 +1,22 @@
+struct Website {
+    url: String,
+    title: Option<String> ,//~ NOTE defined here
+}
+
+fn main() {
+    let website = Website {
+        url: "http://www.example.com".into(),
+        title: Some("Example Domain".into()),
+    };
+
+    if let Website { url, Some(title) } = website { //~ ERROR expected `,`
+        //~^ NOTE while parsing the fields for this pattern
+        println!("[{}]({})", title, url); // we hide the errors for `title` and `url`
+    }
+
+    if let Website { url, .. } = website { //~ NOTE `Website` has a field `title`
+        //~^ NOTE this pattern
+        println!("[{}]({})", title, url); //~ ERROR cannot find value `title` in this scope
+        //~^ NOTE not found in this scope
+    }
+}

tests/ui/pattern/struct-pattern-with-missing-fields-resolve-error.stderr

@@ -0,0 +1,30 @@
+error: expected `,`
+  --> $DIR/struct-pattern-with-missing-fields-resolve-error.rs:12:31
+   |
+LL |     if let Website { url, Some(title) } = website {
+   |            -------            ^
+   |            |
+   |            while parsing the fields for this pattern
+
+error[E0425]: cannot find value `title` in this scope
+  --> $DIR/struct-pattern-with-missing-fields-resolve-error.rs:19:30
+   |
+LL |         println!("[{}]({})", title, url);
+   |                              ^^^^^ not found in this scope
+   |
+note: `Website` has a field `title` which could have been included in this pattern, but it wasn't
+  --> $DIR/struct-pattern-with-missing-fields-resolve-error.rs:17:12
+   |
+LL | / struct Website {
+LL | |     url: String,
+LL | |     title: Option<String> ,
+   | |     ----- defined here
+LL | | }
+   | |_-
+...
+LL |       if let Website { url, .. } = website {
+   |              ^^^^^^^^^^^^^^^^^^^ this pattern doesn't include `title`, which is available in `Website`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0425`.