ICE: assertion failed (left == right) in astconv.rs #38504
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Dec 26, 2016
More systematic error reporting in path resolution
Path resolution for types, expressions and patterns used various heuristics to give more helpful messages on unresolved or incorrectly resolved paths.
This PR combines these heuristics and applies them to all non-import paths.
First a path is resolved in all namespaces, starting from its primary namespace (to give messages like "expected function, found macro, you probably forgot `!`").
If this resolution doesn't give a desired result we create a base error - either "path is not resolved" or "path is resolved, but the resolution is not acceptable in this context".
Other helps and notes are applied to this base error using heuristics.
Here's the list of heuristics for a path with a last segment `name` in order.
First we issue special messages for unresolved `Self` and `self`.
Second we try to find free items named `name` in other modules and suggest to import them.
Then we try to find fields and associated items named `name` and suggest `self.name` or `Self::name`.
After that we try several deterministic context dependent heuristics like "expected value, found struct, you probably forgot `{}`".
If nothing of the above works we try to find candidates with other names using Levenshtein distance.
---
Some alternatives/notes/unresolved questions:
- ~~I had a strong desire to migrate all affected tests to `test/ui`, diagnostics comparison becomes much more meaningful, but I did this only for few tests so far.~~ (Done)
- ~~Labels for "unresolved path" errors are mostly useless now, it may make sense to move some help/notes to these labels, help becomes closer to the error this way.~~ (Done)
- ~~Currently only the first successful heuristic results in additional message shown to the user, it may make sense to print them all, they are rarely compatible, so the diagnostics bloat is unlikely.~~ (Done)
- Now when #38014 landed `resolve_path` can potentially be replaced with `smart_resolve_path` in couple more places - e.g. ~~visibilities~~ (done), ~~import prefixes~~ (done), HIR paths.
---
Some additional fixes:
- Associated suggestions and typo suggestions are filtered with a context specific predicate to avoid inapplicable suggestions.
- `adjust_local_def` works properly in speculative resolution.
- I also fixed a recently introduced ICE in partially resolved UFCS paths (see test `ufcs-partially-resolved.rs`). Minimal reproduction:
```
enum E {}
fn main() {
<u8 as E>::A;
}
```
Fixes #38409, fixes #38504 (duplicates).
- Some bugs in resolution of visibilities are fixed - `pub(Enum)`, `pub(Trait)`, `pub(non::local::path)`.
- Fixes #38012.
---
r? @jseyfried for technical details + @jonathandturner for diagnostics changes
How to read the patch: `smart_resolve_path(_fragment)/resolve_qpath_anywhere` are written anew and replace `resolve_trait_reference`/`resolve_type`/`resolve_pattern_path`/`resolve_struct_path`/`resolve_expr` for `ExprKind::Path`, everything else can be read as a diff.
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The following code:
Produces the following error only on nightly. Stable and beta fail compilation without an ICE.
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