Auto merge of #15003 - WaffleLapkin:become_unuwuable, r=Veykril

feature: Add basic support for `become` expr/tail calls

This follows https://github.com/rust-lang/rfcs/pull/3407 and my WIP implementation in the compiler.

Notice that I haven't even *opened* a compiler PR (although I plan to soon), so this feature doesn't really exist outside of my WIP branches. I've used this to help me test my implementation; opening a PR before I forget.

(feel free to ignore this for now, given all of the above)

crates/hir-def/src/body/lower.rs

@@ -416,6 +416,11 @@ fn maybe_collect_expr(&mut self, expr: ast::Expr) -> Option<ExprId> {
                 let expr = e.expr().map(|e| self.collect_expr(e));
                 self.alloc_expr(Expr::Return { expr }, syntax_ptr)
             }
+            ast::Expr::BecomeExpr(e) => {
+                let expr =
+                    e.expr().map(|e| self.collect_expr(e)).unwrap_or_else(|| self.missing_expr());
+                self.alloc_expr(Expr::Become { expr }, syntax_ptr)
+            }
             ast::Expr::YieldExpr(e) => {
                 self.is_lowering_coroutine = true;
                 let expr = e.expr().map(|e| self.collect_expr(e));

crates/hir-def/src/body/pretty.rs

@@ -261,6 +261,11 @@ fn print_expr(&mut self, expr: ExprId) {
                     self.print_expr(*expr);
                 }
             }
+            Expr::Become { expr } => {
+                w!(self, "become");
+                self.whitespace();
+                self.print_expr(*expr);
+            }
             Expr::Yield { expr } => {
                 w!(self, "yield");
                 if let Some(expr) = expr {

crates/hir-def/src/hir.rs

@@ -216,6 +216,9 @@ pub enum Expr {
     Return {
         expr: Option<ExprId>,
     },
+    Become {
+        expr: ExprId,
+    },
     Yield {
         expr: Option<ExprId>,
     },
@@ -410,6 +413,7 @@ pub fn walk_child_exprs(&self, mut f: impl FnMut(ExprId)) {
                     f(expr);
                 }
             }
+            Expr::Become { expr } => f(*expr),
             Expr::RecordLit { fields, spread, .. } => {
                 for field in fields.iter() {
                     f(field.expr);

crates/hir-ty/src/infer/closure.rs

@@ -531,6 +531,9 @@ fn walk_expr_without_adjust(&mut self, tgt_expr: ExprId) {
                     self.consume_expr(expr);
                 }
             }
+            &Expr::Become { expr } => {
+                self.consume_expr(expr);
+            }
             Expr::RecordLit { fields, spread, .. } => {
                 if let &Some(expr) = spread {
                     self.consume_expr(expr);

crates/hir-ty/src/infer/expr.rs

@@ -502,6 +502,7 @@ fn infer_expr_inner(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
                 self.result.standard_types.never.clone()
             }
             &Expr::Return { expr } => self.infer_expr_return(tgt_expr, expr),
+            &Expr::Become { expr } => self.infer_expr_become(expr),
             Expr::Yield { expr } => {
                 if let Some((resume_ty, yield_ty)) = self.resume_yield_tys.clone() {
                     if let Some(expr) = expr {
@@ -1084,6 +1085,27 @@ fn infer_expr_return(&mut self, ret: ExprId, expr: Option<ExprId>) -> Ty {
         self.result.standard_types.never.clone()
     }
 
+    fn infer_expr_become(&mut self, expr: ExprId) -> Ty {
+        match &self.return_coercion {
+            Some(return_coercion) => {
+                let ret_ty = return_coercion.expected_ty();
+
+                let call_expr_ty =
+                    self.infer_expr_inner(expr, &Expectation::HasType(ret_ty.clone()));
+
+                // NB: this should *not* coerce.
+                //     tail calls don't support any coercions except lifetimes ones (like `&'static u8 -> &'a u8`).
+                self.unify(&call_expr_ty, &ret_ty);
+            }
+            None => {
+                // FIXME: diagnose `become` outside of functions
+                self.infer_expr_no_expect(expr);
+            }
+        }
+
+        self.result.standard_types.never.clone()
+    }
+
     fn infer_expr_box(&mut self, inner_expr: ExprId, expected: &Expectation) -> Ty {
         if let Some(box_id) = self.resolve_boxed_box() {
             let table = &mut self.table;

crates/hir-ty/src/infer/mutability.rs

@@ -93,6 +93,9 @@ fn infer_mut_expr_without_adjust(&mut self, tgt_expr: ExprId, mutability: Mutabi
                     self.infer_mut_expr(expr, Mutability::Not);
                 }
             }
+            Expr::Become { expr } => {
+                self.infer_mut_expr(*expr, Mutability::Not);
+            }
             Expr::RecordLit { path: _, fields, spread, ellipsis: _, is_assignee_expr: _ } => {
                 self.infer_mut_not_expr_iter(fields.iter().map(|it| it.expr).chain(*spread))
             }

crates/hir-ty/src/mir/lower.rs

@@ -775,6 +775,7 @@ fn lower_expr_to_place_without_adjust(
                 self.set_terminator(current, TerminatorKind::Return, expr_id.into());
                 Ok(None)
             }
+            Expr::Become { .. } => not_supported!("tail-calls"),
             Expr::Yield { .. } => not_supported!("yield"),
             Expr::RecordLit { fields, path, spread, ellipsis: _, is_assignee_expr: _ } => {
                 let spread_place = match spread {

crates/ide-db/src/syntax_helpers/node_ext.rs

@@ -329,6 +329,7 @@ pub fn for_each_tail_expr(expr: &ast::Expr, cb: &mut dyn FnMut(&ast::Expr)) {
         | ast::Expr::RecordExpr(_)
         | ast::Expr::RefExpr(_)
         | ast::Expr::ReturnExpr(_)
+        | ast::Expr::BecomeExpr(_)
         | ast::Expr::TryExpr(_)
         | ast::Expr::TupleExpr(_)
         | ast::Expr::LetExpr(_)

crates/parser/src/grammar/expressions/atom.rs

@@ -58,6 +58,7 @@ pub(crate) fn literal(p: &mut Parser<'_>) -> Option<CompletedMarker> {
         T![match],
         T![move],
         T![return],
+        T![become],
         T![static],
         T![try],
         T![unsafe],
@@ -102,6 +103,7 @@ pub(super) fn atom_expr(
         T![try] => try_block_expr(p, None),
         T![match] => match_expr(p),
         T![return] => return_expr(p),
+        T![become] => become_expr(p),
         T![yield] => yield_expr(p),
         T![do] if p.nth_at_contextual_kw(1, T![yeet]) => yeet_expr(p),
         T![continue] => continue_expr(p),
@@ -621,6 +623,18 @@ fn return_expr(p: &mut Parser<'_>) -> CompletedMarker {
     m.complete(p, RETURN_EXPR)
 }
 
+// test become_expr
+// fn foo() {
+//     become foo();
+// }
+fn become_expr(p: &mut Parser<'_>) -> CompletedMarker {
+    assert!(p.at(T![become]));
+    let m = p.start();
+    p.bump(T![become]);
+    expr(p);
+    m.complete(p, BECOME_EXPR)
+}
+
 // test yield_expr
 // fn foo() {
 //     yield;

crates/parser/test_data/parser/inline/ok/0209_become_expr.rast

@@ -0,0 +1,31 @@
+SOURCE_FILE
+  FN
+    FN_KW "fn"
+    WHITESPACE " "
+    NAME
+      IDENT "foo"
+    PARAM_LIST
+      L_PAREN "("
+      R_PAREN ")"
+    WHITESPACE " "
+    BLOCK_EXPR
+      STMT_LIST
+        L_CURLY "{"
+        WHITESPACE "\n    "
+        EXPR_STMT
+          BECOME_EXPR
+            BECOME_KW "become"
+            WHITESPACE " "
+            CALL_EXPR
+              PATH_EXPR
+                PATH
+                  PATH_SEGMENT
+                    NAME_REF
+                      IDENT "foo"
+              ARG_LIST
+                L_PAREN "("
+                R_PAREN ")"
+          SEMICOLON ";"
+        WHITESPACE "\n"
+        R_CURLY "}"
+  WHITESPACE "\n"

crates/parser/test_data/parser/inline/ok/0209_become_expr.rs

@@ -0,0 +1,3 @@
+fn foo() {
+    become foo();
+}

crates/syntax/rust.ungram

@@ -367,6 +367,7 @@ Expr =
 | RecordExpr
 | RefExpr
 | ReturnExpr
+| BecomeExpr
 | TryExpr
 | TupleExpr
 | WhileExpr
@@ -528,6 +529,9 @@ MatchGuard =
 ReturnExpr =
   Attr* 'return' Expr?
 
+BecomeExpr =
+  Attr* 'become' Expr
+
 YieldExpr =
   Attr* 'yield' Expr?
 

crates/syntax/src/ast/generated/nodes.rs

@@ -1095,6 +1095,16 @@ pub fn return_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax,
     pub fn expr(&self) -> Option<Expr> { support::child(&self.syntax) }
 }
 
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct BecomeExpr {
+    pub(crate) syntax: SyntaxNode,
+}
+impl ast::HasAttrs for BecomeExpr {}
+impl BecomeExpr {
+    pub fn become_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![become]) }
+    pub fn expr(&self) -> Option<Expr> { support::child(&self.syntax) }
+}
+
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct TryExpr {
     pub(crate) syntax: SyntaxNode,
@@ -1633,6 +1643,7 @@ pub enum Expr {
     RecordExpr(RecordExpr),
     RefExpr(RefExpr),
     ReturnExpr(ReturnExpr),
+    BecomeExpr(BecomeExpr),
     TryExpr(TryExpr),
     TupleExpr(TupleExpr),
     WhileExpr(WhileExpr),
@@ -2792,6 +2803,17 @@ fn cast(syntax: SyntaxNode) -> Option<Self> {
     }
     fn syntax(&self) -> &SyntaxNode { &self.syntax }
 }
+impl AstNode for BecomeExpr {
+    fn can_cast(kind: SyntaxKind) -> bool { kind == BECOME_EXPR }
+    fn cast(syntax: SyntaxNode) -> Option<Self> {
+        if Self::can_cast(syntax.kind()) {
+            Some(Self { syntax })
+        } else {
+            None
+        }
+    }
+    fn syntax(&self) -> &SyntaxNode { &self.syntax }
+}
 impl AstNode for TryExpr {
     fn can_cast(kind: SyntaxKind) -> bool { kind == TRY_EXPR }
     fn cast(syntax: SyntaxNode) -> Option<Self> {
@@ -3540,6 +3562,9 @@ fn from(node: RefExpr) -> Expr { Expr::RefExpr(node) }
 impl From<ReturnExpr> for Expr {
     fn from(node: ReturnExpr) -> Expr { Expr::ReturnExpr(node) }
 }
+impl From<BecomeExpr> for Expr {
+    fn from(node: BecomeExpr) -> Expr { Expr::BecomeExpr(node) }
+}
 impl From<TryExpr> for Expr {
     fn from(node: TryExpr) -> Expr { Expr::TryExpr(node) }
 }
@@ -3593,6 +3618,7 @@ fn can_cast(kind: SyntaxKind) -> bool {
                 | RECORD_EXPR
                 | REF_EXPR
                 | RETURN_EXPR
+                | BECOME_EXPR
                 | TRY_EXPR
                 | TUPLE_EXPR
                 | WHILE_EXPR
@@ -3632,6 +3658,7 @@ fn cast(syntax: SyntaxNode) -> Option<Self> {
             RECORD_EXPR => Expr::RecordExpr(RecordExpr { syntax }),
             REF_EXPR => Expr::RefExpr(RefExpr { syntax }),
             RETURN_EXPR => Expr::ReturnExpr(ReturnExpr { syntax }),
+            BECOME_EXPR => Expr::BecomeExpr(BecomeExpr { syntax }),
             TRY_EXPR => Expr::TryExpr(TryExpr { syntax }),
             TUPLE_EXPR => Expr::TupleExpr(TupleExpr { syntax }),
             WHILE_EXPR => Expr::WhileExpr(WhileExpr { syntax }),
@@ -3673,6 +3700,7 @@ fn syntax(&self) -> &SyntaxNode {
             Expr::RecordExpr(it) => &it.syntax,
             Expr::RefExpr(it) => &it.syntax,
             Expr::ReturnExpr(it) => &it.syntax,
+            Expr::BecomeExpr(it) => &it.syntax,
             Expr::TryExpr(it) => &it.syntax,
             Expr::TupleExpr(it) => &it.syntax,
             Expr::WhileExpr(it) => &it.syntax,
@@ -4150,6 +4178,7 @@ fn can_cast(kind: SyntaxKind) -> bool {
                 | RANGE_EXPR
                 | REF_EXPR
                 | RETURN_EXPR
+                | BECOME_EXPR
                 | TRY_EXPR
                 | TUPLE_EXPR
                 | WHILE_EXPR
@@ -4851,6 +4880,11 @@ fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         std::fmt::Display::fmt(self.syntax(), f)
     }
 }
+impl std::fmt::Display for BecomeExpr {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        std::fmt::Display::fmt(self.syntax(), f)
+    }
+}
 impl std::fmt::Display for TryExpr {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         std::fmt::Display::fmt(self.syntax(), f)

crates/syntax/src/ast/prec.rs

@@ -130,8 +130,8 @@ fn binding_power(&self) -> (u8, u8) {
             //
             ContinueExpr(_) => (0, 0),
 
-            ClosureExpr(_) | ReturnExpr(_) | YieldExpr(_) | YeetExpr(_) | BreakExpr(_)
+            ClosureExpr(_) | ReturnExpr(_) | BecomeExpr(_) | YieldExpr(_) | YeetExpr(_)
-            | OffsetOfExpr(_) | FormatArgsExpr(_) | AsmExpr(_) => (0, 1),
+            | BreakExpr(_) | OffsetOfExpr(_) | FormatArgsExpr(_) | AsmExpr(_) => (0, 1),
 
             RangeExpr(_) => (5, 5),
 
@@ -288,6 +288,7 @@ fn order(this: &Expr) -> rowan::TextSize {
                 PrefixExpr(e) => e.op_token(),
                 RefExpr(e) => e.amp_token(),
                 ReturnExpr(e) => e.return_token(),
+                BecomeExpr(e) => e.become_token(),
                 TryExpr(e) => e.question_mark_token(),
                 YieldExpr(e) => e.yield_token(),
                 YeetExpr(e) => e.do_token(),
@@ -316,7 +317,8 @@ fn child_is_followed_by_a_block(&self) -> bool {
 
             // For BinExpr and RangeExpr this is technically wrong -- the child can be on the left...
             BinExpr(_) | RangeExpr(_) | BreakExpr(_) | ContinueExpr(_) | PrefixExpr(_)
-            | RefExpr(_) | ReturnExpr(_) | YieldExpr(_) | YeetExpr(_) | LetExpr(_) => self
+            | RefExpr(_) | ReturnExpr(_) | BecomeExpr(_) | YieldExpr(_) | YeetExpr(_)
+            | LetExpr(_) => self
                 .syntax()
                 .parent()
                 .and_then(Expr::cast)

crates/syntax/src/tests/ast_src.rs

@@ -67,8 +67,9 @@ pub(crate) struct KindsSrc<'a> {
     keywords: &[
         "as", "async", "await", "box", "break", "const", "continue", "crate", "do", "dyn", "else",
         "enum", "extern", "false", "fn", "for", "if", "impl", "in", "let", "loop", "macro",
-        "match", "mod", "move", "mut", "pub", "ref", "return", "self", "Self", "static", "struct",
+        "match", "mod", "move", "mut", "pub", "ref", "return", "become", "self", "Self", "static",
-        "super", "trait", "true", "try", "type", "unsafe", "use", "where", "while", "yield",
+        "struct", "super", "trait", "true", "try", "type", "unsafe", "use", "where", "while",
+        "yield",
     ],
     contextual_keywords: &[
         "auto",
@@ -154,6 +155,7 @@ pub(crate) struct KindsSrc<'a> {
         "BLOCK_EXPR",
         "STMT_LIST",
         "RETURN_EXPR",
+        "BECOME_EXPR",
         "YIELD_EXPR",
         "YEET_EXPR",
         "LET_EXPR",