lang: give better errors for init when programs are missing

CHANGELOG.md

@@ -11,6 +11,10 @@ incremented for features.
 
 ## [Unreleased]
 
+#### Fixes
+
+*lang: Improved error msgs when required programs are missing when using the `init` constraint([#1257](https://github.com/project-serum/anchor/pull/1257))
+
 ## [0.20.0] - 2022-01-06
 
 ### Fixes

lang/syn/src/lib.rs

@@ -154,6 +154,15 @@ pub enum AccountField {
     CompositeField(CompositeField),
 }
 
+impl AccountField {
+    fn ident(&self) -> &Ident {
+        match self {
+            AccountField::Field(field) => &field.ident,
+            AccountField::CompositeField(c_field) => &c_field.ident,
+        }
+    }
+}
+
 #[derive(Debug)]
 pub struct Field {
     pub ident: Ident,

lang/syn/src/parser/accounts/mod.rs

@@ -31,9 +31,71 @@ pub fn parse(strct: &syn::ItemStruct) -> ParseResult<AccountsStruct> {
             ))
         }
     };
+
+    let _ = constraints_cross_checks(&fields)?;
+
     Ok(AccountsStruct::new(strct.clone(), fields, instruction_api))
 }
 
+fn constraints_cross_checks(fields: &[AccountField]) -> ParseResult<()> {
+    // INIT
+    let init_field = fields.iter().find(|f| {
+        if let AccountField::Field(field) = f {
+            field.constraints.init.is_some()
+        } else {
+            false
+        }
+    });
+    if let Some(init_field) = init_field {
+        // init needs system program
+        if fields.iter().all(|f| f.ident() != "system_program") {
+            return Err(ParseError::new(
+                init_field.ident().span(),
+                "the init constraint requires \
+                the system_program field to exist in the account \
+                validation struct. Use the program type to add \
+                the system_program field to your validation struct.",
+            ));
+        }
+        if let AccountField::Field(field) = init_field {
+            let kind = &field.constraints.init.as_ref().unwrap().kind;
+            // init token/a_token/mint needs token program
+            match kind {
+                InitKind::Program { .. } => (),
+                InitKind::Token { .. }
+                | InitKind::AssociatedToken { .. }
+                | InitKind::Mint { .. } => {
+                    if fields.iter().all(|f| f.ident() != "token_program") {
+                        return Err(ParseError::new(
+                            init_field.ident().span(),
+                            "the init constraint requires \
+                            the token_program field to exist in the account \
+                            validation struct. Use the program type to add \
+                            the token_program field to your validation struct.",
+                        ));
+                    }
+                }
+            }
+            // a_token needs associated token program
+            if let InitKind::AssociatedToken { .. } = kind {
+                if fields
+                    .iter()
+                    .all(|f| f.ident() != "associated_token_program")
+                {
+                    return Err(ParseError::new(
+                        init_field.ident().span(),
+                        "the init constraint requires \
+                    the associated_token_program field to exist in the account \
+                    validation struct. Use the program type to add \
+                    the associated_token_program field to your validation struct.",
+                    ));
+                }
+            }
+        }
+    }
+    Ok(())
+}
+
 pub fn parse_account_field(f: &syn::Field, has_instruction_api: bool) -> ParseResult<AccountField> {
     let ident = f.ident.clone().unwrap();
     let account_field = match is_field_primitive(f)? {