From 43057698c12e8ceab1f49eff8a3e5a38cc05f7db Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Esteban=20K=C3=BCber?= Date: Wed, 24 Sep 2025 21:06:54 +0000 Subject: [PATCH 1/2] Tweak handling of "struct like start" where a struct isn't supported This improves the case where someone tries to write a `match` expr where the patterns have type ascription syntax. Makes them less verbose, by giving up on the first encounter in the block, and makes them more accurate by only treating them as a struct literal if successfuly parsed as such. --- .../rustc_parse/src/parser/diagnostics.rs | 40 ++++------ compiler/rustc_parse/src/parser/expr.rs | 61 +++++++++----- .../issues/issue-87086-colon-path-sep.rs | 3 +- .../issues/issue-87086-colon-path-sep.stderr | 20 ++--- tests/ui/parser/type-ascription-in-pattern.rs | 17 ++-- .../parser/type-ascription-in-pattern.stderr | 80 +++++-------------- 6 files changed, 93 insertions(+), 128 deletions(-) diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs index a28af7833c387..a9fbd0fa33d5f 100644 --- a/compiler/rustc_parse/src/parser/diagnostics.rs +++ b/compiler/rustc_parse/src/parser/diagnostics.rs @@ -2748,28 +2748,7 @@ impl<'a> Parser<'a> { if token::Colon != self.token.kind { return first_pat; } - if !matches!(first_pat.kind, PatKind::Ident(_, _, None) | PatKind::Path(..)) - || !self.look_ahead(1, |token| token.is_non_reserved_ident()) - { - let mut snapshot_type = self.create_snapshot_for_diagnostic(); - snapshot_type.bump(); // `:` - match snapshot_type.parse_ty() { - Err(inner_err) => { - inner_err.cancel(); - } - Ok(ty) => { - let Err(mut err) = self.expected_one_of_not_found(&[], &[]) else { - return first_pat; - }; - err.span_label(ty.span, "specifying the type of a pattern isn't supported"); - self.restore_snapshot(snapshot_type); - let span = first_pat.span.to(ty.span); - first_pat = self.mk_pat(span, PatKind::Wild); - err.emit(); - } - } - return first_pat; - } + // The pattern looks like it might be a path with a `::` -> `:` typo: // `match foo { bar:baz => {} }` let colon_span = self.token.span; @@ -2857,7 +2836,13 @@ impl<'a> Parser<'a> { Applicability::MaybeIncorrect, ); } else { - first_pat = self.mk_pat(new_span, PatKind::Wild); + first_pat = self.mk_pat( + new_span, + PatKind::Err( + self.dcx() + .span_delayed_bug(colon_span, "recovered bad path pattern"), + ), + ); } self.restore_snapshot(snapshot_pat); } @@ -2870,7 +2855,14 @@ impl<'a> Parser<'a> { err.span_label(ty.span, "specifying the type of a pattern isn't supported"); self.restore_snapshot(snapshot_type); let new_span = first_pat.span.to(ty.span); - first_pat = self.mk_pat(new_span, PatKind::Wild); + first_pat = + self.mk_pat( + new_span, + PatKind::Err(self.dcx().span_delayed_bug( + colon_span, + "recovered bad pattern with type", + )), + ); } } err.emit(); diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs index 81a5d48d94e8c..2f4158450d838 100644 --- a/compiler/rustc_parse/src/parser/expr.rs +++ b/compiler/rustc_parse/src/parser/expr.rs @@ -3612,36 +3612,55 @@ impl<'a> Parser<'a> { self.token.is_keyword(kw::Async) && self.is_gen_block(kw::Gen, 1) } - fn is_certainly_not_a_block(&self) -> bool { - // `{ ident, ` and `{ ident: ` cannot start a block. - self.look_ahead(1, |t| t.is_ident()) - && self.look_ahead(2, |t| t == &token::Comma || t == &token::Colon) - } - fn maybe_parse_struct_expr( &mut self, qself: &Option>, path: &ast::Path, ) -> Option>> { let struct_allowed = !self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL); - if struct_allowed || self.is_certainly_not_a_block() { - if let Err(err) = self.expect(exp!(OpenBrace)) { - return Some(Err(err)); + let is_ident = self.look_ahead(1, |t| t.is_ident()); + let is_comma = self.look_ahead(2, |t| t == &token::Comma); + let is_colon = self.look_ahead(2, |t| t == &token::Colon); + match (struct_allowed, is_ident, is_comma, is_colon) { + (false, true, true, _) | (false, true, _, true) => { + // We have something like `match foo { bar,` or `match foo { bar:`, which means the + // user might have meant to write a struct literal as part of the `match` + // discriminant. + let snapshot = self.create_snapshot_for_diagnostic(); + if let Err(err) = self.expect(exp!(OpenBrace)) { + return Some(Err(err)); + } + match self.parse_expr_struct(qself.clone(), path.clone(), false) { + Ok(expr) => { + // This is a struct literal, but we don't accept them here. + self.dcx().emit_err(errors::StructLiteralNotAllowedHere { + span: expr.span, + sub: errors::StructLiteralNotAllowedHereSugg { + left: path.span.shrink_to_lo(), + right: expr.span.shrink_to_hi(), + }, + }); + Some(Ok(expr)) + } + Err(err) => { + // We couldn't parse a valid struct, rollback and let the parser emit an + // error elsewhere. + err.cancel(); + self.restore_snapshot(snapshot); + None + } + } } - let expr = self.parse_expr_struct(qself.clone(), path.clone(), true); - if let (Ok(expr), false) = (&expr, struct_allowed) { - // This is a struct literal, but we don't can't accept them here. - self.dcx().emit_err(errors::StructLiteralNotAllowedHere { - span: expr.span, - sub: errors::StructLiteralNotAllowedHereSugg { - left: path.span.shrink_to_lo(), - right: expr.span.shrink_to_hi(), - }, - }); + (true, _, _, _) => { + // A struct is accepted here, try to parse it and rely on `parse_expr_struct` for + // any kind of recovery. + if let Err(err) = self.expect(exp!(OpenBrace)) { + return Some(Err(err)); + } + Some(self.parse_expr_struct(qself.clone(), path.clone(), true)) } - return Some(expr); + (false, _, _, _) => None, } - None } pub(super) fn parse_struct_fields( diff --git a/tests/ui/parser/issues/issue-87086-colon-path-sep.rs b/tests/ui/parser/issues/issue-87086-colon-path-sep.rs index d081c06044f14..e1ea38f2795df 100644 --- a/tests/ui/parser/issues/issue-87086-colon-path-sep.rs +++ b/tests/ui/parser/issues/issue-87086-colon-path-sep.rs @@ -37,10 +37,9 @@ fn g1() { //~| HELP: maybe write a path separator here _ => {} } - if let Foo:Bar = f() { //~ WARN: irrefutable `if let` pattern + if let Foo:Bar = f() { //~^ ERROR: expected one of //~| HELP: maybe write a path separator here - //~| HELP: consider replacing the `if let` with a `let` } } diff --git a/tests/ui/parser/issues/issue-87086-colon-path-sep.stderr b/tests/ui/parser/issues/issue-87086-colon-path-sep.stderr index a9bad96f9af76..061586882e0c9 100644 --- a/tests/ui/parser/issues/issue-87086-colon-path-sep.stderr +++ b/tests/ui/parser/issues/issue-87086-colon-path-sep.stderr @@ -64,7 +64,7 @@ LL | if let Foo::Bar = f() { | + error: expected one of `@` or `|`, found `:` - --> $DIR/issue-87086-colon-path-sep.rs:49:16 + --> $DIR/issue-87086-colon-path-sep.rs:48:16 | LL | ref qux: Foo::Baz => {} | ^ -------- specifying the type of a pattern isn't supported @@ -77,7 +77,7 @@ LL | ref qux::Foo::Baz => {} | ~~ error: expected one of `@` or `|`, found `:` - --> $DIR/issue-87086-colon-path-sep.rs:58:16 + --> $DIR/issue-87086-colon-path-sep.rs:57:16 | LL | mut qux: Foo::Baz => {} | ^ -------- specifying the type of a pattern isn't supported @@ -90,7 +90,7 @@ LL | mut qux::Foo::Baz => {} | ~~ error: expected one of `@` or `|`, found `:` - --> $DIR/issue-87086-colon-path-sep.rs:69:12 + --> $DIR/issue-87086-colon-path-sep.rs:68:12 | LL | Foo:Bar::Baz => {} | ^-------- specifying the type of a pattern isn't supported @@ -103,7 +103,7 @@ LL | Foo::Bar::Baz => {} | + error: expected one of `@` or `|`, found `:` - --> $DIR/issue-87086-colon-path-sep.rs:75:12 + --> $DIR/issue-87086-colon-path-sep.rs:74:12 | LL | Foo:Bar => {} | ^--- specifying the type of a pattern isn't supported @@ -115,15 +115,5 @@ help: maybe write a path separator here LL | Foo::Bar => {} | + -warning: irrefutable `if let` pattern - --> $DIR/issue-87086-colon-path-sep.rs:40:8 - | -LL | if let Foo:Bar = f() { - | ^^^^^^^^^^^^^^^^^ - | - = note: this pattern will always match, so the `if let` is useless - = help: consider replacing the `if let` with a `let` - = note: `#[warn(irrefutable_let_patterns)]` on by default - -error: aborting due to 9 previous errors; 1 warning emitted +error: aborting due to 9 previous errors diff --git a/tests/ui/parser/type-ascription-in-pattern.rs b/tests/ui/parser/type-ascription-in-pattern.rs index 18d7061d69c8d..75059d33db645 100644 --- a/tests/ui/parser/type-ascription-in-pattern.rs +++ b/tests/ui/parser/type-ascription-in-pattern.rs @@ -1,15 +1,16 @@ fn foo(x: bool) -> i32 { - match x { //~ ERROR struct literals are not allowed here - x: i32 => x, //~ ERROR expected - true => 42., //~ ERROR expected identifier - false => 0.333, //~ ERROR expected identifier + match x { + x: i32 => x, //~ ERROR: expected + //~^ ERROR: mismatched types + true => 42., + false => 0.333, } -} //~ ERROR expected one of +} fn main() { match foo(true) { - 42: i32 => (), //~ ERROR expected - _: f64 => (), //~ ERROR expected - x: i32 => (), //~ ERROR expected + 42: i32 => (), //~ ERROR: expected + _: f64 => (), //~ ERROR: expected + x: i32 => (), //~ ERROR: expected } } diff --git a/tests/ui/parser/type-ascription-in-pattern.stderr b/tests/ui/parser/type-ascription-in-pattern.stderr index 135879f208b2b..0919075499368 100644 --- a/tests/ui/parser/type-ascription-in-pattern.stderr +++ b/tests/ui/parser/type-ascription-in-pattern.stderr @@ -1,64 +1,18 @@ -error: expected one of `!`, `,`, `.`, `::`, `?`, `{`, `}`, or an operator, found `=>` - --> $DIR/type-ascription-in-pattern.rs:3:16 - | -LL | match x { - | - while parsing this struct -LL | x: i32 => x, - | -^^ expected one of 8 possible tokens - | | - | help: try adding a comma: `,` - -error: expected identifier, found keyword `true` - --> $DIR/type-ascription-in-pattern.rs:4:9 - | -LL | match x { - | - while parsing this struct -LL | x: i32 => x, -LL | true => 42., - | ^^^^ expected identifier, found keyword - -error: expected identifier, found keyword `false` - --> $DIR/type-ascription-in-pattern.rs:5:9 - | -LL | match x { - | - while parsing this struct -... -LL | false => 0.333, - | ^^^^^ expected identifier, found keyword - -error: struct literals are not allowed here - --> $DIR/type-ascription-in-pattern.rs:2:11 - | -LL | match x { - | ___________^ -LL | | x: i32 => x, -LL | | true => 42., -LL | | false => 0.333, -LL | | } - | |_____^ - | -help: surround the struct literal with parentheses +error: expected one of `@` or `|`, found `:` + --> $DIR/type-ascription-in-pattern.rs:3:10 | -LL ~ match (x { LL | x: i32 => x, -LL | true => 42., -LL | false => 0.333, -LL ~ }) + | ^ --- specifying the type of a pattern isn't supported + | | + | expected one of `@` or `|` | - -error: expected one of `.`, `?`, `{`, or an operator, found `}` - --> $DIR/type-ascription-in-pattern.rs:7:1 +help: maybe write a path separator here | -LL | match x { - | ----- while parsing this `match` expression -... -LL | } - | - expected one of `.`, `?`, `{`, or an operator -LL | } - | ^ unexpected token +LL | x::i32 => x, + | ~~ error: expected one of `...`, `..=`, `..`, or `|`, found `:` - --> $DIR/type-ascription-in-pattern.rs:11:11 + --> $DIR/type-ascription-in-pattern.rs:12:11 | LL | 42: i32 => (), | ^ --- specifying the type of a pattern isn't supported @@ -66,7 +20,7 @@ LL | 42: i32 => (), | expected one of `...`, `..=`, `..`, or `|` error: expected `|`, found `:` - --> $DIR/type-ascription-in-pattern.rs:12:10 + --> $DIR/type-ascription-in-pattern.rs:13:10 | LL | _: f64 => (), | ^ --- specifying the type of a pattern isn't supported @@ -74,7 +28,7 @@ LL | _: f64 => (), | expected `|` error: expected one of `@` or `|`, found `:` - --> $DIR/type-ascription-in-pattern.rs:13:10 + --> $DIR/type-ascription-in-pattern.rs:14:10 | LL | x: i32 => (), | ^ --- specifying the type of a pattern isn't supported @@ -86,5 +40,15 @@ help: maybe write a path separator here LL | x::i32 => (), | ~~ -error: aborting due to 8 previous errors +error[E0308]: mismatched types + --> $DIR/type-ascription-in-pattern.rs:3:19 + | +LL | fn foo(x: bool) -> i32 { + | --- expected `i32` because of return type +LL | match x { +LL | x: i32 => x, + | ^ expected `i32`, found `bool` + +error: aborting due to 5 previous errors +For more information about this error, try `rustc --explain E0308`. From bb48c162b62ce20c5f51b6a435ca2ba7bb918d7e Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Esteban=20K=C3=BCber?= Date: Wed, 24 Sep 2025 23:25:29 +0000 Subject: [PATCH 2/2] Simplify logic slightly --- compiler/rustc_parse/src/parser/expr.rs | 37 +++++++++++++++---------- 1 file changed, 22 insertions(+), 15 deletions(-) diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs index 2f4158450d838..8046abcd70bd0 100644 --- a/compiler/rustc_parse/src/parser/expr.rs +++ b/compiler/rustc_parse/src/parser/expr.rs @@ -3612,20 +3612,36 @@ impl<'a> Parser<'a> { self.token.is_keyword(kw::Async) && self.is_gen_block(kw::Gen, 1) } + fn is_likely_struct_lit(&self) -> bool { + // `{ ident, ` and `{ ident: ` cannot start a block. + self.look_ahead(1, |t| t.is_ident()) + && self.look_ahead(2, |t| t == &token::Comma || t == &token::Colon) + } + fn maybe_parse_struct_expr( &mut self, qself: &Option>, path: &ast::Path, ) -> Option>> { let struct_allowed = !self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL); - let is_ident = self.look_ahead(1, |t| t.is_ident()); - let is_comma = self.look_ahead(2, |t| t == &token::Comma); - let is_colon = self.look_ahead(2, |t| t == &token::Colon); - match (struct_allowed, is_ident, is_comma, is_colon) { - (false, true, true, _) | (false, true, _, true) => { + match (struct_allowed, self.is_likely_struct_lit()) { + // A struct literal isn't expected and one is pretty much assured not to be present. The + // only situation that isn't detected is when a struct with a single field was attempted + // in a place where a struct literal wasn't expected, but regular parser errors apply. + // Happy path. + (false, false) => None, + (true, _) => { + // A struct is accepted here, try to parse it and rely on `parse_expr_struct` for + // any kind of recovery. Happy path. + if let Err(err) = self.expect(exp!(OpenBrace)) { + return Some(Err(err)); + } + Some(self.parse_expr_struct(qself.clone(), path.clone(), true)) + } + (false, true) => { // We have something like `match foo { bar,` or `match foo { bar:`, which means the // user might have meant to write a struct literal as part of the `match` - // discriminant. + // discriminant. This is done purely for error recovery. let snapshot = self.create_snapshot_for_diagnostic(); if let Err(err) = self.expect(exp!(OpenBrace)) { return Some(Err(err)); @@ -3651,15 +3667,6 @@ impl<'a> Parser<'a> { } } } - (true, _, _, _) => { - // A struct is accepted here, try to parse it and rely on `parse_expr_struct` for - // any kind of recovery. - if let Err(err) = self.expect(exp!(OpenBrace)) { - return Some(Err(err)); - } - Some(self.parse_expr_struct(qself.clone(), path.clone(), true)) - } - (false, _, _, _) => None, } }