1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192

use crate::grammar::parse_tree::{self, Lifetime, TypeParameter};
use crate::grammar::repr;
use std::iter;
use string_cache::DefaultAtom as Atom;

mod test;

/// Finds the set of "free variables" in something -- that is, the
/// type/lifetime parameters that appear and are not bound. For
/// example, `T: Foo<U>` would return `[T, U]`.
pub trait FreeVariables {
    fn free_variables(&self, type_parameters: &[TypeParameter]) -> Vec<TypeParameter>;
}

/// Subtle: the free-variables code sometimes encounter ambiguous
/// names.  For example, we might see `Vec<Foo>` -- in that case, we
/// look at the list of declared type parameters to decide whether
/// `Foo` is a type parameter or just some other type name.
fn free_type(type_parameters: &[TypeParameter], id: &Atom) -> Vec<TypeParameter> {
    let tp = TypeParameter::Id(id.clone());
    if type_parameters.contains(&tp) {
        vec![tp]
    } else {
        vec![]
    }
}

/// Same as above: really, the only lifetime where this is relevant is
/// `'static`, but it doesn't hurt to be careful.
fn free_lifetime(type_parameters: &[TypeParameter], lt: &Lifetime) -> Vec<TypeParameter> {
    let tp = TypeParameter::Lifetime(lt.clone());
    if type_parameters.contains(&tp) {
        vec![tp]
    } else {
        vec![]
    }
}

impl<T: FreeVariables> FreeVariables for Option<T> {
    fn free_variables(&self, type_parameters: &[TypeParameter]) -> Vec<TypeParameter> {
        match self {
            None => vec![],
            Some(t) => t.free_variables(type_parameters),
        }
    }
}

impl<T: FreeVariables> FreeVariables for Vec<T> {
    fn free_variables(&self, type_parameters: &[TypeParameter]) -> Vec<TypeParameter> {
        self.iter()
            .flat_map(|e| e.free_variables(type_parameters))
            .collect()
    }
}

impl FreeVariables for repr::TypeRepr {
    fn free_variables(&self, type_parameters: &[TypeParameter]) -> Vec<TypeParameter> {
        match self {
            repr::TypeRepr::Tuple(tys) => tys.free_variables(type_parameters),
            repr::TypeRepr::Slice(ty) => ty.free_variables(type_parameters),
            repr::TypeRepr::Nominal(data) | repr::TypeRepr::TraitObject(data) => {
                data.free_variables(type_parameters)
            }
            repr::TypeRepr::Associated { type_parameter, .. } => {
                free_type(type_parameters, type_parameter)
            }
            repr::TypeRepr::Lifetime(l) => free_lifetime(type_parameters, l),
            repr::TypeRepr::Ref {
                lifetime, referent, ..
            } => lifetime
                .iter()
                .map(|id| TypeParameter::Lifetime(id.clone()))
                .chain(referent.free_variables(type_parameters))
                .collect(),
            repr::TypeRepr::Fn {
                forall,
                path,
                parameters,
                ret,
            } => path
                .free_variables(type_parameters)
                .into_iter()
                .chain(
                    parameters
                        .iter()
                        .flat_map(|param| param.free_variables(type_parameters)),
                )
                .chain(
                    ret.iter()
                        .flat_map(|ret| ret.free_variables(type_parameters)),
                )
                .filter(|tp| !forall.contains(tp))
                .collect(),
        }
    }
}

impl FreeVariables for repr::WhereClause {
    fn free_variables(&self, type_parameters: &[TypeParameter]) -> Vec<TypeParameter> {
        match self {
            repr::WhereClause::Forall { binder, clause } => clause
                .free_variables(type_parameters)
                .into_iter()
                .filter(|tp| !binder.contains(tp))
                .collect(),

            repr::WhereClause::Bound { subject, bound } => subject
                .free_variables(type_parameters)
                .into_iter()
                .chain(bound.free_variables(type_parameters))
                .collect(),
        }
    }
}

impl FreeVariables for parse_tree::Path {
    fn free_variables(&self, type_parameters: &[TypeParameter]) -> Vec<TypeParameter> {
        // A path like `foo::Bar` is considered no free variables; a
        // single identifier like `T` is a free variable `T`. Note
        // that we can't distinguish type parameters from random names
        // like `String`.
        match self.as_id() {
            Some(id) => free_type(type_parameters, &id),
            None => vec![],
        }
    }
}

impl FreeVariables for repr::NominalTypeRepr {
    fn free_variables(&self, type_parameters: &[TypeParameter]) -> Vec<TypeParameter> {
        let repr::NominalTypeRepr { path, types } = self;
        path.free_variables(type_parameters)
            .into_iter()
            .chain(types.free_variables(type_parameters))
            .collect()
    }
}

impl<T: FreeVariables> FreeVariables for parse_tree::WhereClause<T> {
    fn free_variables(&self, type_parameters: &[TypeParameter]) -> Vec<TypeParameter> {
        match self {
            parse_tree::WhereClause::Lifetime { lifetime, bounds } => {
                iter::once(TypeParameter::Lifetime(lifetime.clone()))
                    .chain(bounds.iter().map(|l| TypeParameter::Lifetime(l.clone())))
                    .collect()
            }

            parse_tree::WhereClause::Type { forall, ty, bounds } => ty
                .free_variables(type_parameters)
                .into_iter()
                .chain(bounds.free_variables(type_parameters))
                .filter(|tp| !forall.contains(tp))
                .collect(),
        }
    }
}

impl<T: FreeVariables> FreeVariables for parse_tree::TypeBoundParameter<T> {
    fn free_variables(&self, type_parameters: &[TypeParameter]) -> Vec<TypeParameter> {
        match self {
            parse_tree::TypeBoundParameter::Lifetime(l) => free_lifetime(type_parameters, l),
            parse_tree::TypeBoundParameter::TypeParameter(t) => t.free_variables(type_parameters),
            parse_tree::TypeBoundParameter::Associated(..) => vec![],
        }
    }
}

impl<T: FreeVariables> FreeVariables for parse_tree::TypeBound<T> {
    fn free_variables(&self, type_parameters: &[TypeParameter]) -> Vec<TypeParameter> {
        match self {
            parse_tree::TypeBound::Lifetime(l) => free_lifetime(type_parameters, l),
            parse_tree::TypeBound::Fn {
                forall,
                parameters,
                ret,
                ..
            } => parameters
                .free_variables(type_parameters)
                .into_iter()
                .chain(ret.free_variables(type_parameters))
                .filter(|tp| !forall.contains(tp))
                .collect(),
            parse_tree::TypeBound::Trait {
                forall, parameters, ..
            } => parameters
                .free_variables(type_parameters)
                .into_iter()
                .filter(|tp| !forall.contains(tp))
                .collect(),
        }
    }
}