Merge pull request #2846 from rust-lang/tshepang/sembr

sembr some files

Author: tshepang

src/coherence.md

@@ -8,13 +8,14 @@ Coherence checking is what detects both of trait impls and inherent impls overla
 Overlapping trait impls always produce an error,
 while overlapping inherent impls result in an error only if they have methods with the same name.
 
-Checking for overlaps is split in two parts. First there's the [overlap check(s)](#overlap-checks), 
+Checking for overlaps is split in two parts.
+First there's the [overlap check(s)](#overlap-checks),
 which finds overlaps between traits and inherent implementations that the compiler currently knows about.
 
 However, Coherence also results in an error if any other impls **could** exist,
-even if they are currently unknown. 
+even if they are currently unknown.
 This affects impls which may get added to upstream crates in a backwards compatible way,
-and impls from downstream crates. 
+and impls from downstream crates.
 This is called the Orphan check.
 
 ## Overlap checks
@@ -25,7 +26,7 @@ Overlap checks always consider pairs of implementations, comparing them to each
 
 Overlap checking for inherent impl blocks is done through `fn check_item` (in coherence/inherent_impls_overlap.rs),
 where you can very clearly see that (at least for small `n`), the check really performs `n^2`
-comparisons between impls. 
+comparisons between impls.
 
 In the case of traits, this check is currently done as part of building the [specialization graph](traits/specialization.md),
 to handle specializing impls overlapping with their parent, but this may change in the future.
@@ -37,7 +38,7 @@ Overlapping is sometimes partially allowed:
 1. for marker traits
 2. under [specialization](traits/specialization.md)
 
-but normally isn't. 
+It normally isn't.
 
 The overlap check has various modes (see [`OverlapMode`]).
 Importantly, there's the explicit negative impl check, and the implicit negative impl check.
@@ -47,9 +48,9 @@ Both try to prove that an overlap is definitely impossible.
 
 ### The explicit negative impl check
 
-This check is done in [`impl_intersection_has_negative_obligation`]. 
+This check is done in [`impl_intersection_has_negative_obligation`].
 
-This check tries to find a negative trait implementation. 
+This check tries to find a negative trait implementation.
 For example:
 
 ```rust
@@ -64,7 +65,7 @@ In this example, we'd get:
 `MyCustomErrorType: From<&str>` and `MyCustomErrorType: From<?E>`, giving `?E = &str`.
 
 And thus, these two implementations would overlap.
-However, libstd provides `&str: !Error`, and therefore guarantees that there 
+However, libstd provides `&str: !Error`, and therefore guarantees that there
 will never be a positive implementation of `&str: Error`, and thus there is no overlap.
 
 Note that for this kind of negative impl check, we must have explicit negative implementations provided.
@@ -77,18 +78,17 @@ This is not currently stable.
 This check is done in [`impl_intersection_has_impossible_obligation`],
 and does not rely on negative trait implementations and is stable.
 
-Let's say there's a 
+Let's say there's a
 ```rust
 impl From<MyLocalType> for Box<dyn Error> {}  // in your own crate
 impl<E> From<E> for Box<dyn Error> where E: Error {} // in std
 ```
 
-This would give: `Box<dyn Error>: From<MyLocalType>`, and `Box<dyn Error>: From<?E>`,  
+This would give: `Box<dyn Error>: From<MyLocalType>`, and `Box<dyn Error>: From<?E>`,
 giving `?E = MyLocalType`.
 
 In your crate there's no `MyLocalType: Error`, downstream crates cannot implement `Error` (a remote trait) for `MyLocalType` (a remote type).
 Therefore, these two impls do not overlap.
 Importantly, this works even if there isn't a `impl !Error for MyLocalType`.
 
 [`impl_intersection_has_impossible_obligation`]: https://doc.rust-lang.org/beta/nightly-rustc/rustc_trait_selection/traits/coherence/fn.impl_intersection_has_impossible_obligation.html
-

src/hir/lowering.md

@@ -2,8 +2,8 @@
 
 The AST lowering step converts AST to [HIR](../hir.md).
 This means many structures are removed if they are irrelevant
-for type analysis or similar syntax agnostic analyses. Examples
-of such structures include but are not limited to
+for type analysis or similar syntax-agnostic analyses.
+Examples of such structures include but are not limited to
 
 * Parenthesis
     * Removed without replacement, the tree structure makes order explicit
@@ -19,37 +19,42 @@ The implementation of AST lowering is in the [`rustc_ast_lowering`] crate.
 The entry point is [`lower_to_hir`], which retrieves the post-expansion AST
 and resolver data from [`TyCtxt`] and builds the [`hir::Crate`] for the whole crate.
 
-Lowering is organized around HIR owners. [`lower_to_hir`] first indexes the
+Lowering is organized around HIR owners.
+[`lower_to_hir`] first indexes the
 crate and then [`ItemLowerer::lower_node`] lowers each crate, item, associated
 item, and foreign item.
 
-Most of the lowering logic lives on [`LoweringContext`]. The implementation is
+Most of the lowering logic lives on [`LoweringContext`].
+The implementation is
 split across multiple files in the [`rustc_ast_lowering`] crate such as `item.rs`,
 `expr.rs`, `pat.rs`, `path.rs`, and others, but they all share the same [`LoweringContext`]
 state and ID‑lowering machinery.
 
-Each owner is lowered in its own [`with_hir_id_owner`] scope. This is why the
+Each owner is lowered in its own [`with_hir_id_owner`] scope.
+This is why the
 `HirId` invariants below matter: `lower_node_id` maps AST `NodeId`s into the
 current owner, while `next_id` creates fresh HIR-only nodes introduced during
 desugaring.
 
 Lowering needs to uphold several invariants in order to not trigger the
 sanity checks in [`compiler/rustc_passes/src/hir_id_validator.rs`][hir_id_validator]:
 
-1. A `HirId` must be used if created. So if you use the `lower_node_id`,
-  you *must* use the resulting `NodeId` or `HirId` (either is fine, since
-  any `NodeId`s in the `HIR` are checked for existing `HirId`s)
+1. A `HirId` must be used if created.
+   So, if you use the `lower_node_id`,
+   you *must* use the resulting `NodeId` or `HirId` (either is fine, since
+   any `NodeId`s in the `HIR` are checked for existing `HirId`s).
 2. Lowering a `HirId` must be done in the scope of the *owning* item.
-  This means you need to use `with_hir_id_owner` if you are creating parts
-  of an item other than the one being currently lowered. This happens for
-  example during the lowering of existential `impl Trait`
+   This means you need to use `with_hir_id_owner` if you are creating parts
+   of an item other than the one being currently lowered.
+   This happens, for example, during the lowering of existential `impl Trait`.
 3. A `NodeId` that will be placed into a HIR structure must be lowered,
-  even if its `HirId` is unused. Calling
-  `let _ = self.lower_node_id(node_id);` is perfectly legitimate.
+   even if its `HirId` is unused.
+   Calling `let _ = self.lower_node_id(node_id);` is perfectly legitimate.
 4. If you are creating new nodes that didn't exist in the `AST`, you *must*
-  create new ids for them. This is done by calling the `next_id` method,
-  which produces both a new `NodeId` as well as automatically lowering it
-  for you so you also get the `HirId`.
+   create new ids for them.
+   This is done by calling the `next_id` method,
+   which produces both a new `NodeId` as well as automatically lowering it
+   for you so you also get the `HirId`.
 
 [`rustc_ast_lowering`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast_lowering/index.html
 [`lower_to_hir`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast_lowering/fn.lower_to_hir.html
@@ -62,12 +67,16 @@ sanity checks in [`compiler/rustc_passes/src/hir_id_validator.rs`][hir_id_valida
 
 If you are creating new `DefId`s, since each `DefId` needs to have a
 corresponding `NodeId`, it is advisable to add these `NodeId`s to the
-`AST` so you don't have to generate new ones during lowering. This has
+`AST` so you don't have to generate new ones during lowering.
+This has
 the advantage of creating a way to find the `DefId` of something via its
-`NodeId`. If lowering needs this `DefId` in multiple places, you can't
+`NodeId`.
+If lowering needs this `DefId` in multiple places, you can't
 generate a new `NodeId` in all those places because you'd also get a new
-`DefId` then. With a `NodeId` from the `AST` this is not an issue.
+`DefId` then.
+With a `NodeId` from the `AST`, this is not an issue.
 
 Having the `NodeId` also allows the `DefCollector` to generate the `DefId`s
-instead of lowering having to do it on the fly. Centralizing the `DefId`
+instead of lowering having to do it on the fly.
+Centralizing the `DefId`
 generation in one place makes it easier to refactor and reason about.

src/name-resolution.md

@@ -1,57 +1,68 @@
 # Name resolution
 
 In the previous chapters, we saw how the [*Abstract Syntax Tree* (`AST`)][ast]
-is built with all macros expanded. We saw how doing that requires doing some
-name resolution to resolve imports and macro names. In this chapter, we show
-how this is actually done and more.
+is built with all macros expanded.
+We saw how doing that requires doing some
+name resolution to resolve imports and macro names.
+In this chapter, we show how this is actually done and more.
 
 [ast]: ./ast-validation.md
 
 In fact, we don't do full name resolution during macro expansion -- we only
-resolve imports and macros at that time. This is required to know what to even
-expand. Later, after we have the whole AST, we do full name resolution to
-resolve all names in the crate. This happens in [`rustc_resolve::late`][late].
+resolve imports and macros at that time.
+This is required to know what to even expand.
+Later, after we have the whole AST, we do full name resolution to
+resolve all names in the crate.
+This happens in [`rustc_resolve::late`][late].
 Unlike during macro expansion, in this late expansion, we only need to try to
-resolve a name once, since no new names can be added. If we fail to resolve a
-name, then it is a compiler error.
+resolve a name once, since no new names can be added.
+If we fail to resolve a name, then it is a compiler error.
 
 Name resolution is complex. There are different namespaces (e.g.
 macros, values, types, lifetimes), and names may be valid at different (nested)
-scopes. Also, different types of names can fail resolution differently, and
-failures can happen differently at different scopes. For example, in a module
-scope, failure means no unexpanded macros and no unresolved glob imports in
-that module. On the other hand, in a function body scope, failure requires that a
-name be absent from the block we are in, all outer scopes, and the global
-scope.
+scopes.
+Also, different types of names can fail resolution differently, and
+failures can happen differently at different scopes.
+For example, in a module scope,
+failure means no unexpanded macros and no unresolved glob imports in
+that module.
+On the other hand, in a function body scope, failure requires that a
+name be absent from the block we are in, all outer scopes, and the global scope.
 
 [late]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/index.html
 
 ## Basics
 
 In our programs we refer to variables, types, functions, etc, by giving them
-a name. These names are not always unique. For example, take this valid Rust
-program:
+a name.
+These names are not always unique.
+For example, take this valid Rust program:
 
 ```rust
 type x = u32;
 let x: x = 1;
 let y: x = 2;
 ```
 
-How do we know on line 3 whether `x` is a type (`u32`) or a value (1)? These
-conflicts are resolved during name resolution. In this specific case, name
-resolution defines that type names and variable names live in separate
+How do we know on line 3 whether `x` is a type (`u32`) or a value (1)?
+These conflicts are resolved during name resolution.
+In this specific case,
+name resolution defines that type names and variable names live in separate
 namespaces and therefore can co-exist.
 
-The name resolution in Rust is a two-phase process. In the first phase, which runs
-during `macro` expansion, we build a tree of modules and resolve imports. Macro
-expansion and name resolution communicate with each other via the
+The name resolution in Rust is a two-phase process.
+In the first phase,
+which runs during `macro` expansion,
+we build a tree of modules and resolve imports.
+Macro expansion and name resolution communicate with each other via the
 [`ResolverAstLoweringExt`] trait.
 
 The input to the second phase is the syntax tree, produced by parsing input
-files and expanding `macros`. This phase produces links from all the names in the
-source to relevant places where the name was introduced. It also generates
-helpful error messages, like typo suggestions, traits to import or lints about
+files and expanding `macros`.
+This phase produces links from all the names in the
+source to relevant places where the name was introduced.
+It also generates helpful error messages,
+like typo suggestions, traits to import or lints about
 unused items.
 
 A successful run of the second phase ([`Resolver::resolve_crate`]) creates kind
@@ -68,9 +79,11 @@ The name resolution lives in the [`rustc_resolve`] crate, with the bulk in
 ## Namespaces
 
 Different kind of symbols live in different namespaces ‒ e.g. types don't
-clash with variables. This usually doesn't happen, because variables start with
+clash with variables.
+This usually doesn't happen, because variables start with
 lower-case letter while types with upper-case one, but this is only a
-convention. This is legal Rust code that will compile (with warnings):
+convention.
+This is legal Rust code that will compile (with warnings):
 
 ```rust
 type x = u32;
@@ -83,19 +96,21 @@ namespaces, the resolver keeps them separated and builds separate structures for
 them.
 
 In other words, when the code talks about namespaces, it doesn't mean the module
-hierarchy, it's types vs. values vs. macros.
+hierarchy, it's types versus values versus macros.
 
 ## Scopes and ribs
 
-A name is visible only in certain area in the source code. This forms a
-hierarchical structure, but not necessarily a simple one ‒ if one scope is
+A name is visible only in certain area in the source code.
+This forms a hierarchical structure,
+but not necessarily a simple one ‒ if one scope is
 part of another, it doesn't mean a name visible in the outer scope is also
 visible in the inner scope, or that it refers to the same thing.
 
-To cope with that, the compiler introduces the concept of [`Rib`]s. This is
-an abstraction of a scope. Every time the set of visible names potentially changes,
-a new [`Rib`] is pushed onto a stack. The places where this can happen include for
-example:
+To cope with that, the compiler introduces the concept of [`Rib`]s.
+This is an abstraction of a scope.
+Every time the set of visible names potentially changes,
+a new [`Rib`] is pushed onto a stack.
+The places where this can happen include for example:
 
 [`Rib`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/struct.Rib.html
 
@@ -106,11 +121,14 @@ example:
 * Macro expansion border ‒ to cope with macro hygiene.
 
 When searching for a name, the stack of [`ribs`] is traversed from the innermost
-outwards. This helps to find the closest meaning of the name (the one not
-shadowed by anything else). The transition to outer [`Rib`] may also affect
+outwards.
+This helps to find the closest meaning of the name (the one not
+shadowed by anything else).
+The transition to outer [`Rib`] may also affect
 what names are usable ‒ if there are nested functions (not closures),
 the inner one can't access parameters and local bindings of the outer one,
-even though they should be visible by ordinary scoping rules. An example:
+even though they should be visible by ordinary scoping rules.
+An example:
 
 [`ribs`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/struct.LateResolutionVisitor.html#structfield.ribs
 
@@ -139,11 +157,13 @@ blocks), which isn't a full namespace in its own right.
 ## Overall strategy
 
 To perform the name resolution of the whole crate, the syntax tree is traversed
-top-down and every encountered name is resolved. This works for most kinds of
-names, because at the point of use of a name it is already introduced in the [`Rib`]
+top-down and every encountered name is resolved.
+This works for most kinds of names,
+because at the point of use of a name it is already introduced in the [`Rib`]
 hierarchy.
 
-There are some exceptions to this. Items are bit tricky, because they can be
+There are some exceptions to this.
+Items are bit tricky, because they can be
 used even before encountered ‒ therefore every block needs to be first scanned
 for items to fill in its [`Rib`].
 
@@ -156,14 +176,15 @@ Therefore, the resolution is performed in multiple stages.
 ## Speculative crate loading
 
 To give useful errors, rustc suggests importing paths into scope if they're
-not found. How does it do this? It looks through every module of every crate
-and looks for possible matches. This even includes crates that haven't yet
-been loaded!
+not found.
+How does it do this?
+It looks through every module of every crate and looks for possible matches.
+This even includes crates that haven't yet been loaded!
 
 Eagerly loading crates to include import suggestions that haven't yet been
 loaded is called _speculative crate loading_, because any errors it encounters
-shouldn't be reported: [`rustc_resolve`] decided to load them, not the user. The function
-that does this is [`lookup_import_candidates`] and lives in
+shouldn't be reported: [`rustc_resolve`] decided to load them, not the user.
+The function that does this is [`lookup_import_candidates`] and lives in
 [`rustc_resolve::diagnostics`].
 
 [`rustc_resolve`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/index.html
@@ -176,8 +197,9 @@ the load is speculative.
 
 ## TODO: [#16](https://github.com/rust-lang/rustc-dev-guide/issues/16)
 
-This is a result of the first pass of learning the code. It is definitely
-incomplete and not detailed enough. It also might be inaccurate in places.
+This is a result of the first pass of learning the code.
+It is definitely incomplete and not detailed enough.
+It also might be inaccurate in places.
 Still, it probably provides useful first guidepost to what happens in there.
 
 * What exactly does it link to and how is that published and consumed by