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thir::pattern: update some comments and error type names #115887

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merged 2 commits into from
Sep 27, 2023
Merged

thir::pattern: update some comments and error type names #115887

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d34e15e
thir::pattern: update some comments and error type names
RalfJung Sep 16, 2023
12a1b55
better ICE than sorry
RalfJung Sep 16, 2023
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16 changes: 10 additions & 6 deletions compiler/rustc_middle/src/thir.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -732,12 +732,16 @@ pub enum PatKind<'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 exhaustiveness 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`.
/// * `&str` (represented as a valtree), 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 (represented as a valtree), which will be handled by
/// exhaustiveness to cover exactly its own value, similar to `&str`, but these values are
/// much simpler.
/// * Opaque constants (represented as `mir::ConstValue`), that must not be matched
/// structurally. So anything that does not derive `PartialEq` and `Eq`.
///
/// These are always compared with the matched place using (the semantics of) `PartialEq`.
Constant {
value: mir::ConstantKind<'tcx>,
},
Expand Down
28 changes: 17 additions & 11 deletions compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs
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Expand Up @@ -24,6 +24,8 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
/// Converts an evaluated constant to a pattern (if possible).
/// This means aggregate values (like structs and enums) are converted
/// to a pattern that matches the value (as if you'd compared via structural equality).
///
/// `cv` must be a valtree or a `mir::ConstValue`.
#[instrument(level = "debug", skip(self), ret)]
pub(super) fn const_to_pat(
&self,
Expand Down Expand Up @@ -64,12 +66,10 @@ struct ConstToPat<'tcx> {
}

/// This error type signals that we encountered a non-struct-eq situation.
/// We bubble this up in order to get back to the reference destructuring and make that emit
/// a const pattern instead of a deref pattern. This allows us to simply call `PartialEq::eq`
/// on such patterns (since that function takes a reference) and not have to jump through any
/// hoops to get a reference to the value.
/// We will fall back to calling `PartialEq::eq` on such patterns,
/// and exhaustiveness checking will consider them as matching nothing.
#[derive(Debug)]
struct FallbackToConstRef;
struct FallbackToOpaqueConst;

impl<'tcx> ConstToPat<'tcx> {
fn new(
Expand Down Expand Up @@ -136,7 +136,7 @@ impl<'tcx> ConstToPat<'tcx> {
}
ty::ConstKind::Value(valtree) => self
.recur(valtree, cv.ty(), mir_structural_match_violation.unwrap_or(false))
.unwrap_or_else(|_| {
.unwrap_or_else(|_: FallbackToOpaqueConst| {
Box::new(Pat {
span: self.span,
ty: cv.ty(),
Expand Down Expand Up @@ -266,7 +266,7 @@ impl<'tcx> ConstToPat<'tcx> {
fn field_pats(
&self,
vals: impl Iterator<Item = (ValTree<'tcx>, Ty<'tcx>)>,
) -> Result<Vec<FieldPat<'tcx>>, FallbackToConstRef> {
) -> Result<Vec<FieldPat<'tcx>>, FallbackToOpaqueConst> {
vals.enumerate()
.map(|(idx, (val, ty))| {
let field = FieldIdx::new(idx);
Expand All @@ -284,7 +284,7 @@ impl<'tcx> ConstToPat<'tcx> {
cv: ValTree<'tcx>,
ty: Ty<'tcx>,
mir_structural_match_violation: bool,
) -> Result<Box<Pat<'tcx>>, FallbackToConstRef> {
) -> Result<Box<Pat<'tcx>>, FallbackToOpaqueConst> {
let id = self.id;
let span = self.span;
let tcx = self.tcx();
Expand All @@ -299,7 +299,7 @@ impl<'tcx> ConstToPat<'tcx> {
span,
FloatPattern,
);
return Err(FallbackToConstRef);
return Err(FallbackToOpaqueConst);
}
// If the type is not structurally comparable, just emit the constant directly,
// causing the pattern match code to treat it opaquely.
Expand All @@ -323,18 +323,20 @@ impl<'tcx> ConstToPat<'tcx> {
// Since we are behind a reference, we can just bubble the error up so we get a
// constant at reference type, making it easy to let the fallback call
// `PartialEq::eq` on it.
return Err(FallbackToConstRef);
return Err(FallbackToOpaqueConst);
}
ty::FnDef(..) => {
self.saw_const_match_error.set(true);
tcx.sess.emit_err(InvalidPattern { span, non_sm_ty: ty });
// We errored, so the pattern we generate is irrelevant.
PatKind::Wild
}
ty::Adt(adt_def, _) if !self.type_marked_structural(ty) => {
debug!("adt_def {:?} has !type_marked_structural for cv.ty: {:?}", adt_def, ty,);
self.saw_const_match_error.set(true);
let err = TypeNotStructural { span, non_sm_ty: ty };
tcx.sess.emit_err(err);
// We errored, so the pattern we generate is irrelevant.
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That comment was much needed. This always bothered me because if we forgot to error this would cause unsoundness (non exhaustive match considered exhaustive)

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Yeah I'd much prefer something that ICEs later... in particular in that one case where we don't actually emit the error, we check that the flag says that we emitted an error earlier.

In fact, let me add a delay_span_bug in that case just to be sure.

PatKind::Wild
}
ty::Adt(adt_def, args) if adt_def.is_enum() => {
Expand Down Expand Up @@ -404,13 +406,15 @@ impl<'tcx> ConstToPat<'tcx> {
IndirectStructuralMatch { non_sm_ty: *pointee_ty },
);
}
return Err(FallbackToConstRef);
return Err(FallbackToOpaqueConst);
} else {
if !self.saw_const_match_error.get() {
self.saw_const_match_error.set(true);
let err = TypeNotStructural { span, non_sm_ty: *pointee_ty };
tcx.sess.emit_err(err);
}
tcx.sess.delay_span_bug(span, "`saw_const_match_error` set but no error?");
// We errored, so the pattern we generate is irrelevant.
PatKind::Wild
}
}
Expand All @@ -423,6 +427,7 @@ impl<'tcx> ConstToPat<'tcx> {
tcx.sess.emit_err(err);

// FIXME: introduce PatKind::Error to silence follow up diagnostics due to unreachable patterns.
// We errored, so the pattern we generate is irrelevant.
PatKind::Wild
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Should this one set saw_const_match_error too?

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@RalfJung RalfJung Sep 17, 2023 •
edited
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Hm, it doesn't check for saw_const_match_error either. But I guess yes? Not sure. This PR was not meant to have any behavior changes in it, just comments and clarifications.

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Ah fair; I was just thinking that should also trigger a delay_span_bug

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The emit_err is directly above here so theres'a definitely a hard error. In the other case where I added a delay_span_bug, the emit_err is conditional on something, so it becomes a non-local property, and I felt it was worth simplifying the reasoning, so that the failure mode for if saw_const_match_error gets set incorrectly is an ICE rather than a weird miscompilation.

} else {
let old = self.behind_reference.replace(true);
Expand Down Expand Up @@ -453,6 +458,7 @@ impl<'tcx> ConstToPat<'tcx> {
self.saw_const_match_error.set(true);
let err = InvalidPattern { span, non_sm_ty: ty };
tcx.sess.emit_err(err);
// We errored, so the pattern we generate is irrelevant.
PatKind::Wild
}
};
Expand Down
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