Add check for duplicate variable definition (fixes #320)

crates/yuck/src/config/config.rs

@@ -9,6 +9,7 @@ use simplexpr::SimplExpr;
 use super::{
     file_provider::{FilesError, YuckFiles},
     script_var_definition::ScriptVarDefinition,
+    validate::ValidationError,
     var_definition::VarDefinition,
     widget_definition::WidgetDefinition,
     widget_use::WidgetUse,
@@ -91,11 +92,25 @@ impl Config {
     fn append_toplevel(&mut self, files: &mut YuckFiles, toplevel: TopLevel) -> AstResult<()> {
         match toplevel {
             TopLevel::VarDefinition(x) => {
+                if self.var_definitions.contains_key(&x.name) || self.script_vars.contains_key(&x.name) {
+                    return Err(AstError::ValidationError(ValidationError::VariableDefinedTwice {
+                        name: x.name.clone(),
+                        span: x.span,
+                    }));
+                } else {
                     self.var_definitions.insert(x.name.clone(), x);
                 }
+            }
             TopLevel::ScriptVarDefinition(x) => {
+                if self.var_definitions.contains_key(x.name()) || self.script_vars.contains_key(x.name()) {
+                    return Err(AstError::ValidationError(ValidationError::VariableDefinedTwice {
+                        name: x.name().clone(),
+                        span: x.name_span(),
+                    }));
+                } else {
                     self.script_vars.insert(x.name().clone(), x);
                 }
+            }
             TopLevel::WidgetDefinition(x) => {
                 self.widget_definitions.insert(x.name.clone(), x);
             }

crates/yuck/src/config/validate.rs

@@ -32,12 +32,16 @@ pub enum ValidationError {
         /// True if the error occurred inside a widget definition, false if it occurred in a window definition
         in_definition: bool,
     },
+
+    #[error("Variable named `{name}` defined twice")]
+    VariableDefinedTwice { span: Span, name: VarName },
 }
 
 impl Spanned for ValidationError {
     fn span(&self) -> Span {
         match self {
             ValidationError::UnknownWidget(span, _) => *span,
+            ValidationError::VariableDefinedTwice { span, .. } => *span,
             ValidationError::MissingAttr { use_span, .. } => *use_span,
             ValidationError::UnknownVariable { span, .. } => *span,
             ValidationError::AccidentalBuiltinOverride(span, ..) => *span,

crates/yuck/src/format_diagnostic.rs

@@ -199,6 +199,10 @@ impl ToDiagnostic for ValidationError {
                 label = span => "Defined here",
                 note = "Hint: Give your widget a different name. You could call it \"John\" for example. That's a cool name."
             },
+            ValidationError::VariableDefinedTwice { span, name } => gen_diagnostic! {
+                msg = self,
+                label = span => "Defined again here"
+            },
         }
     }
 }