Detect missing if let or let-else

[estebank]

During let binding parse error and encountering a block, detect if there is a likely missing if or else:

error: expected one of `.`, `;`, `?`, `else`, or an operator, found `{`
  --> $DIR/missing-if-let-or-let-else.rs:14:25
   |
LL |     let Some(x) = foo() {
   |                         ^ expected one of `.`, `;`, `?`, `else`, or an operator
   |
help: you might have meant to use `if let`
   |
LL |     if let Some(x) = foo() {
   |     ++
help: alternatively, you might have meant to use `let else`
   |
LL |     let Some(x) = foo() else {
   |                         ++++

Fix #107806.

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[chenyukang]

I'd like to have a review on it
r? @chenyukang

[estebank]

If we wanted to improve the handling of these, then we would have to actually add a new error recovery AST node, with all the information that an if-let would have and perform validation at the resolve or HIR stage. The purely parser approach I think is likely at the right trade-off between complexity and results.

[chenyukang]

if we suggest add else here, a following error missing ; will comes after applying the fix.

[chenyukang]

so the correct code maybe:

 let Some(x) = foo() else { 
        return;
 };  // need `;` here

i'm not sure whether we should also suggest the ; at the same time, maybe goes too far.

[chenyukang]

emm, this code is also valid 😀

fn b() {
    if let Some(x) = foo() {
        return;
    };
}

[chenyukang]

nit: maybe we should move the definition of IdentFinder to here, since it's only used in this function. Otherwise IdentFinder is a too general visitor name.