Rollup merge of rust-lang#78072 - Nadrieril:cleanup-constant-matching, r=varkor

Cleanup constant matching in exhaustiveness checking

This supercedes rust-lang#77390. I made the `Opaque` constructor work.
I have opened two issues rust-lang#78071 and rust-lang#78057 from the discussion we had on the previous PR. They are not regressions nor directly related to the current PR so I thought we'd deal with them separately.

I left a FIXME somewhere because I didn't know how to compare string constants for equality. There might even be some unicode things that need to happen there. In the meantime I preserved previous behavior.

EDIT: I accidentally fixed rust-lang#78071

Author: Dylan-DPC

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

@@ -137,7 +137,7 @@ impl<'tcx> MatchVisitor<'_, 'tcx> {
         patcx.include_lint_checks();
         let pattern = patcx.lower_pattern(pat);
         let pattern_ty = pattern.ty;
-        let pattern: &_ = cx.pattern_arena.alloc(expand_pattern(cx, pattern));
+        let pattern: &_ = cx.pattern_arena.alloc(expand_pattern(pattern));
         if !patcx.errors.is_empty() {
             *have_errors = true;
             patcx.report_inlining_errors(pat.span);

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

@@ -387,14 +387,16 @@ impl<'a, 'tcx> ConstToPat<'a, 'tcx> {
                 // `&str` and `&[u8]` are represented as `ConstValue::Slice`, let's keep using this
                 // optimization for now.
                 ty::Str => PatKind::Constant { value: cv },
-                ty::Slice(elem_ty) if elem_ty == tcx.types.u8 => PatKind::Constant { value: cv },
                 // `b"foo"` produces a `&[u8; 3]`, but you can't use constants of array type when
                 // matching against references, you can only use byte string literals.
-                // FIXME: clean this up, likely by permitting array patterns when matching on slices
-                ty::Array(elem_ty, _) if elem_ty == tcx.types.u8 => PatKind::Constant { value: cv },
+                // The typechecker has a special case for byte string literals, by treating them
+                // as slices. This means we turn `&[T; N]` constants into slice patterns, which
+                // has no negative effects on pattern matching, even if we're actually matching on
+                // arrays.
+                ty::Array(..) |
                 // Cannot merge this with the catch all branch below, because the `const_deref`
-                // changes the type from slice to array, and slice patterns behave differently from
-                // array patterns.
+                // changes the type from slice to array, we need to keep the original type in the
+                // pattern.
                 ty::Slice(..) => {
                     let old = self.behind_reference.replace(true);
                     let array = tcx.deref_const(self.param_env.and(cv));

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

@@ -158,6 +158,13 @@ crate enum PatKind<'tcx> {
         subpattern: Pat<'tcx>,
     },
 
+    /// One of the following:
+    /// * `&str`, which will be handled as a string pattern and thus exhaustiveness
+    ///   checking will detect if you use the same string twice in different patterns.
+    /// * integer, bool, char or float, which will be handled by exhaustivenes to cover exactly
+    ///   its own value, similar to `&str`, but these values are much simpler.
+    /// * Opaque constants, that must not be matched structurally. So anything that does not derive
+    ///   `PartialEq` and `Eq`.
     Constant {
         value: &'tcx ty::Const<'tcx>,
     },

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

@@ -0,0 +1,11 @@
+// check-pass
+// https://github.com/rust-lang/rust/issues/53708
+#[derive(PartialEq, Eq)]
+struct S;
+
+fn main() {
+    const C: &S = &S;
+    match C {
+        C => {}
+    }
+}

src/test/ui/issues/issue-44333.rs renamed to src/test/ui/consts/const_in_pattern/issue-44333.rs

@@ -0,0 +1,17 @@
+#![deny(unreachable_patterns)]
+
+#[derive(PartialEq)]
+struct Opaque(i32);
+
+impl Eq for Opaque {}
+
+const FOO: Opaque = Opaque(42);
+
+fn main() {
+    match FOO {
+        FOO => {},
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+}

src/test/ui/issues/issue-44333.stderr renamed to src/test/ui/consts/const_in_pattern/issue-44333.stderr

@@ -0,0 +1,20 @@
+error: to use a constant of type `Opaque` in a pattern, `Opaque` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/issue-78057.rs:12:9
+   |
+LL |         FOO => {},
+   |         ^^^
+
+error: unreachable pattern
+  --> $DIR/issue-78057.rs:14:9
+   |
+LL |         _ => {}
+   |         ^
+   |
+note: the lint level is defined here
+  --> $DIR/issue-78057.rs:1:9
+   |
+LL | #![deny(unreachable_patterns)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

src/test/ui/consts/const_in_pattern/issue-53708.rs

@@ -0,0 +1,114 @@
+// This file tests the exhaustiveness algorithm on opaque constants. Most of the examples give
+// unnecessary warnings because const_to_pat.rs converts a constant pattern to a wildcard when the
+// constant is not allowed as a pattern. This is an edge case so we may not care to fix it.
+// See also https://github.com/rust-lang/rust/issues/78057
+
+#![deny(unreachable_patterns)]
+
+#[derive(PartialEq)]
+struct Foo(i32);
+impl Eq for Foo {}
+const FOO: Foo = Foo(42);
+const FOO_REF: &Foo = &Foo(42);
+const FOO_REF_REF: &&Foo = &&Foo(42);
+
+#[derive(PartialEq)]
+struct Bar;
+impl Eq for Bar {}
+const BAR: Bar = Bar;
+
+#[derive(PartialEq)]
+enum Baz {
+    Baz1,
+    Baz2
+}
+impl Eq for Baz {}
+const BAZ: Baz = Baz::Baz1;
+
+type Quux = fn(usize, usize) -> usize;
+fn quux(a: usize, b: usize) -> usize { a + b }
+const QUUX: Quux = quux;
+
+fn main() {
+    match FOO {
+        FOO => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        _ => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+    }
+
+    match FOO_REF {
+        FOO_REF => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        Foo(_) => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+    }
+
+    // This used to cause an ICE (https://github.com/rust-lang/rust/issues/78071)
+    match FOO_REF_REF {
+        FOO_REF_REF => {}
+        //~^ WARNING must be annotated with `#[derive(PartialEq, Eq)]`
+        //~| WARNING this was previously accepted by the compiler but is being phased out
+        Foo(_) => {}
+    }
+
+    match BAR {
+        Bar => {}
+        BAR => {} // should not be emitting unreachable warning
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        //~| ERROR unreachable pattern
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+
+    match BAR {
+        BAR => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        Bar => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+
+    match BAR {
+        BAR => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        BAR => {} // should not be emitting unreachable warning
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        //~| ERROR unreachable pattern
+        _ => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+    }
+
+    match BAZ {
+        BAZ => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        Baz::Baz1 => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+
+    match BAZ {
+        Baz::Baz1 => {}
+        BAZ => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+
+    match BAZ {
+        BAZ => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        Baz::Baz2 => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+        _ => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+    }
+
+    match QUUX {
+        QUUX => {}
+        QUUX => {}
+        _ => {}
+    }
+}