Keep last redundant linker flag, not first
When a library (L1) is passed to the linker multiple times, this is sometimes purposeful: there might be several other libraries in the linker command (L2 and L3) that all depend on L1. You'd end up with a (simplified) linker command that looks like:
```
-l2 -l1 -l3 -l1
```
With the previous behavior, when rustc encountered a redundant library, it would keep the first instance, and remove the later ones, resulting in:
```
-l2 -l1 -l3
```
This can cause a linker error, because on some platforms (e.g. Linux), the linker will only include symbols from L1 that are needed *at the point it's referenced in the command line*. So if L3 depends on additional symbols from L1, which aren't needed by L2, the linker won't know to include them, and you'll end up with "undefined symbols" errors.
A better behavior is to keep the *last* instance of the library:
```
-l2 -l3 -l1
```
This ensures that all "downstream" libraries have been included in the linker command before the "upstream" library is referenced.
Fixes rust-lang#47989
When a library (L1) is passed to the linker multiple times, this is
sometimes purposeful: there might be several other libraries in the
linker command (L2 and L3) that all depend on L1. You'd end up with a
(simplified) linker command that looks like:
-l2 -l1 -l3 -l1
With the previous behavior, when rustc encountered a redundant library,
it would keep the first instance, and remove the later ones, resulting
in:
-l2 -l1 -l3
This can cause a linker error, because on some platforms (e.g. Linux),
the linker will only include symbols from L1 that are needed *at the
point it's referenced in the command line*. So if L3 depends on
additional symbols from L1, which aren't needed by L2, the linker won't
know to include them, and you'll end up with "undefined symbols" errors.
A better behavior is to keep the *last* instance of the library:
-l2 -l3 -l1
This ensures that all "downstream" libraries have been included in the
linker command before the "upstream" library is referenced.
Fixes rust-lang#47989
This commit stabilizes the `#[wasm_import_module]` attribute as
`#[link(wasm_import_module = "...")]`. Tracked by #52090 this new directive in
the `#[link]` attribute is used to configured the module name that the imports
are listed with. The WebAssembly specification indicates two utf-8 names are
associated with all imported items, one for the module the item comes from and
one for the item itself. The item itself is configurable in Rust via its
identifier or `#[link_name = "..."]`, but the module name was previously not
configurable and defaulted to `"env"`. This commit ensures that this is also
configurable.
Closes#52090
This commit adds a new attribute to the Rust compiler specific to the wasm
target (and no other targets). The `#[wasm_import_module]` attribute is used to
specify the module that a name is imported from, and is used like so:
#[wasm_import_module = "./foo.js"]
extern {
fn some_js_function();
}
Here the import of the symbol `some_js_function` is tagged with the `./foo.js`
module in the wasm output file. Wasm-the-format includes two fields on all
imports, a module and a field. The field is the symbol name (`some_js_function`
above) and the module has historically unconditionally been `"env"`. I'm not
sure if this `"env"` convention has asm.js or LLVM roots, but regardless we'd
like the ability to configure it!
The proposed ES module integration with wasm (aka a wasm module is "just another
ES module") requires that the import module of wasm imports is interpreted as an
ES module import, meaning that you'll need to encode paths, NPM packages, etc.
As a result, we'll need this to be something other than `"env"`!
Unfortunately neither our version of LLVM nor LLD supports custom import modules
(aka anything not `"env"`). My hope is that by the time LLVM 7 is released both
will have support, but in the meantime this commit adds some primitive
encoding/decoding of wasm files to the compiler. This way rustc postprocesses
the wasm module that LLVM emits to ensure it's got all the imports we'd like to
have in it.
Eventually I'd ideally like to unconditionally require this attribute to be
placed on all `extern { ... }` blocks. For now though it seemed prudent to add
it as an unstable attribute, so for now it's not required (as that'd force usage
of a feature gate). Hopefully it doesn't take too long to "stabilize" this!
cc rust-lang-nursery/rust-wasm#29
- Don't hash traits in scope as part of HIR hashing any more.
- Some queries returned DefIndexes from other crates.
- Provide a generic way of stably hashing maps (not used everywhere yet).
This commit started by moving methods from `CrateStore` to queries, but it ended
up necessitating some deeper refactorings to move more items in general to
queries.
Before this commit the *resolver* would walk over the AST and process foreign
modules (`extern { .. }` blocks) and collect `#[link]` annotations. It would
then also process the command line `-l` directives and such. This information
was then stored as precalculated lists in the `CrateStore` object for iterating
over later.
After this, commit, however, this pass no longer happens during resolution but
now instead happens through queries. A query for the linked libraries of a crate
will crawl the crate for `extern` blocks and then process the linkage
annotations at that time.
