Don't lint irrefutable_let_patterns on leading patterns if else if let-chains.

Author: Jarcho

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -79,6 +79,8 @@ enum LetSource {
     IfLetGuard,
     LetElse,
     WhileLet,
+    Else,
+    ElseIfLet,
 }
 
 struct MatchVisitor<'p, 'tcx> {
@@ -129,15 +131,20 @@ impl<'p, 'tcx> Visitor<'p, 'tcx> for MatchVisitor<'p, 'tcx> {
                 // Give a specific `let_source` for the condition.
                 let let_source = match ex.span.desugaring_kind() {
                     Some(DesugaringKind::WhileLoop) => LetSource::WhileLet,
-                    _ => LetSource::IfLet,
+                    _ => match self.let_source {
+                        LetSource::Else => LetSource::ElseIfLet,
+                        _ => LetSource::IfLet,
+                    },
                 };
                 self.with_let_source(let_source, |this| this.visit_expr(&self.thir[cond]));
                 self.with_let_source(LetSource::None, |this| {
                     this.visit_expr(&this.thir[then]);
-                    if let Some(else_) = else_opt {
-                        this.visit_expr(&this.thir[else_]);
-                    }
                 });
+                if let Some(else_) = else_opt {
+                    self.with_let_source(LetSource::Else, |this| {
+                        this.visit_expr(&this.thir[else_])
+                    });
+                }
                 return;
             }
             ExprKind::Match { scrutinee, scrutinee_hir_id: _, box ref arms, match_source } => {
@@ -569,9 +576,13 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
             // and we shouldn't lint.
             // For let guards inside a match, prefixes might use bindings of the match pattern,
             // so can't always be moved out.
+            // For `else if let`, an extra indentation level would be required to move the bindings.
             // FIXME: Add checking whether the bindings are actually used in the prefix,
             // and lint if they are not.
-            if !matches!(self.let_source, LetSource::WhileLet | LetSource::IfLetGuard) {
+            if !matches!(
+                self.let_source,
+                LetSource::WhileLet | LetSource::IfLetGuard | LetSource::ElseIfLet
+            ) {
                 // Emit the lint
                 let prefix = &chain_refutabilities[..until];
                 let span_start = prefix[0].unwrap().0;
@@ -902,8 +913,8 @@ fn report_irrefutable_let_patterns(
     }
 
     match source {
-        LetSource::None | LetSource::PlainLet => bug!(),
-        LetSource::IfLet => emit_diag!(IrrefutableLetPatternsIfLet),
+        LetSource::None | LetSource::PlainLet | LetSource::Else => bug!(),
+        LetSource::IfLet | LetSource::ElseIfLet => emit_diag!(IrrefutableLetPatternsIfLet),
         LetSource::IfLetGuard => emit_diag!(IrrefutableLetPatternsIfLetGuard),
         LetSource::LetElse => emit_diag!(IrrefutableLetPatternsLetElse),
         LetSource::WhileLet => emit_diag!(IrrefutableLetPatternsWhileLet),

tests/ui/rfcs/rfc-2497-if-let-chains/irrefutable-lets.disallowed.stderr

@@ -111,5 +111,14 @@ LL |     while let Some(ref first) = opt && let second = first && let _third = s
    = note: these patterns will always match
    = help: consider moving them into the body
 
-error: aborting due to 12 previous errors
+error: trailing irrefutable pattern in let chain
+  --> $DIR/irrefutable-lets.rs:87:12
+   |
+LL |         && let x = &opt
+   |            ^^^^^^^^^^^^
+   |
+   = note: this pattern will always match
+   = help: consider moving it into the body
+
+error: aborting due to 13 previous errors
 

tests/ui/rfcs/rfc-2497-if-let-chains/irrefutable-lets.rs

@@ -75,4 +75,16 @@ fn main() {
         && let Range { start: local_start, end: _ } = first
         && let None = local_start {
     }
+
+    // No error. An extra nesting level would be required for the `else if`.
+    if opt == Some(None..None) {
+    } else if let x = opt.clone().map(|_| 1)
+        && x == Some(1)
+    {}
+
+    if opt == Some(None..None) {
+    } else if opt.is_some()
+        && let x = &opt
+        //[disallowed]~^ ERROR trailing irrefutable pattern in let chain
+    {}
 }