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loop_match: suggest extracting to a const item

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if the expression cannot be evaluated in a straightforward way
This commit is contained in:
Folkert de Vries 2025-07-07 14:37:50 +02:00
parent 29a58723b0
commit 730d33dd64
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GPG key ID: 1F17F6FFD112B97C
5 changed files with 297 additions and 4 deletions
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8
compiler/rustc_mir_build/messages.ftl
View file

@ -86,10 +86,16 @@ mir_build_confused = missing patterns are not covered because `{$variable}` is i
mir_build_const_continue_bad_const = could not determine the target branch for this `#[const_continue]`
.label = this value is too generic
.note = the value must be a literal or a monomorphic const
mir_build_const_continue_missing_value = a `#[const_continue]` must break to a label with a value
mir_build_const_continue_not_const = could not determine the target branch for this `#[const_continue]`
.help = try extracting the expression into a `const` item
mir_build_const_continue_not_const_const_block = `const` blocks may use generics, and are not evaluated early enough
mir_build_const_continue_not_const_const_other = this value must be a literal or a monomorphic const
mir_build_const_continue_not_const_constant_parameter = constant parameters may use generics, and are not evaluated early enough
mir_build_const_continue_unknown_jump_target = the target of this `#[const_continue]` is not statically known
.label = this value must be a literal or a monomorphic const

29
compiler/rustc_mir_build/src/builder/scope.rs
View file

@ -100,7 +100,9 @@ use tracing::{debug, instrument};
use super::matches::BuiltMatchTree;
use crate::builder::{BlockAnd, BlockAndExtension, BlockFrame, Builder, CFG};
use crate::errors::{ConstContinueBadConst, ConstContinueUnknownJumpTarget};
use crate::errors::{
ConstContinueBadConst, ConstContinueNotMonomorphicConst, ConstContinueUnknownJumpTarget,
};
#[derive(Debug)]
pub(crate) struct Scopes<'tcx> {
@ -867,7 +869,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
span_bug!(span, "break value must be a scope")
};
let constant = match &self.thir[value].kind {
let expr = &self.thir[value];
let constant = match &expr.kind {
ExprKind::Adt(box AdtExpr { variant_index, fields, base, .. }) => {
assert!(matches!(base, AdtExprBase::None));
assert!(fields.is_empty());
@ -887,7 +890,27 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
),
}
}
_ => self.as_constant(&self.thir[value]),
ExprKind::Literal { .. }
| ExprKind::NonHirLiteral { .. }
| ExprKind::ZstLiteral { .. }
| ExprKind::NamedConst { .. } => self.as_constant(&self.thir[value]),
other => {
use crate::errors::ConstContinueNotMonomorphicConstReason as Reason;
let span = expr.span;
let reason = match other {
ExprKind::ConstParam { .. } => Reason::ConstantParameter { span },
ExprKind::ConstBlock { .. } => Reason::ConstBlock { span },
_ => Reason::Other { span },
};
self.tcx
.dcx()
.emit_err(ConstContinueNotMonomorphicConst { span: expr.span, reason });
return block.unit();
}
};
let break_index = self

32
compiler/rustc_mir_build/src/errors.rs
View file

@ -1213,6 +1213,38 @@ pub(crate) struct LoopMatchArmWithGuard {
pub span: Span,
}
#[derive(Diagnostic)]
#[diag(mir_build_const_continue_not_const)]
#[help]
pub(crate) struct ConstContinueNotMonomorphicConst {
#[primary_span]
pub span: Span,
#[subdiagnostic]
pub reason: ConstContinueNotMonomorphicConstReason,
}
#[derive(Subdiagnostic)]
pub(crate) enum ConstContinueNotMonomorphicConstReason {
#[label(mir_build_const_continue_not_const_constant_parameter)]
ConstantParameter {
#[primary_span]
span: Span,
},
#[label(mir_build_const_continue_not_const_const_block)]
ConstBlock {
#[primary_span]
span: Span,
},
#[label(mir_build_const_continue_not_const_const_other)]
Other {
#[primary_span]
span: Span,
},
}
#[derive(Diagnostic)]
#[diag(mir_build_const_continue_bad_const)]
pub(crate) struct ConstContinueBadConst {

174
tests/ui/loop-match/suggest-const-item.rs Normal file
View file

@ -0,0 +1,174 @@
#![allow(incomplete_features)]
#![feature(loop_match)]
#![feature(generic_const_items)]
#![crate_type = "lib"]
const fn const_fn() -> i32 {
1
}
#[unsafe(no_mangle)]
fn suggest_const_block<const N: i32>() -> i32 {
let mut state = 0;
#[loop_match]
loop {
state = 'blk: {
match state {
0 => {
#[const_continue]
break 'blk const_fn();
//~^ ERROR could not determine the target branch for this `#[const_continue]`
}
1 => {
#[const_continue]
break 'blk const { const_fn() };
//~^ ERROR could not determine the target branch for this `#[const_continue]`
}
2 => {
#[const_continue]
break 'blk N;
//~^ ERROR could not determine the target branch for this `#[const_continue]`
}
_ => {
#[const_continue]
break 'blk 1 + 1;
//~^ ERROR could not determine the target branch for this `#[const_continue]`
}
}
}
}
state
}
struct S;
impl S {
const M: usize = 42;
fn g() {
let mut state = 0;
#[loop_match]
loop {
state = 'blk: {
match state {
0 => {
#[const_continue]
break 'blk Self::M;
}
_ => panic!(),
}
}
}
}
}
trait T {
const N: usize;
fn f() {
let mut state = 0;
#[loop_match]
loop {
state = 'blk: {
match state {
0 => {
#[const_continue]
break 'blk Self::N;
//~^ ERROR could not determine the target branch for this `#[const_continue]`
}
_ => panic!(),
}
}
}
}
}
impl T for S {
const N: usize = 1;
}
impl S {
fn h() {
let mut state = 0;
#[loop_match]
loop {
state = 'blk: {
match state {
0 => {
#[const_continue]
break 'blk Self::N;
}
_ => panic!(),
}
}
}
}
}
trait T2<U> {
const L: u32;
fn p() {
let mut state = 0;
#[loop_match]
loop {
state = 'blk: {
match state {
0 => {
#[const_continue]
break 'blk Self::L;
//~^ ERROR could not determine the target branch for this `#[const_continue]`
}
_ => panic!(),
}
}
}
}
}
const SIZE_OF<T>: usize = size_of::<T>();
fn q<T>() {
let mut state = 0;
#[loop_match]
loop {
state = 'blk: {
match state {
0 => {
#[const_continue]
break 'blk SIZE_OF::<T>;
//~^ ERROR could not determine the target branch for this `#[const_continue]`
}
_ => panic!(),
}
}
}
}
trait Trait<T> {
const X: usize = 9000;
const Y: usize = size_of::<T>();
}
impl<T> Trait<T> for () {}
fn r<T>() {
let mut state = 0;
#[loop_match]
loop {
state = 'blk: {
match state {
0 => {
#[const_continue]
break 'blk <() as Trait<T>>::X;
}
1 => {
#[const_continue]
break 'blk <() as Trait<T>>::Y;
//~^ ERROR could not determine the target branch for this `#[const_continue]`
}
_ => panic!(),
}
}
}
}

58
tests/ui/loop-match/suggest-const-item.stderr Normal file
View file

@ -0,0 +1,58 @@
error: could not determine the target branch for this `#[const_continue]`
--> $DIR/suggest-const-item.rs:19:32
|
LL | break 'blk const_fn();
| ^^^^^^^^^^ this value must be a literal or a monomorphic const
|
= help: try extracting the expression into a `const` item
error: could not determine the target branch for this `#[const_continue]`
--> $DIR/suggest-const-item.rs:24:32
|
LL | break 'blk const { const_fn() };
| ^^^^^^^^^^^^^^^^^^^^ `const` blocks may use generics, and are not evaluated early enough
|
= help: try extracting the expression into a `const` item
error: could not determine the target branch for this `#[const_continue]`
--> $DIR/suggest-const-item.rs:29:32
|
LL | break 'blk N;
| ^ constant parameters may use generics, and are not evaluated early enough
|
= help: try extracting the expression into a `const` item
error: could not determine the target branch for this `#[const_continue]`
--> $DIR/suggest-const-item.rs:34:32
|
LL | break 'blk 1 + 1;
| ^^^^^ this value must be a literal or a monomorphic const
|
= help: try extracting the expression into a `const` item
error: could not determine the target branch for this `#[const_continue]`
--> $DIR/suggest-const-item.rs:76:36
|
LL | break 'blk Self::N;
| ^^^^^^^ this value is too generic
error: could not determine the target branch for this `#[const_continue]`
--> $DIR/suggest-const-item.rs:119:36
|
LL | break 'blk Self::L;
| ^^^^^^^ this value is too generic
error: could not determine the target branch for this `#[const_continue]`
--> $DIR/suggest-const-item.rs:139:32
|
LL | break 'blk SIZE_OF::<T>;
| ^^^^^^^^^^^^ this value is too generic
error: could not determine the target branch for this `#[const_continue]`
--> $DIR/suggest-const-item.rs:167:32
|
LL | break 'blk <() as Trait<T>>::Y;
| ^^^^^^^^^^^^^^^^^^^ this value is too generic
error: aborting due to 8 previous errors