always check for mixed deref pattern and normal constructors

This makes it work for box patterns and in rust-analyzer.

Author: dianne

compiler/rustc_pattern_analysis/src/checks.rs

@@ -0,0 +1,50 @@
+//! Contains checks that must be run to validate matches before performing usefulness analysis.
+
+use crate::constructor::Constructor::*;
+use crate::pat_column::PatternColumn;
+use crate::{MatchArm, PatCx};
+
+/// Validate that deref patterns and normal constructors aren't used to match on the same place.
+pub(crate) fn detect_mixed_deref_pat_ctors<'p, Cx: PatCx>(
+    cx: &Cx,
+    arms: &[MatchArm<'p, Cx>],
+) -> Result<(), Cx::Error> {
+    let pat_column = PatternColumn::new(arms);
+    detect_mixed_deref_pat_ctors_inner(cx, &pat_column)
+}
+
+fn detect_mixed_deref_pat_ctors_inner<'p, Cx: PatCx>(
+    cx: &Cx,
+    column: &PatternColumn<'p, Cx>,
+) -> Result<(), Cx::Error> {
+    let Some(ty) = column.head_ty() else {
+        return Ok(());
+    };
+
+    // Check for a mix of deref patterns and normal constructors.
+    let mut deref_pat = None;
+    let mut normal_pat = None;
+    for pat in column.iter() {
+        match pat.ctor() {
+            // The analysis can handle mixing deref patterns with wildcards and opaque patterns.
+            Wildcard | Opaque(_) => {}
+            DerefPattern(_) => deref_pat = Some(pat),
+            // Nothing else can be compared to deref patterns in `Constructor::is_covered_by`.
+            _ => normal_pat = Some(pat),
+        }
+    }
+    if let Some(deref_pat) = deref_pat
+        && let Some(normal_pat) = normal_pat
+    {
+        return Err(cx.report_mixed_deref_pat_ctors(deref_pat, normal_pat));
+    }
+
+    // Specialize and recurse into the patterns' fields.
+    let set = column.analyze_ctors(cx, &ty)?;
+    for ctor in set.present {
+        for specialized_column in column.specialize(cx, &ty, &ctor).iter() {
+            detect_mixed_deref_pat_ctors_inner(cx, specialized_column)?;
+        }
+    }
+    Ok(())
+}

compiler/rustc_pattern_analysis/src/lib.rs

@@ -8,6 +8,7 @@
 #![allow(unused_crate_dependencies)]
 // tidy-alphabetical-end
 
+pub(crate) mod checks;
 pub mod constructor;
 #[cfg(feature = "rustc")]
 pub mod errors;
@@ -107,6 +108,15 @@ pub trait PatCx: Sized + fmt::Debug {
         _gapped_with: &[&DeconstructedPat<Self>],
     ) {
     }
+
+    /// The current implementation of deref patterns requires that they can't match on the same
+    /// place as a normal constructor. Since this isn't caught by type-checking, we check it in the
+    /// `PatCx` before running the analysis. This reports an error if the check fails.
+    fn report_mixed_deref_pat_ctors(
+        &self,
+        deref_pat: &DeconstructedPat<Self>,
+        normal_pat: &DeconstructedPat<Self>,
+    ) -> Self::Error;
 }
 
 /// The arm of a match expression.

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -1027,6 +1027,21 @@ impl<'p, 'tcx: 'p> PatCx for RustcPatCtxt<'p, 'tcx> {
             );
         }
     }
+
+    fn report_mixed_deref_pat_ctors(
+        &self,
+        deref_pat: &crate::pat::DeconstructedPat<Self>,
+        normal_pat: &crate::pat::DeconstructedPat<Self>,
+    ) -> Self::Error {
+        let deref_pattern_label = deref_pat.data().span;
+        let normal_constructor_label = normal_pat.data().span;
+        self.tcx.dcx().emit_err(errors::MixedDerefPatternConstructors {
+            spans: vec![deref_pattern_label, normal_constructor_label],
+            smart_pointer_ty: deref_pat.ty().inner(),
+            deref_pattern_label,
+            normal_constructor_label,
+        })
+    }
 }
 
 /// Recursively expand this pattern into its subpatterns. Only useful for or-patterns.
@@ -1055,13 +1070,6 @@ pub fn analyze_match<'p, 'tcx>(
 ) -> Result<UsefulnessReport<'p, 'tcx>, ErrorGuaranteed> {
     let scrut_ty = tycx.reveal_opaque_ty(scrut_ty);
 
-    // The analysis doesn't support deref patterns mixed with normal constructors; error if present.
-    // FIXME(deref_patterns): This only needs to run when a deref pattern was found during lowering.
-    if tycx.tcx.features().deref_patterns() {
-        let pat_column = PatternColumn::new(arms);
-        detect_mixed_deref_pat_ctors(tycx, &pat_column)?;
-    }
-
     let scrut_validity = PlaceValidity::from_bool(tycx.known_valid_scrutinee);
     let report = compute_match_usefulness(
         tycx,
@@ -1080,48 +1088,3 @@ pub fn analyze_match<'p, 'tcx>(
 
     Ok(report)
 }
-
-// FIXME(deref_patterns): Currently it's the responsibility of the frontend (rustc or rust-analyzer)
-// to ensure that deref patterns don't appear in the same column as normal constructors. Deref
-// patterns aren't currently implemented in rust-analyzer, but should they be, the columnwise check
-// here could be made generic and shared between frontends.
-fn detect_mixed_deref_pat_ctors<'p, 'tcx>(
-    cx: &RustcPatCtxt<'p, 'tcx>,
-    column: &PatternColumn<'p, RustcPatCtxt<'p, 'tcx>>,
-) -> Result<(), ErrorGuaranteed> {
-    let Some(&ty) = column.head_ty() else {
-        return Ok(());
-    };
-
-    // Check for a mix of deref patterns and normal constructors.
-    let mut normal_ctor_span = None;
-    let mut deref_pat_span = None;
-    for pat in column.iter() {
-        match pat.ctor() {
-            // The analysis can handle mixing deref patterns with wildcards and opaque patterns.
-            Wildcard | Opaque(_) => {}
-            DerefPattern(_) => deref_pat_span = Some(pat.data().span),
-            // Nothing else can be compared to deref patterns in `Constructor::is_covered_by`.
-            _ => normal_ctor_span = Some(pat.data().span),
-        }
-    }
-    if let Some(normal_constructor_label) = normal_ctor_span
-        && let Some(deref_pattern_label) = deref_pat_span
-    {
-        return Err(cx.tcx.dcx().emit_err(errors::MixedDerefPatternConstructors {
-            spans: vec![deref_pattern_label, normal_constructor_label],
-            smart_pointer_ty: ty.inner(),
-            deref_pattern_label,
-            normal_constructor_label,
-        }));
-    }
-
-    // Specialize and recurse into the patterns' fields.
-    let set = column.analyze_ctors(cx, &ty)?;
-    for ctor in set.present {
-        for specialized_column in column.specialize(cx, &ty, &ctor).iter() {
-            detect_mixed_deref_pat_ctors(cx, specialized_column)?;
-        }
-    }
-    Ok(())
-}

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -720,7 +720,7 @@ use tracing::{debug, instrument};
 use self::PlaceValidity::*;
 use crate::constructor::{Constructor, ConstructorSet, IntRange};
 use crate::pat::{DeconstructedPat, PatId, PatOrWild, WitnessPat};
-use crate::{MatchArm, PatCx, PrivateUninhabitedField};
+use crate::{MatchArm, PatCx, PrivateUninhabitedField, checks};
 #[cfg(not(feature = "rustc"))]
 pub fn ensure_sufficient_stack<R>(f: impl FnOnce() -> R) -> R {
     f()
@@ -1836,6 +1836,9 @@ pub fn compute_match_usefulness<'p, Cx: PatCx>(
     scrut_validity: PlaceValidity,
     complexity_limit: usize,
 ) -> Result<UsefulnessReport<'p, Cx>, Cx::Error> {
+    // The analysis doesn't support deref patterns mixed with normal constructors; error if present.
+    checks::detect_mixed_deref_pat_ctors(tycx, arms)?;
+
     let mut cx = UsefulnessCtxt {
         tycx,
         branch_usefulness: FxHashMap::default(),

compiler/rustc_pattern_analysis/tests/common/mod.rs

@@ -1,6 +1,7 @@
 use rustc_pattern_analysis::constructor::{
     Constructor, ConstructorSet, IntRange, MaybeInfiniteInt, RangeEnd, VariantVisibility,
 };
+use rustc_pattern_analysis::pat::DeconstructedPat;
 use rustc_pattern_analysis::usefulness::{PlaceValidity, UsefulnessReport};
 use rustc_pattern_analysis::{MatchArm, PatCx, PrivateUninhabitedField};
 
@@ -184,6 +185,14 @@ impl PatCx for Cx {
     fn complexity_exceeded(&self) -> Result<(), Self::Error> {
         Err(())
     }
+
+    fn report_mixed_deref_pat_ctors(
+        &self,
+        _deref_pat: &DeconstructedPat<Self>,
+        _normal_pat: &DeconstructedPat<Self>,
+    ) -> Self::Error {
+        panic!("`rustc_pattern_analysis::tests` currently doesn't test deref pattern errors")
+    }
 }
 
 /// Construct a single pattern; see `pats!()`.

src/tools/rust-analyzer/crates/hir-ty/src/diagnostics/match_check/pat_analysis.rs

@@ -527,6 +527,14 @@ impl PatCx for MatchCheckCtx<'_> {
     fn complexity_exceeded(&self) -> Result<(), Self::Error> {
         Err(())
     }
+
+    fn report_mixed_deref_pat_ctors(
+        &self,
+        _deref_pat: &DeconstructedPat<'_>,
+        _normal_pat: &DeconstructedPat<'_>,
+    ) {
+        // FIXME(deref_patterns): This could report an error comparable to the one in rustc.
+    }
 }
 
 impl fmt::Debug for MatchCheckCtx<'_> {

tests/ui/pattern/box-pattern-constructor-mismatch.rs

@@ -0,0 +1,11 @@
+//! Test that `box _` patterns and `Box { .. }` patterns can't be used to match on the same place.
+//! This is required for the current implementation of exhaustiveness analysis for deref patterns.
+
+#![feature(box_patterns)]
+
+fn main() {
+    match Box::new(0) {
+        box _ => {} //~ ERROR mix of deref patterns and normal constructors
+        Box { .. } => {}
+    }
+}

tests/ui/pattern/box-pattern-constructor-mismatch.stderr

@@ -0,0 +1,10 @@
+error: mix of deref patterns and normal constructors
+  --> $DIR/box-pattern-constructor-mismatch.rs:8:9
+   |
+LL |         box _ => {}
+   |         ^^^^^ matches on the result of dereferencing `Box<i32>`
+LL |         Box { .. } => {}
+   |         ^^^^^^^^^^ matches directly on `Box<i32>`
+
+error: aborting due to 1 previous error
+