Move increment checks to improve errors

5 files changed, 286 insertions, 79 deletions

diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index 0361f7063fc..99844bef1ff 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -156,6 +156,52 @@ impl AttemptLocalParseRecovery {
     }
 }
 
+#[derive(Debug, Copy, Clone)]
+struct IncDecRecovery {
+    standalone: bool,
+    op: IncOrDec,
+    fixity: UnaryFixity,
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+enum IncOrDec {
+    Inc,
+    // FIXME: `i--` recovery isn't implemented yet
+    #[allow(dead_code)]
+    Dec,
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+enum UnaryFixity {
+    Pre,
+    Post,
+}
+
+impl IncOrDec {
+    fn chr(&self) -> char {
+        match self {
+            Self::Inc => '+',
+            Self::Dec => '-',
+        }
+    }
+
+    fn name(&self) -> &'static str {
+        match self {
+            Self::Inc => "increment",
+            Self::Dec => "decrement",
+        }
+    }
+}
+
+impl std::fmt::Display for UnaryFixity {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Pre => write!(f, "prefix"),
+            Self::Post => write!(f, "postfix"),
+        }
+    }
+}
+
 // SnapshotParser is used to create a snapshot of the parser
 // without causing duplicate errors being emitted when the `Parser`
 // is dropped.
@@ -1167,6 +1213,145 @@ impl<'a> Parser<'a> {
         Ok(())
     }
 
+    pub(super) fn maybe_recover_from_prefix_increment(
+        &mut self,
+        operand_expr: P<Expr>,
+        op_span: Span,
+        prev_is_semi: bool,
+    ) -> PResult<'a, P<Expr>> {
+        let kind = IncDecRecovery {
+            standalone: prev_is_semi,
+            op: IncOrDec::Inc,
+            fixity: UnaryFixity::Pre,
+        };
+
+        self.recover_from_inc_dec(operand_expr, kind, op_span)
+    }
+
+    pub(super) fn maybe_recover_from_postfix_increment(
+        &mut self,
+        operand_expr: P<Expr>,
+        op_span: Span,
+        prev_is_semi: bool,
+    ) -> PResult<'a, P<Expr>> {
+        let kind = IncDecRecovery {
+            standalone: prev_is_semi,
+            op: IncOrDec::Inc,
+            fixity: UnaryFixity::Post,
+        };
+
+        self.recover_from_inc_dec(operand_expr, kind, op_span)
+    }
+
+    fn recover_from_inc_dec(
+        &mut self,
+        base: P<Expr>,
+        kind: IncDecRecovery,
+        op_span: Span,
+    ) -> PResult<'a, P<Expr>> {
+        let mut err = self.struct_span_err(
+            op_span,
+            &format!("Rust has no {} {} operator", kind.fixity, kind.op.name()),
+        );
+        err.span_label(op_span, &format!("not a valid {} operator", kind.fixity));
+
+        if let ExprKind::Path(_, path) = &base.kind {
+            if let [segment] = path.segments.as_slice() {
+                let ident = segment.ident;
+                // (pre, post)
+                let spans = match kind.fixity {
+                    UnaryFixity::Pre => (op_span, ident.span.shrink_to_hi()),
+                    UnaryFixity::Post => (ident.span.shrink_to_lo(), op_span),
+                };
+
+                if !ident.is_reserved() {
+                    if kind.standalone {
+                        return self.inc_dec_standalone_recovery(base, err, kind, ident, spans);
+                    } else {
+                        match kind.fixity {
+                            UnaryFixity::Pre => {
+                                return self.prefix_inc_dec_suggest_and_recover(
+                                    base, err, kind, ident, spans,
+                                );
+                            }
+                            UnaryFixity::Post => {
+                                return self.postfix_inc_dec_suggest_and_recover(
+                                    base, err, kind, ident, spans,
+                                );
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        // base case
+        err.help(&format!("use `{}= 1` instead", kind.op.chr()));
+        err.emit();
+
+        Ok(base)
+    }
+
+    fn prefix_inc_dec_suggest_and_recover(
+        &mut self,
+        base: P<Expr>,
+        mut err: DiagnosticBuilder<'_>,
+        kind: IncDecRecovery,
+        ident: Ident,
+        (pre_span, post_span): (Span, Span),
+    ) -> PResult<'a, P<Expr>> {
+        err.multipart_suggestion(
+            &format!("use `{}= 1` instead", kind.op.chr()),
+            vec![
+                (pre_span, "{ ".to_string()),
+                (post_span, format!(" {}= 1; {} }}", kind.op.chr(), ident)),
+            ],
+            Applicability::MachineApplicable,
+        );
+        err.emit();
+        // TODO: recover
+        Ok(base)
+    }
+
+    fn postfix_inc_dec_suggest_and_recover(
+        &mut self,
+        base: P<Expr>,
+        mut err: DiagnosticBuilder<'_>,
+        kind: IncDecRecovery,
+        ident: Ident,
+        (pre_span, post_span): (Span, Span),
+    ) -> PResult<'a, P<Expr>> {
+        err.multipart_suggestion(
+            &format!("use `{}= 1` instead", kind.op.chr()),
+            vec![
+                (pre_span, "{ let tmp = ".to_string()),
+                (post_span, format!("; {} {}= 1; tmp }}", ident, kind.op.chr())),
+            ],
+            Applicability::MachineApplicable,
+        );
+        err.emit();
+        // TODO: recover
+        Ok(base)
+    }
+
+    fn inc_dec_standalone_recovery(
+        &mut self,
+        base: P<Expr>,
+        mut err: DiagnosticBuilder<'_>,
+        kind: IncDecRecovery,
+        _ident: Ident,
+        (pre_span, post_span): (Span, Span),
+    ) -> PResult<'a, P<Expr>> {
+        err.multipart_suggestion(
+            &format!("use `{}= 1` instead", kind.op.chr()),
+            vec![(pre_span, String::new()), (post_span, format!(" {}= 1", kind.op.chr()))],
+            Applicability::MachineApplicable,
+        );
+        err.emit();
+        // TODO: recover
+        Ok(base)
+    }
+
     /// Tries to recover from associated item paths like `[T]::AssocItem` / `(T, U)::AssocItem`.
     /// Attempts to convert the base expression/pattern/type into a type, parses the `::AssocItem`
     /// tail, and combines them into a `<Ty>::AssocItem` expression/pattern/type.
@@ -1882,49 +2067,6 @@ impl<'a> Parser<'a> {
             self.sess.expr_parentheses_needed(&mut err, *sp);
         }
         err.span_label(span, "expected expression");
-        if self.prev_token.kind == TokenKind::BinOp(token::Plus)
-            && self.token.kind == TokenKind::BinOp(token::Plus)
-            && self.look_ahead(1, |t| !t.is_lit())
-        {
-            let span = self.prev_token.span.to(self.token.span);
-            err.note("Rust has no dedicated increment operator");
-            err.span_suggestion_verbose(
-                span,
-                "try using `+= 1` instead",
-                " += 1".into(),
-                Applicability::Unspecified,
-            );
-        } else if self.token.kind == TokenKind::BinOp(token::Plus)
-            && self.look_ahead(1, |t| t.kind == TokenKind::BinOp(token::Plus))
-            && self.look_ahead(2, |t| !t.is_lit())
-        {
-            let target_span = self.look_ahead(2, |t| t.span);
-            let left_brace_span = target_span.shrink_to_lo();
-            let pre_span = self.token.span.to(self.look_ahead(1, |t| t.span));
-            let post_span = target_span.shrink_to_hi();
-
-            err.note("Rust has no dedicated increment operator");
-
-            if self.prev_token.kind == TokenKind::Semi {
-                err.multipart_suggestion(
-                    "try using `+= 1` instead",
-                    vec![(pre_span, String::new()), (post_span, " += 1".into())],
-                    Applicability::MachineApplicable,
-                );
-            } else if let Ok(target_snippet) = self.span_to_snippet(target_span) {
-                err.multipart_suggestion(
-                    "try using `+= 1` instead",
-                    vec![
-                        (left_brace_span, "{ ".to_string()),
-                        (pre_span, String::new()),
-                        (post_span, format!(" += 1; {} }}", target_snippet)),
-                    ],
-                    Applicability::MachineApplicable,
-                );
-            } else {
-                err.span_help(pre_span.to(target_span), "try using `+= 1` instead");
-            }
-        }
         err
     }
 
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 148e0a24ec3..39d96b8a9e3 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -267,6 +267,18 @@ impl<'a> Parser<'a> {
                 self.bump();
             }
 
+            if self.prev_token == token::BinOp(token::Plus)
+                && self.token == token::BinOp(token::Plus)
+            {
+                let op_span = self.prev_token.span.to(self.token.span);
+                // Eat the second `+`
+                self.bump();
+                // TODO: implement
+                let start_is_semi = false;
+                lhs = self.maybe_recover_from_postfix_increment(lhs, op_span, start_is_semi)?;
+                continue;
+            }
+
             let op = op.node;
             // Special cases:
             if op == AssocOp::As {
@@ -586,6 +598,19 @@ impl<'a> Parser<'a> {
             token::Ident(..) if this.is_mistaken_not_ident_negation() => {
                 make_it!(this, attrs, |this, _| this.recover_not_expr(lo))
             }
+            // Recover from `++x`
+            token::BinOp(token::Plus)
+                if this.look_ahead(1, |t| *t == token::BinOp(token::Plus)) =>
+            {
+                let prev_is_semi = this.prev_token == token::Semi;
+                let pre_span = this.token.span.to(this.look_ahead(1, |t| t.span));
+                // Eat both `+`s.
+                this.bump();
+                this.bump();
+
+                let operand_expr = this.parse_path_start_expr(Default::default())?;
+                this.maybe_recover_from_prefix_increment(operand_expr, pre_span, prev_is_semi)
+            }
             _ => return this.parse_dot_or_call_expr(Some(attrs)),
         }
     }
diff --git a/src/test/ui/parser/increment.fixed b/src/test/ui/parser/increment.fixed
new file mode 100644
index 00000000000..ad61c4e66d2
--- /dev/null
+++ b/src/test/ui/parser/increment.fixed
@@ -0,0 +1,36 @@
+// run-rustfix
+
+fn post_regular() {
+    let mut i = 0;
+    { let tmp = i; i += 1; tmp }; //~ ERROR Rust has no postfix increment operator
+    println!("{}", i);
+}
+
+fn post_while() {
+    let mut i = 0;
+    while { let tmp = i; i += 1; tmp } < 5 {
+        //~^ ERROR Rust has no postfix increment operator
+        println!("{}", i);
+    }
+}
+
+fn pre_regular() {
+    let mut i = 0;
+    i += 1; //~ ERROR Rust has no prefix increment operator
+    println!("{}", i);
+}
+
+fn pre_while() {
+    let mut i = 0;
+    while { i += 1; i } < 5 {
+        //~^ ERROR Rust has no prefix increment operator
+        println!("{}", i);
+    }
+}
+
+fn main() {
+    post_regular();
+    post_while();
+    pre_regular();
+    pre_while();
+}
diff --git a/src/test/ui/parser/increment.rs b/src/test/ui/parser/increment.rs
index 77a94b65f1f..f31031fed3a 100644
--- a/src/test/ui/parser/increment.rs
+++ b/src/test/ui/parser/increment.rs
@@ -1,27 +1,36 @@
+// run-rustfix
+
 fn post_regular() {
-    let i = 0;
-    i++; //~ ERROR
+    let mut i = 0;
+    i++; //~ ERROR Rust has no postfix increment operator
+    println!("{}", i);
 }
 
 fn post_while() {
-    let i = 0;
+    let mut i = 0;
     while i++ < 5 {
-        //~^ ERROR
+        //~^ ERROR Rust has no postfix increment operator
         println!("{}", i);
     }
 }
 
 fn pre_regular() {
-    let i = 0;
-    ++i; //~ ERROR
+    let mut i = 0;
+    ++i; //~ ERROR Rust has no prefix increment operator
+    println!("{}", i);
 }
 
 fn pre_while() {
-    let i = 0;
+    let mut i = 0;
     while ++i < 5 {
-        //~^ ERROR
+        //~^ ERROR Rust has no prefix increment operator
         println!("{}", i);
     }
 }
 
-fn main() {}
+fn main() {
+    post_regular();
+    post_while();
+    pre_regular();
+    pre_while();
+}
diff --git a/src/test/ui/parser/increment.stderr b/src/test/ui/parser/increment.stderr
index 667956cdd7d..6a2b37e3ddc 100644
--- a/src/test/ui/parser/increment.stderr
+++ b/src/test/ui/parser/increment.stderr
@@ -1,52 +1,47 @@
-error: expected expression, found `+`
-  --> $DIR/increment.rs:3:7
+error: Rust has no postfix increment operator
+  --> $DIR/increment.rs:5:6
    |
 LL |     i++;
-   |       ^ expected expression
+   |      ^^ not a valid postfix operator
    |
-   = note: Rust has no dedicated increment operator
-help: try using `+= 1` instead
+help: use `+= 1` instead
    |
-LL |     i += 1;
-   |       ~~~~
+LL |     { let tmp = i; i += 1; tmp };
+   |     +++++++++++  ~~~~~~~~~~~~~~~
 
-error: expected expression, found `+`
-  --> $DIR/increment.rs:8:13
+error: Rust has no postfix increment operator
+  --> $DIR/increment.rs:11:12
    |
 LL |     while i++ < 5 {
-   |             ^ expected expression
+   |            ^^ not a valid postfix operator
    |
-   = note: Rust has no dedicated increment operator
-help: try using `+= 1` instead
+help: use `+= 1` instead
    |
-LL |     while i += 1 < 5 {
-   |             ~~~~
+LL |     while { let tmp = i; i += 1; tmp } < 5 {
+   |           +++++++++++  ~~~~~~~~~~~~~~~
 
-error: expected expression, found `+`
-  --> $DIR/increment.rs:16:5
+error: Rust has no prefix increment operator
+  --> $DIR/increment.rs:19:5
    |
 LL |     ++i;
-   |     ^ expected expression
+   |     ^^ not a valid prefix operator
    |
-   = note: Rust has no dedicated increment operator
-help: try using `+= 1` instead
+help: use `+= 1` instead
    |
 LL -     ++i;
 LL +     i += 1;
    | 
 
-error: expected expression, found `+`
-  --> $DIR/increment.rs:21:11
+error: Rust has no prefix increment operator
+  --> $DIR/increment.rs:25:11
    |
 LL |     while ++i < 5 {
-   |           ^ expected expression
+   |           ^^ not a valid prefix operator
    |
-   = note: Rust has no dedicated increment operator
-help: try using `+= 1` instead
+help: use `+= 1` instead
    |
-LL -     while ++i < 5 {
-LL +     while { i += 1; i } < 5 {
-   | 
+LL |     while { i += 1; i } < 5 {
+   |           ~   +++++++++
 
 error: aborting due to 4 previous errors