Auto merge of #107290 - Dylan-DPC:rollup-tovojhr, r=Dylan-DPC

Rollup of 9 pull requests

Successful merges:

 - #105552 (Add help message about function pointers)
 - #106583 (Suggest coercion of `Result` using `?`)
 - #106767 (Allow setting CFG_DISABLE_UNSTABLE_FEATURES to 0)
 - #106823 (Allow fmt::Arguments::as_str() to return more Some(_).)
 - #107166 (rustc_metadata: Support non-`Option` nullable values in metadata tables)
 - #107213 (Add suggestion to remove if in let..else block)
 - #107223 (`sub_ptr()` is equivalent to `usize::try_from().unwrap_unchecked()`, not `usize::from().unwrap_unchecked()`)
 - #107227 (`new_outside_solver` ->  `evaluate_root_goal`)
 - #107232 (rustdoc: simplify settings popover DOM, CSS, JS)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

2 files changed, 30 insertions, 14 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 06b970ad979..40763da0bb5 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -337,7 +337,9 @@ pub(crate) struct IfExpressionMissingThenBlock {
     #[primary_span]
     pub if_span: Span,
     #[subdiagnostic]
-    pub sub: IfExpressionMissingThenBlockSub,
+    pub missing_then_block_sub: IfExpressionMissingThenBlockSub,
+    #[subdiagnostic]
+    pub let_else_sub: Option<IfExpressionLetSomeSub>,
 }
 
 #[derive(Subdiagnostic)]
@@ -348,6 +350,13 @@ pub(crate) enum IfExpressionMissingThenBlockSub {
     AddThenBlock(#[primary_span] Span),
 }
 
+#[derive(Subdiagnostic)]
+#[help(parse_extra_if_in_let_else)]
+pub(crate) struct IfExpressionLetSomeSub {
+    #[primary_span]
+    pub if_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_if_expression_missing_condition)]
 pub(crate) struct IfExpressionMissingCondition {
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index bf93a89f065..3225a309a31 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -11,15 +11,15 @@ use crate::errors::{
     ComparisonOrShiftInterpretedAsGenericSugg, DoCatchSyntaxRemoved, DotDotDot, EqFieldInit,
     ExpectedElseBlock, ExpectedEqForLetExpr, ExpectedExpressionFoundLet,
     FieldExpressionWithGeneric, FloatLiteralRequiresIntegerPart, FoundExprWouldBeStmt,
-    IfExpressionMissingCondition, IfExpressionMissingThenBlock, IfExpressionMissingThenBlockSub,
-    InvalidBlockMacroSegment, InvalidComparisonOperator, InvalidComparisonOperatorSub,
-    InvalidInterpolatedExpression, InvalidLiteralSuffixOnTupleIndex, InvalidLogicalOperator,
-    InvalidLogicalOperatorSub, LabeledLoopInBreak, LeadingPlusNotSupported, LeftArrowOperator,
-    LifetimeInBorrowExpression, MacroInvocationWithQualifiedPath, MalformedLoopLabel,
-    MatchArmBodyWithoutBraces, MatchArmBodyWithoutBracesSugg, MissingCommaAfterMatchArm,
-    MissingDotDot, MissingInInForLoop, MissingInInForLoopSub, MissingSemicolonBeforeArray,
-    NoFieldsForFnCall, NotAsNegationOperator, NotAsNegationOperatorSub,
-    OuterAttributeNotAllowedOnIfElse, ParenthesesWithStructFields,
+    IfExpressionLetSomeSub, IfExpressionMissingCondition, IfExpressionMissingThenBlock,
+    IfExpressionMissingThenBlockSub, InvalidBlockMacroSegment, InvalidComparisonOperator,
+    InvalidComparisonOperatorSub, InvalidInterpolatedExpression, InvalidLiteralSuffixOnTupleIndex,
+    InvalidLogicalOperator, InvalidLogicalOperatorSub, LabeledLoopInBreak, LeadingPlusNotSupported,
+    LeftArrowOperator, LifetimeInBorrowExpression, MacroInvocationWithQualifiedPath,
+    MalformedLoopLabel, MatchArmBodyWithoutBraces, MatchArmBodyWithoutBracesSugg,
+    MissingCommaAfterMatchArm, MissingDotDot, MissingInInForLoop, MissingInInForLoopSub,
+    MissingSemicolonBeforeArray, NoFieldsForFnCall, NotAsNegationOperator,
+    NotAsNegationOperatorSub, OuterAttributeNotAllowedOnIfElse, ParenthesesWithStructFields,
     RequireColonAfterLabeledExpression, ShiftInterpretedAsGeneric, StructLiteralNotAllowedHere,
     StructLiteralNotAllowedHereSugg, TildeAsUnaryOperator, UnexpectedIfWithIf,
     UnexpectedTokenAfterLabel, UnexpectedTokenAfterLabelSugg, WrapExpressionInParentheses,
@@ -2251,9 +2251,10 @@ impl<'a> Parser<'a> {
                     if let ExprKind::Block(_, None) = right.kind => {
                         self.sess.emit_err(IfExpressionMissingThenBlock {
                             if_span: lo,
-                            sub: IfExpressionMissingThenBlockSub::UnfinishedCondition(
-                                cond_span.shrink_to_lo().to(*binop_span)
-                            ),
+                            missing_then_block_sub:
+                                IfExpressionMissingThenBlockSub::UnfinishedCondition(cond_span.shrink_to_lo().to(*binop_span)),
+                                let_else_sub: None,
+
                         });
                         std::mem::replace(right, this.mk_expr_err(binop_span.shrink_to_hi()))
                     },
@@ -2279,9 +2280,15 @@ impl<'a> Parser<'a> {
             if let Some(block) = recover_block_from_condition(self) {
                 block
             } else {
+                let let_else_sub = matches!(cond.kind, ExprKind::Let(..))
+                    .then(|| IfExpressionLetSomeSub { if_span: lo });
+
                 self.sess.emit_err(IfExpressionMissingThenBlock {
                     if_span: lo,
-                    sub: IfExpressionMissingThenBlockSub::AddThenBlock(cond_span.shrink_to_hi()),
+                    missing_then_block_sub: IfExpressionMissingThenBlockSub::AddThenBlock(
+                        cond_span.shrink_to_hi(),
+                    ),
+                    let_else_sub,
                 });
                 self.mk_block_err(cond_span.shrink_to_hi())
             }