fix: Don't add braces to 'if' completion in match guard position

When the cursor is in a match arm, but before the fat arrow (=>) token, don't
add braces when autocompleting "if".

fixes rust-lang#12823

Author: DorianListens

crates/ide-completion/src/completions/expr.rs

@@ -21,6 +21,7 @@ pub(crate) fn complete_expr_path(
         ref is_func_update,
         ref innermost_ret_ty,
         ref impl_,
+        in_match_guard,
         ..
     }: &ExprCtx,
 ) {
@@ -195,7 +196,11 @@ pub(crate) fn complete_expr_path(
                 add_keyword("while", "while $1 {\n    $0\n}");
                 add_keyword("while let", "while let $1 = $2 {\n    $0\n}");
                 add_keyword("loop", "loop {\n    $0\n}");
-                add_keyword("if", "if $1 {\n    $0\n}");
+                if in_match_guard {
+                    add_keyword("if", "if $0");
+                } else {
+                    add_keyword("if", "if $1 {\n    $0\n}");
+                }
                 add_keyword("if let", "if let $1 = $2 {\n    $0\n}");
                 add_keyword("for", "for $1 in $2 {\n    $0\n}");
                 add_keyword("true", "true");

crates/ide-completion/src/completions/keyword.rs

@@ -163,4 +163,75 @@ fn main() {
 "#,
         );
     }
+
+    #[test]
+    fn if_completion_in_match_guard() {
+        check_edit(
+            "if",
+            r"
+fn main() {
+    match () {
+        () $0
+    }
+}
+",
+            r"
+fn main() {
+    match () {
+        () if $0
+    }
+}
+",
+        )
+    }
+
+    #[test]
+    fn if_completion_in_match_arm_expr() {
+        check_edit(
+            "if",
+            r"
+fn main() {
+    match () {
+        () => $0
+    }
+}
+",
+            r"
+fn main() {
+    match () {
+        () => if $1 {
+    $0
+}
+    }
+}
+",
+        )
+    }
+
+    #[test]
+    fn if_completion_in_match_arm_expr_block() {
+        check_edit(
+            "if",
+            r"
+fn main() {
+    match () {
+        () => {
+            $0
+        }
+    }
+}
+",
+            r"
+fn main() {
+    match () {
+        () => {
+            if $1 {
+    $0
+}
+        }
+    }
+}
+",
+        )
+    }
 }

crates/ide-completion/src/context.rs

@@ -138,6 +138,9 @@ pub(crate) struct ExprCtx {
     pub(crate) self_param: Option<hir::SelfParam>,
     pub(crate) innermost_ret_ty: Option<hir::Type>,
     pub(crate) impl_: Option<ast::Impl>,
+    /// Whether this expression occurs in match arm guard position: before the
+    /// fat arrow token
+    pub(crate) in_match_guard: bool,
 }
 
 /// Original file ast nodes

crates/ide-completion/src/context/analysis.rs

@@ -763,6 +763,13 @@ impl<'a> CompletionContext<'a> {
                 .map_or(false, |it| it.semicolon_token().is_none());
             let impl_ = fetch_immediate_impl(sema, original_file, expr.syntax());
 
+            let in_match_guard = match it.parent().and_then(ast::MatchArm::cast) {
+                Some(arm) => arm
+                    .fat_arrow_token()
+                    .map_or(true, |arrow| it.text_range().start() < arrow.text_range().start()),
+                None => false,
+            };
+
             PathKind::Expr {
                 expr_ctx: ExprCtx {
                     in_block_expr,
@@ -775,6 +782,7 @@ impl<'a> CompletionContext<'a> {
                     self_param,
                     incomplete_let,
                     impl_,
+                    in_match_guard,
                 },
             }
         };