Suggest turning if let into irrefutable let if appropriate

[estebank]

When encountering an if let tail expression without an else arm for an enum with a single variant, suggest writing an irrefutable let binding instead.

error[E0317]: `if` may be missing an `else` clause
  --> $DIR/irrefutable-if-let-without-else.rs:8:5
   |
LL |   fn foo(x: Enum) -> i32 {
   |                      --- expected `i32` because of this return type
LL | /     if let Enum::Variant(value) = x {
LL | |         value
LL | |     }
   | |_____^ expected `i32`, found `()`
   |
   = note: `if` expressions without `else` evaluate to `()`
   = help: consider adding an `else` block that evaluates to the expected type
help: consider using an irrefutable `let` binding instead
   |
LL ~     let Enum::Variant(value) = x;
LL ~         value
   |

Fix #61788.

[rustbot]

r? @wesleywiser

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[estebank]

If this is too much logic for little gain, we can still close #61788, as the help and note that weren't present in the original report can also be claimed to fix the issue.

[wesleywiser]

Just an idle thought but I wonder if we should have an extension trait specifically for use in diagnostics that can provide some of this boilerplate functionality? As another example of a similar thing, the idiom used to test if a type implements a specific trait that sometimes appears in diagnostics is also a bit cryptic and would be neater if factored behind a well named function.

[estebank]

@wesleywiser yeah. There are a couple of methods to check for "type implements trait?", but they are not always available in all stages. We should probably audit all cases and unify functionality behind a handful of easier to use methods. It is a bit complicated by the needs of diagnostics, where sometimes we want to be maximally inclusive (accepting things that could implement, but we don't know for sure) or maximally exclusive (accepting only things that we know for sure implement, to avoid invalid suggestions).

[wesleywiser]

Thanks @estebank 💯

@bors r+ rollup

[bors]

📌 Commit a939bad has been approved by wesleywiser

It is now in the queue for this repository.