Auto merge of #116734 - Nadrieril:lint-per-column, r=cjgillot

Lint `non_exhaustive_omitted_patterns` by columns

This is a rework of the `non_exhaustive_omitted_patterns` lint to make it more consistent. The intent of the lint is to help consumers of `non_exhaustive` enums ensure they stay up-to-date with all upstream variants. This rewrite fixes two cases we didn't handle well before:

First, because of details of exhaustiveness checking, the following wouldn't lint `Enum::C` as missing:
```rust
match Some(x) {
    Some(Enum::A) => {}
    Some(Enum::B) => {}
    _ => {}
}
```

Second, because of the fundamental workings of exhaustiveness checking, the following would treat the `true` and `false` cases separately and thus lint about missing variants:
```rust
match (true, x) {
    (true, Enum::A) => {}
    (true, Enum::B) => {}
    (false, Enum::C) => {}
    _ => {}
}
```
Moreover, it would correctly not lint in the case where the pair is flipped, because of asymmetry in how exhaustiveness checking proceeds.

A drawback is that it no longer makes sense to set the lint level per-arm. This will silently break the lint for current users of it (but it's behind a feature gate so that's ok).

The new approach is now independent of the exhaustiveness algorithm; it's a separate pass that looks at patterns column by column. This is another of the motivations for this: I'm glad to move it out of the algorithm, it was akward there.

This PR is almost identical to #111651. cc `@eholk` who reviewed it at the time. Compared to then, I'm more confident this is the right approach.

compiler/rustc_lint_defs/src/builtin.rs

@@ -3953,8 +3953,13 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// The `non_exhaustive_omitted_patterns` lint detects when a wildcard (`_` or `..`) in a
-    /// pattern for a `#[non_exhaustive]` struct or enum is reachable.
+    /// The `non_exhaustive_omitted_patterns` lint aims to help consumers of a `#[non_exhaustive]`
+    /// struct or enum who want to match all of its fields/variants explicitly.
+    ///
+    /// The `#[non_exhaustive]` annotation forces matches to use wildcards, so exhaustiveness
+    /// checking cannot be used to ensure that all fields/variants are matched explicitly. To remedy
+    /// this, this allow-by-default lint warns the user when a match mentions some but not all of
+    /// the fields/variants of a `#[non_exhaustive]` struct or enum.
     ///
     /// ### Example
     ///
@@ -3968,39 +3973,42 @@ declare_lint! {
     ///
     /// // in crate B
     /// #![feature(non_exhaustive_omitted_patterns_lint)]
+    /// #[warn(non_exhaustive_omitted_patterns)]
     /// match Bar::A {
     ///     Bar::A => {},
-    ///     #[warn(non_exhaustive_omitted_patterns)]
     ///     _ => {},
     /// }
     /// ```
     ///
     /// This will produce:
     ///
     /// ```text
-    /// warning: reachable patterns not covered of non exhaustive enum
+    /// warning: some variants are not matched explicitly
     ///    --> $DIR/reachable-patterns.rs:70:9
     ///    |
-    /// LL |         _ => {}
-    ///    |         ^ pattern `B` not covered
+    /// LL |         match Bar::A {
+    ///    |               ^ pattern `Bar::B` not covered
     ///    |
     ///  note: the lint level is defined here
     ///   --> $DIR/reachable-patterns.rs:69:16
     ///    |
     /// LL |         #[warn(non_exhaustive_omitted_patterns)]
     ///    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-    ///    = help: ensure that all possible cases are being handled by adding the suggested match arms
+    ///    = help: ensure that all variants are matched explicitly by adding the suggested match arms
     ///    = note: the matched value is of type `Bar` and the `non_exhaustive_omitted_patterns` attribute was found
     /// ```
     ///
+    /// Warning: setting this to `deny` will make upstream non-breaking changes (adding fields or
+    /// variants to a `#[non_exhaustive]` struct or enum) break your crate. This goes against
+    /// expected semver behavior.
+    ///
     /// ### Explanation
     ///
-    /// Structs and enums tagged with `#[non_exhaustive]` force the user to add a
-    /// (potentially redundant) wildcard when pattern-matching, to allow for future
-    /// addition of fields or variants. The `non_exhaustive_omitted_patterns` lint
-    /// detects when such a wildcard happens to actually catch some fields/variants.
-    /// In other words, when the match without the wildcard would not be exhaustive.
-    /// This lets the user be informed if new fields/variants were added.
+    /// Structs and enums tagged with `#[non_exhaustive]` force the user to add a (potentially
+    /// redundant) wildcard when pattern-matching, to allow for future addition of fields or
+    /// variants. The `non_exhaustive_omitted_patterns` lint detects when such a wildcard happens to
+    /// actually catch some fields/variants. In other words, when the match without the wildcard
+    /// would not be exhaustive. This lets the user be informed if new fields/variants were added.
     pub NON_EXHAUSTIVE_OMITTED_PATTERNS,
     Allow,
     "detect when patterns of types marked `non_exhaustive` are missed",

compiler/rustc_mir_build/src/errors.rs

@@ -1,6 +1,6 @@
 use crate::{
     fluent_generated as fluent,
-    thir::pattern::{deconstruct_pat::DeconstructedPat, MatchCheckCtxt},
+    thir::pattern::{deconstruct_pat::WitnessPat, MatchCheckCtxt},
 };
 use rustc_errors::{
     error_code, AddToDiagnostic, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
@@ -810,7 +810,7 @@ impl<'tcx> Uncovered<'tcx> {
     pub fn new<'p>(
         span: Span,
         cx: &MatchCheckCtxt<'p, 'tcx>,
-        witnesses: Vec<DeconstructedPat<'p, 'tcx>>,
+        witnesses: Vec<WitnessPat<'tcx>>,
     ) -> Self {
         let witness_1 = witnesses.get(0).unwrap().to_pat(cx);
         Self {

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -1,4 +1,4 @@
-use super::deconstruct_pat::{Constructor, DeconstructedPat};
+use super::deconstruct_pat::{Constructor, DeconstructedPat, WitnessPat};
 use super::usefulness::{
     compute_match_usefulness, MatchArm, MatchCheckCtxt, Reachability, UsefulnessReport,
 };
@@ -279,7 +279,7 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
 
         let scrut = &self.thir[scrut];
         let scrut_ty = scrut.ty;
-        let report = compute_match_usefulness(&cx, &tarms, self.lint_level, scrut_ty);
+        let report = compute_match_usefulness(&cx, &tarms, self.lint_level, scrut_ty, scrut.span);
 
         match source {
             // Don't report arm reachability of desugared `match $iter.into_iter() { iter => .. }`
@@ -473,7 +473,8 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
         let pattern = self.lower_pattern(&mut cx, pat);
         let pattern_ty = pattern.ty();
         let arm = MatchArm { pat: pattern, hir_id: self.lint_level, has_guard: false };
-        let report = compute_match_usefulness(&cx, &[arm], self.lint_level, pattern_ty);
+        let report =
+            compute_match_usefulness(&cx, &[arm], self.lint_level, pattern_ty, pattern.span());
 
         // Note: we ignore whether the pattern is unreachable (i.e. whether the type is empty). We
         // only care about exhaustiveness here.
@@ -662,7 +663,7 @@ fn is_let_irrefutable<'p, 'tcx>(
     pat: &'p DeconstructedPat<'p, 'tcx>,
 ) -> bool {
     let arms = [MatchArm { pat, hir_id: pat_id, has_guard: false }];
-    let report = compute_match_usefulness(&cx, &arms, pat_id, pat.ty());
+    let report = compute_match_usefulness(&cx, &arms, pat_id, pat.ty(), pat.span());
 
     // Report if the pattern is unreachable, which can only occur when the type is uninhabited.
     // This also reports unreachable sub-patterns though, so we can't just replace it with an
@@ -701,8 +702,8 @@ fn report_arm_reachability<'p, 'tcx>(
     }
 }
 
-fn collect_non_exhaustive_tys<'p, 'tcx>(
-    pat: &DeconstructedPat<'p, 'tcx>,
+fn collect_non_exhaustive_tys<'tcx>(
+    pat: &WitnessPat<'tcx>,
     non_exhaustive_tys: &mut FxHashSet<Ty<'tcx>>,
 ) {
     if matches!(pat.ctor(), Constructor::NonExhaustive) {
@@ -718,7 +719,7 @@ fn non_exhaustive_match<'p, 'tcx>(
     thir: &Thir<'tcx>,
     scrut_ty: Ty<'tcx>,
     sp: Span,
-    witnesses: Vec<DeconstructedPat<'p, 'tcx>>,
+    witnesses: Vec<WitnessPat<'tcx>>,
     arms: &[ArmId],
     expr_span: Span,
 ) -> ErrorGuaranteed {
@@ -896,10 +897,10 @@ fn non_exhaustive_match<'p, 'tcx>(
 
 pub(crate) fn joined_uncovered_patterns<'p, 'tcx>(
     cx: &MatchCheckCtxt<'p, 'tcx>,
-    witnesses: &[DeconstructedPat<'p, 'tcx>],
+    witnesses: &[WitnessPat<'tcx>],
 ) -> String {
     const LIMIT: usize = 3;
-    let pat_to_str = |pat: &DeconstructedPat<'p, 'tcx>| pat.to_pat(cx).to_string();
+    let pat_to_str = |pat: &WitnessPat<'tcx>| pat.to_pat(cx).to_string();
     match witnesses {
         [] => bug!(),
         [witness] => format!("`{}`", witness.to_pat(cx)),
@@ -916,7 +917,7 @@ pub(crate) fn joined_uncovered_patterns<'p, 'tcx>(
 }
 
 pub(crate) fn pattern_not_covered_label(
-    witnesses: &[DeconstructedPat<'_, '_>],
+    witnesses: &[WitnessPat<'_>],
     joined_patterns: &str,
 ) -> String {
     format!("pattern{} {} not covered", rustc_errors::pluralize!(witnesses.len()), joined_patterns)
@@ -927,7 +928,7 @@ fn adt_defined_here<'p, 'tcx>(
     cx: &MatchCheckCtxt<'p, 'tcx>,
     err: &mut Diagnostic,
     ty: Ty<'tcx>,
-    witnesses: &[DeconstructedPat<'p, 'tcx>],
+    witnesses: &[WitnessPat<'tcx>],
 ) {
     let ty = ty.peel_refs();
     if let ty::Adt(def, _) = ty.kind() {
@@ -958,7 +959,7 @@ fn adt_defined_here<'p, 'tcx>(
 fn maybe_point_at_variant<'a, 'p: 'a, 'tcx: 'a>(
     cx: &MatchCheckCtxt<'p, 'tcx>,
     def: AdtDef<'tcx>,
-    patterns: impl Iterator<Item = &'a DeconstructedPat<'p, 'tcx>>,
+    patterns: impl Iterator<Item = &'a WitnessPat<'tcx>>,
 ) -> Vec<Span> {
     use Constructor::*;
     let mut covered = vec![];