mirror of
https://github.com/rust-lang/rust.git
synced 2025-12-30 04:25:29 +00:00
e111e99253
match lowering: expand or-candidates mixed with candidates above
This PR tweaks match lowering of or-patterns. Consider this:
```rust
match (x, y) {
(1, true) => 1,
(2, false) => 2,
(1 | 2, true | false) => 3,
(3 | 4, true | false) => 4,
_ => 5,
}
```
One might hope that this can be compiled to a single `SwitchInt` on `x` followed by some boolean checks. Before this PR, we compile this to 3 `SwitchInt`s on `x`, because an arm that contains more than one or-pattern was compiled on its own. This PR groups branch `3` with the two branches above, getting us down to 2 `SwitchInt`s on `x`.
We can't in general expand or-patterns freely, because this interacts poorly with another optimization we do: or-pattern simplification. When an or-pattern doesn't involve bindings, we branch the success paths of all its alternatives to the same block. The drawback is that in a case like:
```rust
match (1, true) {
(1 | 2, false) => unreachable!(),
(2, _) => unreachable!(),
_ => {}
}
```
if we used a single `SwitchInt`, by the time we test `false` we don't know whether we came from the `1` case or the `2` case, so we don't know where to go if `false` doesn't match.
Hence the limitation: we can process or-pattern alternatives alongside candidates that precede it, but not candidates that follow it. (Unless the or-pattern is the only remaining match pair of its candidate, in which case we can process it alongside whatever).
This PR allows the processing of or-pattern alternatives alongside candidates that precede it. One benefit is that we now process or-patterns in a single place in `mod.rs`.
r? ``@matthewjasper``
UI Tests
This folder contains rustc's
UI tests.
Test Directives (Headers)
Typically, a UI test will have some test directives / headers which are special comments that tell compiletest how to build and intepret a test.
As part of an on-going effort to rewrite compiletest
(see https://github.com/rust-lang/compiler-team/issues/536), a major
change proposal to change legacy compiletest-style headers // <directive>
to ui_test-style headers
//@ <directive> was accepted (see
https://github.com/rust-lang/compiler-team/issues/512.
An example directive is ignore-test. In legacy compiletest style, the header
would be written as
// ignore-test
but in ui_test style, the header would be written as
//@ ignore-test
compiletest is changed to accept only //@ directives for UI tests
(currently), and will reject and report an error if it encounters any
comments // <content> that may be parsed as an legacy compiletest-style
test header. To fix this, you should migrate to the ui_test-style header
//@ <content>.