Add wildcard imports

Imports.md

@@ -22,6 +22,10 @@ import my::geom::sphere::{ draw, default_radius as foobar };
 
 // Imports a whole module. Use it with `bevy_ui::name`
 import bevy_ui;
+
+// Import all items from another module
+import bevy::prelude::*;
+import wgsl_test::expect::*;
 ```
 
 These can then be used anywhere in the source code.
@@ -74,6 +78,7 @@ import_relative:
 import_path_or_item:
 | ident '::' (import_collection | import_path_or_item) 
 | ident ('as' ident)?
+| '*'
 
 import_collection:
 | '{' (import_path_or_item) (',' (import_path_or_item))* ','? '}'
@@ -91,6 +96,21 @@ An item import imports a single item. The item can be renamed with the `as` keyw
 
 An import collection imports multiple items, and allows for nested imports.
 
+A wildcard import imports all top-level declarations from a module. Submodule names and submodule contents are not imported.
+
+WESL also extends WGSL's `global_directive` rule with a `module_attribute` form (used by `@wildcardable`; see [Wildcard imports](#wildcard-imports)):
+
+```ebnf
+global_directive:
+| ... // existing WGSL forms
+| module_attribute
+
+module_attribute:
+| '@' ident ';'
+```
+
+The `@` prefix avoids conflict with WGSL identifiers.
+
 ### Import resolution algorithm
 
 To resolve the import, the recursive structure is flattened out. This means turning every `import_collection` into multiple separate imports, ending with the items.
@@ -111,8 +131,9 @@ Then, one iterates over each segment from left to right, and looks it up one by
 2. We take that as the "current module".
 3. We repeatedly look at the next segment.
     1. Item in current module: Take that item. We must be at the last segment, otherwise it's an error.
-    2. (Else if re-exported or inline module in current module: We continue with that module.)
-    3. Else go to `current module path/ident.wesl`
+    2. Wildcard import: Take all items in the current module.
+    3. (Else if re-exported or inline module in current module: We continue with that module.)
+    4. Else go to `current module path/ident.wesl`
        * File found: We take that file as the current module.
        * File not found: We assume an empty module as the current module, and continue with that.
        * (Re-exporting changes the path.)
@@ -121,10 +142,7 @@ Then, one iterates over each segment from left to right, and looks it up one by
 To get an absolute path to a module, one follows the algorithm above. In step 1, one takes the known absolute path of the `super` module, or the package.
 The absolute path of the `super` module is always known, since the first loaded WESL file must always be the root module, and children are only discovered from there.
 
-Once the import has been resolved, the last segment, or its alias, is brought into scope.
-
-The order of the scopes is "user declarations and imported items > package names > predeclared items".
-This lets WGSL add more predeclared items without breaking existing WESL code. Package names can shadow predeclared items, but we recommend that authors avoid doing that.
+Once the import has been resolved, the last segment, or its `as` alias, is brought into scope.
 
 
 ### Example
@@ -245,6 +263,160 @@ const b = a + 1;
 
 Basic linker implementations do not need to check for this. Generating broken code and letting the underlying shader compiler throw an error is fine.
 
+## Wildcard imports
+
+Wildcard imports bring all items from another module into the importing module's
+scope.
+
+Users can wildcard import:
+- from any other module in the local package.
+- from any external library module where the library author has added a
+  `@wildcardable` directive.
+
+Wildcard importing brings some stability risk when the imported module adds to
+its API. The newly introduced names may conflict with other names in the
+importer's namespace (from local definitions and other imports), leading to
+compiler warnings or errors. To help mitigate this risk, WESL provides a
+`@wildcardable` annotation that library authors can place on modules that are
+designed for wildcard importing.
+
+Advanced users who wish to wildcard import from external modules not marked as
+`@wildcardable` can do so by suppressing `wildcard_import` (see
+[Suppressible diagnostics](#suppressible-diagnostics)).
+
+```wesl
+// wildcard import from a @wildcardable external
+import bevy::prelude::*;
+import wgsl_test::expect::*;
+
+// wildcard import from within the local package
+import package::utils::*;
+import super::fun::*;
+```
+
+### `@wildcardable` directive
+
+Library authors mark modules they intend for library consumers to wildcard
+import with `@wildcardable`:
+
+```wesl
+// math.wesl  (in a library)
+@wildcardable;
+
+fn dot2(a: vec2f, b: vec2f) -> f32 { return a.x*b.x + a.y*b.y; }
+fn cross2(a: vec2f, b: vec2f) -> f32 { return a.x*b.y - a.y*b.x; }
+```
+
+`@wildcardable` is a module attribute (see [Grammar](#grammar)). It must
+appear after any imports and before any global declarations, and applies only
+to the module it appears in.
+
+### Recommendations for `@wildcardable` modules
+
+Because every name in a `@wildcardable` module is a potential collision in
+importer code, library authors should curate these modules carefully.
+
+**Add hesitantly.** Additions to a `@wildcardable` module are semver minor
+version bumps but can break users who have local declarations or import other
+`@wildcardable` modules.
+- **Bundle** additions into a major release when one is upcoming.
+- **Document** additions clearly in changelogs so downstream users debugging
+  unexpected name resolution can trace them.
+
+**Compose with re-exports.** See [Re-exports](#re-exports) (TBD) to collect
+items from other modules into a single `@wildcardable` module for user
+convenience.
+
+**Avoid generic names.** Prefer domain-specific names. Common names like
+`Buffer`, `Config`, `Result`, `Vec`, etc. are more likely to collide with user
+applications.
+
+**Don't shadow WGSL builtins.** Names like `vec3`, `clamp`, `inverseSqrt` have
+expected semantics that oughtn't be implicitly overridden with wildcards.
+Similarly, avoid experimental Naga/Dawn/Safari builtins. Consumers of the
+module will see a `builtin_shadow` warning at the import site.
+- If a future WGSL update adds a conflicting builtin name, plan to update the
+  `@wildcardable` module to rename the conflicting item.
+
+### Library-to-library wildcard imports
+
+Libraries that wildcard import from other libraries raise special concerns. If a
+user's package manager chooses a newer version of the imported-from library,
+the user may see a conflict in library code they don't expect to modify.
+
+WESL library publishing tools address this by expanding wildcards to named
+imports in the published version of the module:
+
+```wesl
+// source
+import bevy::prelude::*;
+```
+
+```wesl
+// published artifact
+import bevy::prelude::{Color, Mesh, Transform, /* snapshot at publish time */};
+```
+
+## Import errors and warnings
+
+WESL emits errors for name collisions and for external wildcards from unmarked
+modules, and warnings for shadowing that could surprise readers. Genuine
+collisions cannot be suppressed; other diagnostics are suppressible via
+`@diagnostic`.
+
+| Situation | Behavior |
+| --- | --- |
+| Local declaration conflicts with named import | Error |
+| Named import conflicts with named import | Error |
+| Wildcard import conflicts with wildcard import (when name is referenced) | Error |
+| Wildcard import from a non-`@wildcardable` external module | Error (`wildcard_import`); suppressible |
+| Local declaration or named import shadows a wildcard-imported name | Warning (`wildcard_shadow`); suppressible |
+| Wildcard import shadows a WGSL builtin | Warning (`builtin_shadow`); suppressible |
+
+When multiple wildcard imports are in scope, the same name may be exported by
+more than one module:
+
+```wesl
+import foo::*; // exports clashing_zap
+import bar::*; // exports clashing_zap
+```
+
+If `foo::clashing_zap` and `bar::clashing_zap` re-export the same item, there's
+no conflict. Otherwise the potential conflict is dormant unless `clashing_zap`
+is referenced.
+
+### Suppressible diagnostics
+
+- **`wildcard_import`** fires on a wildcard import from an external module not
+  marked `@wildcardable`. Suppress with `@diagnostic(off, wildcard_import)` on
+  the import statement to accept the upgrade risk that future versions of the
+  imported module may add conflicting names.
+
+- **`wildcard_shadow`** fires when a local declaration or named import shadows a
+  name brought in by a wildcard import. The local wins by precedence (see
+  [Scope precedence](#scope-precedence)).
+
+- **`builtin_shadow`** fires when a wildcard import shadows a WGSL builtin such
+  as `vec3` or `clamp`. Suppress at the import site if the override is
+  intentional; the suppression itself documents documents to readers that the
+  builtins have changed semantics.
+
+### Scope precedence
+
+When a name could resolve to items at multiple precedence levels, the
+highest-precedence one wins. The table above describes whether such a resolution
+is silent, produces a warning, or produces an error.
+
+1. user declarations
+2. named imports (non-wildcard)
+3. wildcard-imported names
+4. package names
+5. predeclared items (WGSL builtins)
+
+Predeclared items rank lowest so that future WGSL spec revisions can add new
+builtins without breaking existing WESL code: any name already bound at a
+higher level continues to resolve as before.
+
 ## Directives
 Under discussion, see: <https://github.com/wgsl-tooling-wg/wesl-spec/issues/71>