Create dedicated error when environment isn't found.

This is less confusing that getting an 'UnknownModule' error reporting
  even a different module name than the one actually being important
  ('env').

  Also, this commit fixes a few errors found in the type-checker
  when reporting 'UnknownModule' errors. About half the time, we would
  actually attached _imported modules_ instead of _importable modules_
  to the error, making the neighboring suggestion quite worse (nay
  useless).

crates/aiken-lang/src/tipo/environment.rs

@@ -91,20 +91,36 @@ impl<'a> Environment<'a> {
     pub fn find_module(&self, fragments: &[String], location: Span) -> Result<&'a TypeInfo, Error> {
         let mut name = fragments.join("/");
 
-        if name == ast::ENV_MODULE {
+        let is_env = name == ast::ENV_MODULE;
+
+        if is_env {
             name = self
                 .target_env
                 .unwrap_or(ast::DEFAULT_ENV_MODULE)
                 .to_string()
         }
 
-        self.importable_modules
-            .get(&name)
-            .ok_or_else(|| Error::UnknownModule {
-                location,
-                name,
-                imported_modules: self.imported_modules.keys().cloned().collect(),
-            })
+        self.importable_modules.get(&name).ok_or_else(|| {
+            if is_env {
+                Error::UnknownEnvironment {
+                    name,
+                    known_environments: self
+                        .importable_modules
+                        .values()
+                        .filter_map(|m| match m.kind {
+                            ModuleKind::Env => Some(m.name.clone()),
+                            ModuleKind::Lib | ModuleKind::Validator => None,
+                        })
+                        .collect(),
+                }
+            } else {
+                Error::UnknownModule {
+                    location,
+                    name,
+                    known_modules: self.importable_modules.keys().cloned().collect(),
+                }
+            }
+        })
     }
 
     pub fn close_scope(&mut self, data: ScopeResetData) {
@@ -373,7 +389,7 @@ impl<'a> Environment<'a> {
                         .ok_or_else(|| Error::UnknownModule {
                             location,
                             name: name.to_string(),
-                            imported_modules: self
+                            known_modules: self
                                 .importable_modules
                                 .keys()
                                 .map(|t| t.to_string())
@@ -419,7 +435,7 @@ impl<'a> Environment<'a> {
                         .get(m)
                         .ok_or_else(|| Error::UnknownModule {
                             name: m.to_string(),
-                            imported_modules: self
+                            known_modules: self
                                 .importable_modules
                                 .keys()
                                 .map(|t| t.to_string())
@@ -1726,7 +1742,7 @@ impl<'a> Environment<'a> {
                 .ok_or_else(|| Error::UnknownModule {
                     location,
                     name: name.to_string(),
-                    imported_modules: self
+                    known_modules: self
                         .importable_modules
                         .keys()
                         .map(|t| t.to_string())

crates/aiken-lang/src/tipo/error.rs

@@ -751,13 +751,39 @@ Perhaps, try the following:
     #[diagnostic(code("unknown::module"))]
     #[diagnostic(help(
         "{}",
-        suggest_neighbor(name, imported_modules.iter(), "Did you forget to add a package as dependency?")
+        suggest_neighbor(name, known_modules.iter(), "Did you forget to add a package as dependency?")
     ))]
     UnknownModule {
         #[label]
         location: Span,
         name: String,
-        imported_modules: Vec<String>,
+        known_modules: Vec<String>,
+    },
+
+    #[error(
+        "I couldn't find any module for the environment: '{}'\n",
+        name.if_supports_color(Stdout, |s| s.purple())
+    )]
+    #[diagnostic(code("unknown::environment"))]
+    #[diagnostic(help(
+        "{}{}",
+        if known_environments.is_empty() {
+            String::new()
+        } else {
+            format!(
+                "I know about the following environments:\n{}\n\n",
+                known_environments
+                    .iter()
+                    .map(|s| format!("─▶ {}", s.if_supports_color(Stdout, |s| s.purple())))
+                    .collect::<Vec<_>>()
+                    .join("\n")
+            )
+        },
+        suggest_neighbor(name, known_environments.iter(), "Did you forget to define this environment?")
+    ))]
+    UnknownEnvironment {
+        name: String,
+        known_environments: Vec<String>,
     },
 
     #[error(
@@ -1066,6 +1092,7 @@ impl ExtraData for Error {
             | Error::UnknownModuleType { .. }
             | Error::UnknownModuleValue { .. }
             | Error::UnknownRecordField { .. }
+            | Error::UnknownEnvironment { .. }
             | Error::UnnecessarySpreadOperator { .. }
             | Error::UpdateMultiConstructorType { .. }
             | Error::ValidatorImported { .. }

crates/aiken-lang/src/tipo/expr.rs

@@ -956,9 +956,9 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
                 .ok_or_else(|| Error::UnknownModule {
                     name: module_alias.to_string(),
                     location: *module_location,
-                    imported_modules: self
+                    known_modules: self
                         .environment
-                        .imported_modules
+                        .importable_modules
                         .keys()
                         .map(|t| t.to_string())
                         .collect(),
@@ -2327,9 +2327,9 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
                     .ok_or_else(|| Error::UnknownModule {
                         location: *location,
                         name: module_name.to_string(),
-                        imported_modules: self
+                        known_modules: self
                             .environment
-                            .imported_modules
+                            .importable_modules
                             .keys()
                             .map(|t| t.to_string())
                             .collect(),