forbid early returns in init

1 files changed, 246 insertions, 1 deletions

diff --git a/macros/src/check.rs b/macros/src/check.rs
index ae2262a8..0bc31c5f 100644
--- a/macros/src/check.rs
+++ b/macros/src/check.rs
@@ -1,7 +1,7 @@
 use std::{collections::HashSet, iter};
 
 use proc_macro2::Span;
-use syn::parse;
+use syn::{parse, spanned::Spanned, Block, Expr, Stmt};
 
 use crate::syntax::App;
 
@@ -112,5 +112,250 @@ pub fn app(app: &App) -> parse::Result<()> {
         }
     }
 
+    // Check that `init` contains no early returns *if* late resources exist
+    if app.resources.values().any(|res| res.expr.is_none()) {
+        for stmt in &app.init.stmts {
+            noreturn_stmt(stmt)?;
+        }
+    }
+
+    Ok(())
+}
+
+// checks that the given block contains no instance of `return`
+fn noreturn_block(block: &Block) -> Result<(), parse::Error> {
+    for stmt in &block.stmts {
+        noreturn_stmt(stmt)?;
+    }
+
+    Ok(())
+}
+
+// checks that the given statement contains no instance of `return`
+fn noreturn_stmt(stmt: &Stmt) -> Result<(), parse::Error> {
+    match stmt {
+        // `let x = ..` -- this may contain a return in the RHS
+        Stmt::Local(local) => {
+            if let Some(ref init) = local.init {
+                noreturn_expr(&init.1)?
+            }
+        }
+
+        // items have no effect on control flow
+        Stmt::Item(..) => {}
+
+        Stmt::Expr(expr) => noreturn_expr(expr)?,
+
+        Stmt::Semi(expr, ..) => noreturn_expr(expr)?,
+    }
+
+    Ok(())
+}
+
+// checks that the given expression contains no `return`
+fn noreturn_expr(expr: &Expr) -> Result<(), parse::Error> {
+    match expr {
+        Expr::Box(b) => noreturn_expr(&b.expr)?,
+
+        Expr::InPlace(ip) => {
+            noreturn_expr(&ip.place)?;
+            noreturn_expr(&ip.value)?;
+        }
+
+        Expr::Array(a) => {
+            for elem in &a.elems {
+                noreturn_expr(elem)?;
+            }
+        }
+
+        Expr::Call(c) => {
+            noreturn_expr(&c.func)?;
+
+            for arg in &c.args {
+                noreturn_expr(arg)?;
+            }
+        }
+
+        Expr::MethodCall(mc) => {
+            noreturn_expr(&mc.receiver)?;
+
+            for arg in &mc.args {
+                noreturn_expr(arg)?;
+            }
+        }
+
+        Expr::Tuple(t) => {
+            for elem in &t.elems {
+                noreturn_expr(elem)?;
+            }
+        }
+
+        Expr::Binary(b) => {
+            noreturn_expr(&b.left)?;
+            noreturn_expr(&b.right)?;
+        }
+
+        Expr::Unary(u) => {
+            noreturn_expr(&u.expr)?;
+        }
+
+        Expr::Lit(..) => {}
+
+        Expr::Cast(c) => {
+            noreturn_expr(&c.expr)?;
+        }
+
+        Expr::Type(t) => {
+            noreturn_expr(&t.expr)?;
+        }
+
+        Expr::Let(l) => {
+            noreturn_expr(&l.expr)?;
+        }
+
+        Expr::If(i) => {
+            noreturn_expr(&i.cond)?;
+
+            noreturn_block(&i.then_branch)?;
+
+            if let Some(ref e) = i.else_branch {
+                noreturn_expr(&e.1)?;
+            }
+        }
+
+        Expr::While(w) => {
+            noreturn_expr(&w.cond)?;
+            noreturn_block(&w.body)?;
+        }
+
+        Expr::ForLoop(fl) => {
+            noreturn_expr(&fl.expr)?;
+            noreturn_block(&fl.body)?;
+        }
+
+        Expr::Loop(l) => {
+            noreturn_block(&l.body)?;
+        }
+
+        Expr::Match(m) => {
+            noreturn_expr(&m.expr)?;
+
+            for arm in &m.arms {
+                if let Some(g) = &arm.guard {
+                    noreturn_expr(&g.1)?;
+                }
+
+                noreturn_expr(&arm.body)?;
+            }
+        }
+
+        // we don't care about `return`s inside closures
+        Expr::Closure(..) => {}
+
+        Expr::Unsafe(u) => {
+            noreturn_block(&u.block)?;
+        }
+
+        Expr::Block(b) => {
+            noreturn_block(&b.block)?;
+        }
+
+        Expr::Assign(a) => {
+            noreturn_expr(&a.left)?;
+            noreturn_expr(&a.right)?;
+        }
+
+        Expr::AssignOp(ao) => {
+            noreturn_expr(&ao.left)?;
+            noreturn_expr(&ao.right)?;
+        }
+
+        Expr::Field(f) => {
+            noreturn_expr(&f.base)?;
+        }
+
+        Expr::Index(i) => {
+            noreturn_expr(&i.expr)?;
+            noreturn_expr(&i.index)?;
+        }
+
+        Expr::Range(r) => {
+            if let Some(ref f) = r.from {
+                noreturn_expr(f)?;
+            }
+
+            if let Some(ref t) = r.to {
+                noreturn_expr(t)?;
+            }
+        }
+
+        Expr::Path(..) => {}
+
+        Expr::Reference(r) => {
+            noreturn_expr(&r.expr)?;
+        }
+
+        Expr::Break(b) => {
+            if let Some(ref e) = b.expr {
+                noreturn_expr(e)?;
+            }
+        }
+
+        Expr::Continue(..) => {}
+
+        Expr::Return(r) => {
+            return Err(parse::Error::new(
+                r.span(),
+                "`init` is *not* allowed to early return",
+            ));
+        }
+
+        // we can not analyze this
+        Expr::Macro(..) => {}
+
+        Expr::Struct(s) => {
+            for field in &s.fields {
+                noreturn_expr(&field.expr)?;
+            }
+
+            if let Some(ref rest) = s.rest {
+                noreturn_expr(rest)?;
+            }
+        }
+
+        Expr::Repeat(r) => {
+            noreturn_expr(&r.expr)?;
+            noreturn_expr(&r.len)?;
+        }
+
+        Expr::Paren(p) => {
+            noreturn_expr(&p.expr)?;
+        }
+
+        Expr::Group(g) => {
+            noreturn_expr(&g.expr)?;
+        }
+
+        Expr::Try(t) => {
+            noreturn_expr(&t.expr)?;
+        }
+
+        // we don't care about `return`s inside async blocks
+        Expr::Async(..) => {}
+
+        Expr::TryBlock(tb) => {
+            noreturn_block(&tb.block)?;
+        }
+
+        Expr::Yield(y) => {
+            if let Some(expr) = &y.expr {
+                noreturn_expr(expr)?;
+            }
+        }
+
+        // we can not analyze this
+        Expr::Verbatim(..) => {}
+    }
+
     Ok(())
 }