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Auto merge of #118527 - Nadrieril:never_patterns_parse, r=compiler-errors

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never_patterns: Parse match arms with no body

Never patterns are meant to signal unreachable cases, and thus don't take bodies:
```rust
let ptr: *const Option<!> = ...;
match *ptr {
    None => { foo(); }
    Some(!),
}
```
This PR makes rustc accept the above, and enforces that an arm has a body xor is a never pattern. This affects parsing of match arms even with the feature off, so this is delicate. (Plus this is my first non-trivial change to the parser).

~~The last commit is optional; it introduces a bit of churn to allow the new suggestions to be machine-applicable. There may be a better solution? I'm not sure.~~ EDIT: I removed that commit

r? `@compiler-errors`
This commit is contained in:
bors
2023-12-08 17:08:52 +00:00
parent ae612bedcb 431cc4a3b0
commit 2b399b5275
47 changed files with 777 additions and 241 deletions
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50
compiler/rustc_ast_lowering/src/expr.rs
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@@ -1,8 +1,9 @@
use super::errors::{
AsyncCoroutinesNotSupported, AsyncNonMoveClosureNotSupported, AwaitOnlyInAsyncFnAndBlocks,
BaseExpressionDoubleDot, ClosureCannotBeStatic, CoroutineTooManyParameters,
FunctionalRecordUpdateDestructuringAssignment, InclusiveRangeWithNoEnd,
NotSupportedForLifetimeBinderAsyncClosure, UnderscoreExprLhsAssign,
FunctionalRecordUpdateDestructuringAssignment, InclusiveRangeWithNoEnd, MatchArmWithNoBody,
NeverPatternWithBody, NeverPatternWithGuard, NotSupportedForLifetimeBinderAsyncClosure,
UnderscoreExprLhsAssign,
};
use super::ResolverAstLoweringExt;
use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
@@ -549,7 +550,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
let pat = self.lower_pat(&arm.pat);
let guard = arm.guard.as_ref().map(|cond| {
let mut guard = arm.guard.as_ref().map(|cond| {
if let ExprKind::Let(pat, scrutinee, span, is_recovered) = &cond.kind {
hir::Guard::IfLet(self.arena.alloc(hir::Let {
hir_id: self.next_id(),
@@ -564,14 +565,43 @@ impl<'hir> LoweringContext<'_, 'hir> {
}
});
let hir_id = self.next_id();
let span = self.lower_span(arm.span);
self.lower_attrs(hir_id, &arm.attrs);
hir::Arm {
hir_id,
pat,
guard,
body: self.lower_expr(&arm.body),
span: self.lower_span(arm.span),
}
let is_never_pattern = pat.is_never_pattern();
let body = if let Some(body) = &arm.body
&& !is_never_pattern
{
self.lower_expr(body)
} else {
// Either `body.is_none()` or `is_never_pattern` here.
if !is_never_pattern {
let suggestion = span.shrink_to_hi();
self.tcx.sess.emit_err(MatchArmWithNoBody { span, suggestion });
} else if let Some(body) = &arm.body {
self.tcx.sess.emit_err(NeverPatternWithBody { span: body.span });
guard = None;
} else if let Some(g) = &arm.guard {
self.tcx.sess.emit_err(NeverPatternWithGuard { span: g.span });
guard = None;
}
// We add a fake `loop {}` arm body so that it typecks to `!`.
// FIXME(never_patterns): Desugar into a call to `unreachable_unchecked`.
let block = self.arena.alloc(hir::Block {
stmts: &[],
expr: None,
hir_id: self.next_id(),
rules: hir::BlockCheckMode::DefaultBlock,
span,
targeted_by_break: false,
});
self.arena.alloc(hir::Expr {
hir_id: self.next_id(),
kind: hir::ExprKind::Loop(block, None, hir::LoopSource::Loop, span),
span,
})
};
hir::Arm { hir_id, pat, guard, body, span }
}
/// Lower an `async` construct to a coroutine that implements `Future`.