Struct rustc::mir::LocalDecl

pub struct LocalDecl<'tcx> {
    pub mutability: Mutability,
    pub is_user_variable: Option<ClearCrossCrate<BindingForm<'tcx>>>,
    pub internal: bool,
    pub ty: Ty<'tcx>,
    pub user_ty: Option<CanonicalTy<'tcx>>,
    pub name: Option<Name>,
    pub source_info: SourceInfo,
    pub visibility_scope: SourceScope,
}

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A MIR local.

This can be a binding declared by the user, a temporary inserted by the compiler, a function argument, or the return place.

Fields

mutability: Mutability

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let mut x vs let x.

Temporaries and the return place are always mutable.

is_user_variable: Option<ClearCrossCrate<BindingForm<'tcx>>>

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Some(binding_mode) if this corresponds to a user-declared local variable.

This is solely used for local diagnostics when generating warnings/errors when compiling the current crate, and therefore it need not be visible across crates. pnkfelix currently hypothesized we need to wrap this in a ClearCrossCrate as long as it carries as HirId.

internal: bool

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True if this is an internal local

These locals are not based on types in the source code and are only used for a few desugarings at the moment.

The generator transformation will sanity check the locals which are live across a suspension point against the type components of the generator which type checking knows are live across a suspension point. We need to flag drop flags to avoid triggering this check as they are introduced after typeck.

Unsafety checking will also ignore dereferences of these locals, so they can be used for raw pointers only used in a desugaring.

This should be sound because the drop flags are fully algebraic, and therefore don't affect the OIBIT or outlives properties of the generator.

ty: Ty<'tcx>

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Type of this local.

user_ty: Option<CanonicalTy<'tcx>>

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If the user manually ascribed a type to this variable, e.g. via let x: T, then we carry that type here. The MIR borrow checker needs this information since it can affect region inference.

name: Option<Name>

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Name of the local, used in debuginfo and pretty-printing.

Note that function arguments can also have this set to Some(_) to generate better debuginfo.

source_info: SourceInfo

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The syntactic (i.e. not visibility) source scope the local is defined in. If the local was defined in a let-statement, this is within the let-statement, rather than outside of it.

This is needed because the visibility source scope of locals within a let-statement is weird.

The reason is that we want the local to be within the let-statement for lint purposes, but we want the local to be after the let-statement for names-in-scope purposes.

That's it, if we have a let-statement like the one in this function:

fn foo(x: &str) {
    #[allow(unused_mut)]
    let mut x: u32 = { // <- one unused mut
        let mut y: u32 = x.parse().unwrap();
        y + 2
    };
    drop(x);
}

Then, from a lint point of view, the declaration of x: u32 (and y: u32) are within the #[allow(unused_mut)] scope - the lint scopes are the same as the AST/HIR nesting.

However, from a name lookup point of view, the scopes look more like as if the let-statements were match expressions:

fn foo(x: &str) {
    match {
        match x.parse().unwrap() {
            y => y + 2
        }
    } {
        x => drop(x)
    };
}

We care about the name-lookup scopes for debuginfo - if the debuginfo instruction pointer is at the call to x.parse(), we want x to refer to x: &str, but if it is at the call to drop(x), we want it to refer to x: u32.

To allow both uses to work, we need to have more than a single scope for a local. We have the source_info.scope represent the "syntactic" lint scope (with a variable being under its let block) while the visibility_scope represents the "local variable" scope (where the "rest" of a block is under all prior let-statements).

The end result looks like this:

ROOT SCOPE
 │{ argument x: &str }
 │
 │ │{ #[allow(unused_mut)] } // this is actually split into 2 scopes
 │ │                        // in practice because I'm lazy.
 │ │
 │ │← x.source_info.scope
 │ │← `x.parse().unwrap()`
 │ │
 │ │ │← y.source_info.scope
 │ │
 │ │ │{ let y: u32 }
 │ │ │
 │ │ │← y.visibility_scope
 │ │ │← `y + 2`
 │
 │ │{ let x: u32 }
 │ │← x.visibility_scope
 │ │← `drop(x)` // this accesses `x: u32`

visibility_scope: SourceScope

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Source scope within which the local is visible (for debuginfo) (see source_info for more details).

Methods

impl<'tcx> LocalDecl<'tcx>

pub fn can_be_made_mutable(&self) -> bool

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Returns true only if local is a binding that can itself be made mutable via the addition of the mut keyword, namely something like the occurrences of x in:

• fn foo(x: Type) { ... },
• let x = ...,
• or match ... { C(x) => ... }

pub fn is_nonref_binding(&self) -> bool

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Returns true if local is definitely not a ref ident or ref mut ident binding. (Such bindings cannot be made into mutable bindings, but the inverse does not necessarily hold).

pub fn new_temp(ty: Ty<'tcx>, span: Span) -> Self

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Create a new LocalDecl for a temporary.

pub fn new_immutable_temp(ty: Ty<'tcx>, span: Span) -> Self

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Create a new immutable LocalDecl for a temporary.

pub fn new_internal(ty: Ty<'tcx>, span: Span) -> Self

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Create a new LocalDecl for a internal temporary.

fn new_local(     ty: Ty<'tcx>,     mutability: Mutability,     internal: bool,     span: Span ) -> Self

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pub fn new_return_place(return_ty: Ty, span: Span) -> LocalDecl

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Builds a LocalDecl for the return place.

This must be inserted into the local_decls list as the first local.

Trait Implementations

impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for LocalDecl<'tcx>

fn hash_stable<W: StableHasherResult>(     &self,     __ctx: &mut StableHashingContext<'a>,     __hasher: &mut StableHasher<W> )

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impl<'tcx> Clone for LocalDecl<'tcx>

fn clone(&self) -> LocalDecl<'tcx>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<'tcx> Debug for LocalDecl<'tcx>

fn fmt(&self, f: &mut Formatter) -> Result

Formats the value using the given formatter.

impl<'tcx> Encodable for LocalDecl<'tcx>

fn encode<__S: Encoder>(&self, s: &mut __S) -> Result<(), __S::Error>

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impl<'tcx> Decodable for LocalDecl<'tcx>

fn decode<__D: Decoder>(d: &mut __D) -> Result<LocalDecl<'tcx>, __D::Error>

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impl<'tcx> TypeFoldable<'tcx> for LocalDecl<'tcx>

fn super_fold_with<'gcx: 'tcx, V: TypeFolder<'gcx, 'tcx>>(     &self,     folder: &mut V ) -> Self

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fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool

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fn fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(     &self,     folder: &mut F ) -> Self

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fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool

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fn has_regions_bound_at_or_above(&self, binder: DebruijnIndex) -> bool

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True if self has any late-bound regions that are either bound by binder or bound by some binder outside of binder. If binder is ty::INNERMOST, this indicates whether there are any late-bound regions that appear free.

fn has_regions_bound_above(&self, binder: DebruijnIndex) -> bool

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True if this self has any regions that escape binder (and hence are not bound by it).

fn has_escaping_regions(&self) -> bool

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fn has_type_flags(&self, flags: TypeFlags) -> bool

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fn has_projections(&self) -> bool

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fn references_error(&self) -> bool

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fn has_param_types(&self) -> bool

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fn has_self_ty(&self) -> bool

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fn has_infer_types(&self) -> bool

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fn needs_infer(&self) -> bool

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fn has_skol(&self) -> bool

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fn needs_subst(&self) -> bool

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fn has_re_skol(&self) -> bool

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fn has_closure_types(&self) -> bool

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fn has_free_regions(&self) -> bool

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"Free" regions in this context means that it has any region that is not (a) erased or (b) late-bound.

fn has_erasable_regions(&self) -> bool

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True if there any any un-erased free regions.

fn is_global(&self) -> bool

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Indicates whether this value references only 'global' types/lifetimes that are the same regardless of what fn we are in. This is used for caching.

fn has_late_bound_regions(&self) -> bool

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True if there are any late-bound regions

fn visit_tys_shallow(     &self,     visit: impl FnMut(Ty<'tcx>) -> bool ) -> bool

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A visitor that does not recurse into types, works like fn walk_shallow in Ty.