diff options
Diffstat (limited to 'compiler/rustc_mir_build/src/thir/pattern/usefulness.rs')
| -rw-r--r-- | compiler/rustc_mir_build/src/thir/pattern/usefulness.rs | 19 |
1 files changed, 17 insertions, 2 deletions
diff --git a/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs b/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs index d8f66a175..e5b635069 100644 --- a/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs +++ b/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs @@ -288,6 +288,22 @@ //! //! The details are not necessary to understand this file, so we explain them in //! [`super::deconstruct_pat`]. Splitting is done by the [`Constructor::split`] function. +//! +//! # Constants in patterns +//! +//! There are two kinds of constants in patterns: +//! +//! * literals (`1`, `true`, `"foo"`) +//! * named or inline consts (`FOO`, `const { 5 + 6 }`) +//! +//! The latter are converted into other patterns with literals at the leaves. For example +//! `const_to_pat(const { [1, 2, 3] })` becomes an `Array(vec![Const(1), Const(2), Const(3)])` +//! pattern. This gets problematic when comparing the constant via `==` would behave differently +//! from matching on the constant converted to a pattern. Situations like that can occur, when +//! the user implements `PartialEq` manually, and thus could make `==` behave arbitrarily different. +//! In order to honor the `==` implementation, constants of types that implement `PartialEq` manually +//! stay as a full constant and become an `Opaque` pattern. These `Opaque` patterns do not participate +//! in exhaustiveness, specialization or overlap checking. use self::ArmType::*; use self::Usefulness::*; @@ -685,10 +701,9 @@ enum ArmType { /// For example, if we are constructing a witness for the match against /// /// ```compile_fail,E0004 -/// # #![feature(type_ascription)] /// struct Pair(Option<(u32, u32)>, bool); /// # fn foo(p: Pair) { -/// match (p: Pair) { +/// match p { /// Pair(None, _) => {} /// Pair(_, false) => {} /// } |