Make the ordering of non-terminal binds in ambiguity error messages deterministic by bal-e · Pull Request #158894 · rust-lang/rust

bal-e · 2026-07-07T04:17:45Z

My next PR will rework macro parsing to use a DFS approach instead of a BFS. It will change the order in which the parsing space is explored, and while this does not change the behavior of parsing in success cases, it did cause changes to some error messages; in particular, when an ambiguity error occurs, it would sometimes report non-terminal binds in a different order (e.g. in error: local ambiguity when calling macro `foo`: multiple parsing options: built-in NTs ident ('i') or ident ('j').). I realized that the current macro parsing code does not enforce a particular order for those binds, so this PR 1) adds a test case for strange bind orderings and 2) makes the bind ordering deterministic (in line with the ordering of those binds in the macro definition, which was how binds tended to be ordered anyway).

Ideally reviewed commit-by-commit.

r? @nnethercote

None of the existing tests captured a case where non-terminal binds appeared differently ordered than their definition in their rule, so I wrote a test for it. The existing macro parsing code is quite resilient to this kind of mis-ordering; it took quite a convoluted macro to trigger a case where the ordering is incorrect today. I've documented the parse tree (based on my own mental model) that the convoluted macro causes.

The order of `bb_mps` and `next_mps` depends on arbitrary implementation choices, e.g. the order in which `$(a)? b` causes `a b` and `b` to be explored. To stop depending on these implementation details, this commit sorts `bb_mps` and `next_mps` by `mp.idx` (corresponding to the position in the rule) before presenting error messages. This makes it easier to refactor the macro parsing implementation.

rustbot · 2026-07-07T04:17:50Z

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @nnethercote (or someone else) some time within the next two weeks.

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nnethercote

Looks good. Thanks for breaking up the commits and the detailed commit messages and comments, it makes reviewing really easy.

View changes since this review

nnethercote · 2026-07-08T07:49:33Z

@bors r+ rollup

rust-bors · 2026-07-08T07:49:37Z

📌 Commit 87c76f9 has been approved by nnethercote

It is now in the queue for this repository.

@nnethercote

…thercote Make the ordering of non-terminal binds in ambiguity error messages deterministic My next PR will rework macro parsing to use a DFS approach instead of a BFS. It will change the order in which the parsing space is explored, and while this does not change the behavior of parsing in success cases, it did cause changes to some error messages; in particular, when an ambiguity error occurs, it would sometimes report non-terminal binds in a different order (e.g. in ```error: local ambiguity when calling macro `foo`: multiple parsing options: built-in NTs ident ('i') or ident ('j').```). I realized that the current macro parsing code does not enforce a particular order for those binds, so this PR 1) adds a test case for strange bind orderings and 2) makes the bind ordering deterministic (in line with the ordering of those binds in the macro definition, which was how binds tended to be ordered anyway). Ideally reviewed commit-by-commit. r? @nnethercote

@nnethercote

…thercote Make the ordering of non-terminal binds in ambiguity error messages deterministic My next PR will rework macro parsing to use a DFS approach instead of a BFS. It will change the order in which the parsing space is explored, and while this does not change the behavior of parsing in success cases, it did cause changes to some error messages; in particular, when an ambiguity error occurs, it would sometimes report non-terminal binds in a different order (e.g. in ```error: local ambiguity when calling macro `foo`: multiple parsing options: built-in NTs ident ('i') or ident ('j').```). I realized that the current macro parsing code does not enforce a particular order for those binds, so this PR 1) adds a test case for strange bind orderings and 2) makes the bind ordering deterministic (in line with the ordering of those binds in the macro definition, which was how binds tended to be ordered anyway). Ideally reviewed commit-by-commit. r? @nnethercote

…uwer Rollup of 18 pull requests Successful merges: - #158871 (add relnotes for 1.97.0) - #150946 (intrinsics: Add a fallback for non-const libm float functions) - #158617 (allow mGCA const arguments to fall back to anon consts) - #158645 (Fix splat ICEs and ban it in closures) - #158655 (Fix coroutine MIR saved local remapping) - #158666 (Carry the `b_offset` inside `BackendRepr::ScalarPair`) - #158920 (Update wasm-component-ld to 0.5.26) - #158926 (wrapping_sh* methods: clarify underspecified reference) - #158927 (add core test run with `-Zforce-intrinsic-fallback`) - #151379 (Stabilize `VecDeque::retain_back` from `truncate_front`) - #158807 (Add regression test for CString::clone_into unwind safety) - #158862 (Fix the span for parameter suggestion ) - #158883 (tests: fix enum-match.rs to handle LLVM 23) - #158894 (Make the ordering of non-terminal binds in ambiguity error messages deterministic) - #158902 (add codegen test for range length bound propagation) - #158913 (Update `browser-ui-test` version to `0.24.1`) - #158935 (std: support real fd methods on Emscripten) - #158951 (Merge three `MaxUniverse`s into one)

@nnethercote

…thercote Make the ordering of non-terminal binds in ambiguity error messages deterministic My next PR will rework macro parsing to use a DFS approach instead of a BFS. It will change the order in which the parsing space is explored, and while this does not change the behavior of parsing in success cases, it did cause changes to some error messages; in particular, when an ambiguity error occurs, it would sometimes report non-terminal binds in a different order (e.g. in ```error: local ambiguity when calling macro `foo`: multiple parsing options: built-in NTs ident ('i') or ident ('j').```). I realized that the current macro parsing code does not enforce a particular order for those binds, so this PR 1) adds a test case for strange bind orderings and 2) makes the bind ordering deterministic (in line with the ordering of those binds in the macro definition, which was how binds tended to be ordered anyway). Ideally reviewed commit-by-commit. r? @nnethercote

Rollup of 23 pull requests Successful merges: - #158968 (stdarch subtree update) - #154445 (rustdoc: Represent `--output-format=json` coverage and ir differently) - #158495 (Rename HAS_CT_PROJECTION to HAS_CONST_ALIAS) - #158666 (Carry the `b_offset` inside `BackendRepr::ScalarPair`) - #158870 (std: merge the unix-like io::error modules into one file) - #158920 (Update wasm-component-ld to 0.5.26) - #158926 (wrapping_sh* methods: clarify underspecified reference) - #158927 (add core test run with `-Zforce-intrinsic-fallback`) - #158932 (Do not build the compiler when invoking `x perf compare`) - #158937 (Emit the emscripten entry point as `__main_argc_argv`) - #151379 (Stabilize `VecDeque::retain_back` from `truncate_front`) - #156144 (Better docs for PartialEq (includes macro rename)) - #156548 ( Library support for aarch64-unknown-linux-pauthtest target) - #157995 (`Vec::dedup_by` docs explicit function argument order) - #158307 (CI job for parallel frontend ui tests) - #158741 (Simplify `Option::into_flat_iter` signature) - #158807 (Add regression test for CString::clone_into unwind safety) - #158862 (Fix the span for parameter suggestion ) - #158894 (Make the ordering of non-terminal binds in ambiguity error messages deterministic) - #158902 (add codegen test for range length bound propagation) - #158913 (Update `browser-ui-test` version to `0.24.1`) - #158935 (std: support real fd methods on Emscripten) - #158978 (Add regression test for too-big by-value ABI args)

@nnethercote

Rollup merge of #158894 - bal-e:ambiguity-nt-ordering, r=nnethercote Make the ordering of non-terminal binds in ambiguity error messages deterministic My next PR will rework macro parsing to use a DFS approach instead of a BFS. It will change the order in which the parsing space is explored, and while this does not change the behavior of parsing in success cases, it did cause changes to some error messages; in particular, when an ambiguity error occurs, it would sometimes report non-terminal binds in a different order (e.g. in ```error: local ambiguity when calling macro `foo`: multiple parsing options: built-in NTs ident ('i') or ident ('j').```). I realized that the current macro parsing code does not enforce a particular order for those binds, so this PR 1) adds a test case for strange bind orderings and 2) makes the bind ordering deterministic (in line with the ordering of those binds in the macro definition, which was how binds tended to be ordered anyway). Ideally reviewed commit-by-commit. r? @nnethercote

Rollup of 23 pull requests Successful merges: - rust-lang/rust#158968 (stdarch subtree update) - rust-lang/rust#154445 (rustdoc: Represent `--output-format=json` coverage and ir differently) - rust-lang/rust#158495 (Rename HAS_CT_PROJECTION to HAS_CONST_ALIAS) - rust-lang/rust#158666 (Carry the `b_offset` inside `BackendRepr::ScalarPair`) - rust-lang/rust#158870 (std: merge the unix-like io::error modules into one file) - rust-lang/rust#158920 (Update wasm-component-ld to 0.5.26) - rust-lang/rust#158926 (wrapping_sh* methods: clarify underspecified reference) - rust-lang/rust#158927 (add core test run with `-Zforce-intrinsic-fallback`) - rust-lang/rust#158932 (Do not build the compiler when invoking `x perf compare`) - rust-lang/rust#158937 (Emit the emscripten entry point as `__main_argc_argv`) - rust-lang/rust#151379 (Stabilize `VecDeque::retain_back` from `truncate_front`) - rust-lang/rust#156144 (Better docs for PartialEq (includes macro rename)) - rust-lang/rust#156548 ( Library support for aarch64-unknown-linux-pauthtest target) - rust-lang/rust#157995 (`Vec::dedup_by` docs explicit function argument order) - rust-lang/rust#158307 (CI job for parallel frontend ui tests) - rust-lang/rust#158741 (Simplify `Option::into_flat_iter` signature) - rust-lang/rust#158807 (Add regression test for CString::clone_into unwind safety) - rust-lang/rust#158862 (Fix the span for parameter suggestion ) - rust-lang/rust#158894 (Make the ordering of non-terminal binds in ambiguity error messages deterministic) - rust-lang/rust#158902 (add codegen test for range length bound propagation) - rust-lang/rust#158913 (Update `browser-ui-test` version to `0.24.1`) - rust-lang/rust#158935 (std: support real fd methods on Emscripten) - rust-lang/rust#158978 (Add regression test for too-big by-value ABI args)

@nnethercote

Preparatory changes for macro parsing BFS->DFS This PR contains some miscellaneous commits I accumulated while working on the BFS->DFS change. Their goal is to simplify the code and clarifying existing behavior. It is a conceptual follow-up to #158577. High-level overview: - Adds context about the current match arm to `Tracker` through the new `Tracker::prepare()`, so that the `WhichMatcher` parameter is available implicitly. In a later PR, this will be used to reference `MatcherLoc`s by index. - Reformulates matching failures to work more like ambiguity errors wrt. `Tracker`; `Tracker::build_failure()` (which would build a failure consumed by `Tracker::after_arm()`) becomes `Tracker::failure()` which eagerly processes the error. This removes the need for `ParseResult::Failure` to store any data at all. This relies on the match arm context provided by `Tracker::prepare()`. - There is a subtle edge case involving `token::Eof` and non-terminal parsing; `Parser::nonterminal_may_begin_with()` would sometimes return `true` for `token::Eof`, even though the non-terminal parse would never be attempted. `TtParser` did not check `bb_mps` when handling `token::Eof`, so non-terminal parses at EOF were being silently dropped. I changed `Parser::nonterminal_may_begin_with()` to always return `false` for `token::Eof`, making this behavior much more visible. I added a test case to make sure meta-variables return the same error uniformly when parsed against EOF. Contains #158894, which should be merged soon (after which I'll rebase onto `main`). Best reviewed commit-by-commit. r? @nnethercote

@nnethercote

Preparatory changes for macro parsing BFS->DFS This PR contains some miscellaneous commits I accumulated while working on the BFS->DFS change. Their goal is to simplify the code and clarifying existing behavior. It is a conceptual follow-up to #158577. High-level overview: - Adds context about the current match arm to `Tracker` through the new `Tracker::prepare()`, so that the `WhichMatcher` parameter is available implicitly. In a later PR, this will be used to reference `MatcherLoc`s by index. - Reformulates matching failures to work more like ambiguity errors wrt. `Tracker`; `Tracker::build_failure()` (which would build a failure consumed by `Tracker::after_arm()`) becomes `Tracker::failure()` which eagerly processes the error. This removes the need for `ParseResult::Failure` to store any data at all. This relies on the match arm context provided by `Tracker::prepare()`. - There is a subtle edge case involving `token::Eof` and non-terminal parsing; `Parser::nonterminal_may_begin_with()` would sometimes return `true` for `token::Eof`, even though the non-terminal parse would never be attempted. `TtParser` did not check `bb_mps` when handling `token::Eof`, so non-terminal parses at EOF were being silently dropped. I changed `Parser::nonterminal_may_begin_with()` to always return `false` for `token::Eof`, making this behavior much more visible. I added a test case to make sure meta-variables return the same error uniformly when parsed against EOF. Contains #158894, which should be merged soon (after which I'll rebase onto `main`). Best reviewed commit-by-commit. r? @nnethercote

@nnethercote

Preparatory changes for macro parsing BFS->DFS This PR contains some miscellaneous commits I accumulated while working on the BFS->DFS change. Their goal is to simplify the code and clarifying existing behavior. It is a conceptual follow-up to #158577. High-level overview: - Adds context about the current match arm to `Tracker` through the new `Tracker::prepare()`, so that the `WhichMatcher` parameter is available implicitly. In a later PR, this will be used to reference `MatcherLoc`s by index. - Reformulates matching failures to work more like ambiguity errors wrt. `Tracker`; `Tracker::build_failure()` (which would build a failure consumed by `Tracker::after_arm()`) becomes `Tracker::failure()` which eagerly processes the error. This removes the need for `ParseResult::Failure` to store any data at all. This relies on the match arm context provided by `Tracker::prepare()`. - There is a subtle edge case involving `token::Eof` and non-terminal parsing; `Parser::nonterminal_may_begin_with()` would sometimes return `true` for `token::Eof`, even though the non-terminal parse would never be attempted. `TtParser` did not check `bb_mps` when handling `token::Eof`, so non-terminal parses at EOF were being silently dropped. I changed `Parser::nonterminal_may_begin_with()` to always return `false` for `token::Eof`, making this behavior much more visible. I added a test case to make sure meta-variables return the same error uniformly when parsed against EOF. Contains #158894, which should be merged soon (after which I'll rebase onto `main`). Best reviewed commit-by-commit. r? @nnethercote

@nnethercote

Preparatory changes for macro parsing BFS->DFS This PR contains some miscellaneous commits I accumulated while working on the BFS->DFS change. Their goal is to simplify the code and clarifying existing behavior. It is a conceptual follow-up to rust-lang/rust#158577. High-level overview: - Adds context about the current match arm to `Tracker` through the new `Tracker::prepare()`, so that the `WhichMatcher` parameter is available implicitly. In a later PR, this will be used to reference `MatcherLoc`s by index. - Reformulates matching failures to work more like ambiguity errors wrt. `Tracker`; `Tracker::build_failure()` (which would build a failure consumed by `Tracker::after_arm()`) becomes `Tracker::failure()` which eagerly processes the error. This removes the need for `ParseResult::Failure` to store any data at all. This relies on the match arm context provided by `Tracker::prepare()`. - There is a subtle edge case involving `token::Eof` and non-terminal parsing; `Parser::nonterminal_may_begin_with()` would sometimes return `true` for `token::Eof`, even though the non-terminal parse would never be attempted. `TtParser` did not check `bb_mps` when handling `token::Eof`, so non-terminal parses at EOF were being silently dropped. I changed `Parser::nonterminal_may_begin_with()` to always return `false` for `token::Eof`, making this behavior much more visible. I added a test case to make sure meta-variables return the same error uniformly when parsed against EOF. Contains rust-lang/rust#158894, which should be merged soon (after which I'll rebase onto `main`). Best reviewed commit-by-commit. r? @nnethercote

@nnethercote

Eagerly check for ambiguity in macro parsing This PR implements an important step leading up to the BFS->DFS change: it makes ambiguity detection (as occurs when parsing meta-variables and reaching EOF) eager. Rather than accumulating the `bb_mps` and `eof_mps` lists, then checking that a single valid parse exists, this PR introduces a `check_for_ambiguity()` method that gets called as soon as the relevant `MatcherLoc` is observed. `check_for_ambiguity()` immediately drains `cur_mps` instead of waiting for the outer loop to do so, and evaluates the mps within a "checking for ambiguity" context. This change was complicated by the fact that `bb_mps` was relied on for error messages; it was passed along to `Tracker::ambiguity()`. The same is true for `next_mps`. The first few commits of this PR (after pending rebases) add state to `Tracker` so that it can compute the relevant diagnostic information independently of `bb_mps`. This PR complicates the control flow around `parse_tt_inner()` in subtle ways. I'm not super happy about it, but I think it will become easier to understand again once a DFS approach is implemented. ~This PR contains #158894 and #158974. I'll rebase on top of `main` once they're both merged.~ Best reviewed commit-by-commit. r? @nnethercote

@nnethercote

Eagerly check for ambiguity in macro parsing This PR implements an important step leading up to the BFS->DFS change: it makes ambiguity detection (as occurs when parsing meta-variables and reaching EOF) eager. Rather than accumulating the `bb_mps` and `eof_mps` lists, then checking that a single valid parse exists, this PR introduces a `check_for_ambiguity()` method that gets called as soon as the relevant `MatcherLoc` is observed. `check_for_ambiguity()` immediately drains `cur_mps` instead of waiting for the outer loop to do so, and evaluates the mps within a "checking for ambiguity" context. This change was complicated by the fact that `bb_mps` was relied on for error messages; it was passed along to `Tracker::ambiguity()`. The same is true for `next_mps`. The first few commits of this PR (after pending rebases) add state to `Tracker` so that it can compute the relevant diagnostic information independently of `bb_mps`. This PR complicates the control flow around `parse_tt_inner()` in subtle ways. I'm not super happy about it, but I think it will become easier to understand again once a DFS approach is implemented. ~This PR contains rust-lang/rust#158894 and rust-lang/rust#158974. I'll rebase on top of `main` once they're both merged.~ Best reviewed commit-by-commit. r? @nnethercote

Rollup of 23 pull requests Successful merges: - rust-lang/rust#158968 (stdarch subtree update) - rust-lang/rust#154445 (rustdoc: Represent `--output-format=json` coverage and ir differently) - rust-lang/rust#158495 (Rename HAS_CT_PROJECTION to HAS_CONST_ALIAS) - rust-lang/rust#158666 (Carry the `b_offset` inside `BackendRepr::ScalarPair`) - rust-lang/rust#158870 (std: merge the unix-like io::error modules into one file) - rust-lang/rust#158920 (Update wasm-component-ld to 0.5.26) - rust-lang/rust#158926 (wrapping_sh* methods: clarify underspecified reference) - rust-lang/rust#158927 (add core test run with `-Zforce-intrinsic-fallback`) - rust-lang/rust#158932 (Do not build the compiler when invoking `x perf compare`) - rust-lang/rust#158937 (Emit the emscripten entry point as `__main_argc_argv`) - rust-lang/rust#151379 (Stabilize `VecDeque::retain_back` from `truncate_front`) - rust-lang/rust#156144 (Better docs for PartialEq (includes macro rename)) - rust-lang/rust#156548 ( Library support for aarch64-unknown-linux-pauthtest target) - rust-lang/rust#157995 (`Vec::dedup_by` docs explicit function argument order) - rust-lang/rust#158307 (CI job for parallel frontend ui tests) - rust-lang/rust#158741 (Simplify `Option::into_flat_iter` signature) - rust-lang/rust#158807 (Add regression test for CString::clone_into unwind safety) - rust-lang/rust#158862 (Fix the span for parameter suggestion ) - rust-lang/rust#158894 (Make the ordering of non-terminal binds in ambiguity error messages deterministic) - rust-lang/rust#158902 (add codegen test for range length bound propagation) - rust-lang/rust#158913 (Update `browser-ui-test` version to `0.24.1`) - rust-lang/rust#158935 (std: support real fd methods on Emscripten) - rust-lang/rust#158978 (Add regression test for too-big by-value ABI args)

@nnethercote

Eagerly check for ambiguity in macro parsing This PR implements an important step leading up to the BFS->DFS change: it makes ambiguity detection (as occurs when parsing meta-variables and reaching EOF) eager. Rather than accumulating the `bb_mps` and `eof_mps` lists, then checking that a single valid parse exists, this PR introduces a `check_for_ambiguity()` method that gets called as soon as the relevant `MatcherLoc` is observed. `check_for_ambiguity()` immediately drains `cur_mps` instead of waiting for the outer loop to do so, and evaluates the mps within a "checking for ambiguity" context. This change was complicated by the fact that `bb_mps` was relied on for error messages; it was passed along to `Tracker::ambiguity()`. The same is true for `next_mps`. The first few commits of this PR (after pending rebases) add state to `Tracker` so that it can compute the relevant diagnostic information independently of `bb_mps`. This PR complicates the control flow around `parse_tt_inner()` in subtle ways. I'm not super happy about it, but I think it will become easier to understand again once a DFS approach is implemented. ~This PR contains rust-lang/rust#158894 and rust-lang/rust#158974. I'll rebase on top of `main` once they're both merged.~ Best reviewed commit-by-commit. r? @nnethercote

Rollup of 23 pull requests Successful merges: - rust-lang/rust#158968 (stdarch subtree update) - rust-lang/rust#154445 (rustdoc: Represent `--output-format=json` coverage and ir differently) - rust-lang/rust#158495 (Rename HAS_CT_PROJECTION to HAS_CONST_ALIAS) - rust-lang/rust#158666 (Carry the `b_offset` inside `BackendRepr::ScalarPair`) - rust-lang/rust#158870 (std: merge the unix-like io::error modules into one file) - rust-lang/rust#158920 (Update wasm-component-ld to 0.5.26) - rust-lang/rust#158926 (wrapping_sh* methods: clarify underspecified reference) - rust-lang/rust#158927 (add core test run with `-Zforce-intrinsic-fallback`) - rust-lang/rust#158932 (Do not build the compiler when invoking `x perf compare`) - rust-lang/rust#158937 (Emit the emscripten entry point as `__main_argc_argv`) - rust-lang/rust#151379 (Stabilize `VecDeque::retain_back` from `truncate_front`) - rust-lang/rust#156144 (Better docs for PartialEq (includes macro rename)) - rust-lang/rust#156548 ( Library support for aarch64-unknown-linux-pauthtest target) - rust-lang/rust#157995 (`Vec::dedup_by` docs explicit function argument order) - rust-lang/rust#158307 (CI job for parallel frontend ui tests) - rust-lang/rust#158741 (Simplify `Option::into_flat_iter` signature) - rust-lang/rust#158807 (Add regression test for CString::clone_into unwind safety) - rust-lang/rust#158862 (Fix the span for parameter suggestion ) - rust-lang/rust#158894 (Make the ordering of non-terminal binds in ambiguity error messages deterministic) - rust-lang/rust#158902 (add codegen test for range length bound propagation) - rust-lang/rust#158913 (Update `browser-ui-test` version to `0.24.1`) - rust-lang/rust#158935 (std: support real fd methods on Emscripten) - rust-lang/rust#158978 (Add regression test for too-big by-value ABI args)

arya dradjica added 2 commits July 3, 2026 09:29

rustbot assigned nnethercote Jul 7, 2026

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Rollup of 16 pull requests #158953

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Rollup of 18 pull requests #158958

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Preparatory changes for macro parsing BFS->DFS #158974

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Rollup of 25 pull requests #158989

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rust-bors Bot merged commit 2bb944e into rust-lang:main Jul 9, 2026
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bal-e deleted the ambiguity-nt-ordering branch July 9, 2026 13:33

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Use depth-first traversal for macro parsing #159808

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Make the ordering of non-terminal binds in ambiguity error messages deterministic#158894

Make the ordering of non-terminal binds in ambiguity error messages deterministic#158894
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