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Fix #35825.
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This attempts to bring better error messages to invalid method calls, by applying some heuristics to identify common mistakes.
The algorithm is inspired by Levenshtein distance and longest common sub-sequence. In essence, we treat the types of the function, and the types of the arguments you provided as two "words" and compute the edits to get from one to the other.
We then modify that algorithm to detect 4 cases:
- A function input is missing
- An extra argument was provided
- The type of an argument is straight up invalid
- Two arguments have been swapped
- A subset of the arguments have been shuffled
(We detect the last two as separate cases so that we can detect two swaps, instead of 4 parameters permuted.)
It helps to understand this argument by paying special attention to terminology: "inputs" refers to the inputs being *expected* by the function, and "arguments" refers to what has been provided at the call site.
The basic sketch of the algorithm is as follows:
- Construct a boolean grid, with a row for each argument, and a column for each input. The cell [i, j] is true if the i'th argument could satisfy the j'th input.
- If we find an argument that could satisfy no inputs, provided for an input that can't be satisfied by any other argument, we consider this an "invalid type".
- Extra arguments are those that can't satisfy any input, provided for an input that *could* be satisfied by another argument.
- Missing inputs are inputs that can't be satisfied by any argument, where the provided argument could satisfy another input
- Swapped / Permuted arguments are identified with a cycle detection algorithm.
As each issue is found, we remove the relevant inputs / arguments and check for more issues. If we find no issues, we match up any "valid" arguments, and start again.
Note that there's a lot of extra complexity:
- We try to stay efficient on the happy path, only computing the diagonal until we find a problem, and then filling in the rest of the matrix.
- Closure arguments are wrapped in a tuple and need to be unwrapped
- We need to resolve closure types after the rest, to allow the most specific type constraints
- We need to handle imported C functions that might be variadic in their inputs.
I tried to document a lot of this in comments in the code and keep the naming clear.
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When encountering an unsatisfied trait bound, if there are no other
suggestions, mention all the types that *do* implement that trait:
```
error[E0277]: the trait bound `f32: Foo` is not satisfied
--> $DIR/impl_wf.rs:22:6
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LL | impl Baz<f32> for f32 { }
| ^^^^^^^^ the trait `Foo` is not implemented for `f32`
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= help: the following other types implement trait `Foo`:
Option<T>
i32
str
note: required by a bound in `Baz`
--> $DIR/impl_wf.rs:18:31
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LL | trait Baz<U: ?Sized> where U: Foo { }
| ^^^ required by this bound in `Baz`
```
Mention implementers of traits in `ImplObligation`s.
Do not mention other `impl`s for closures, ranges and `?`.
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Be more thorough in using `ItemObligation` and `BindingObligation` when
evaluating obligations so that we can point at trait bounds that
introduced unfulfilled obligations. We no longer incorrectly point at
unrelated trait bounds (`substs-ppaux.verbose.stderr`).
In particular, we now point at trait bounds on method calls.
We no longer point at "obvious" obligation sources (we no longer have a
note pointing at `Trait` saying "required by a bound in `Trait`", like
in `associated-types-no-suitable-supertrait*`).
Address part of #89418.
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Link to edition guide instead of issues for 2021 lints.
This changes the 2021 lints to not link to github issues, but to the edition guide instead.
Fixes #86996
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* On suggestions that include deletions, use a diff inspired output format
* When suggesting addition, use `+` as underline
* Color highlight modified span
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* Always point at macros, including derive macros
* Point at non-local items that introduce a trait requirement
* On private associated item, point at definition
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Update BARE_TRAIT_OBJECT and ELLIPSIS_INCLUSIVE_RANGE_PATTERNS to errors in Rust 2021
This addresses https://github.com/rust-lang/rust/pull/81244 by updating two lints to errors in the Rust 2021 edition.
r? `@estebank`
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