Support MarshalText/UnmarshalText for map keys

[gordon-klotho]

This change enables using types with MarshalText/UnmarshalText as keys. This behaviour matches encoding/json and gopkg.in/yaml.v3 (one of, if not the most popular YAML library).

The only potential breaking change is that maps with unsupported key types only fail if there are elements in the map (see updated tests). This behaviour can be restored by adding the validation before iteration, but given the potential overhead, I didn't want to assume it would be desired.

---

Benchstat

goos: linux
goarch: amd64
pkg: github.com/pelletier/go-toml/v2/benchmark
cpu: AMD Ryzen 9 5950X 16-Core Processor
                                  │   old.txt    │              new.txt               │
                                  │    sec/op    │    sec/op     vs base              │
UnmarshalDataset/config-2           18.01m ± ∞ ¹   18.06m ± ∞ ¹       ~ (p=0.841 n=5)
UnmarshalDataset/canada-2           70.13m ± ∞ ¹   70.09m ± ∞ ¹       ~ (p=1.000 n=5)
UnmarshalDataset/citm_catalog-2     24.97m ± ∞ ¹   25.43m ± ∞ ¹       ~ (p=0.151 n=5)
UnmarshalDataset/twitter-2          9.714m ± ∞ ¹   9.404m ± ∞ ¹  -3.18% (p=0.008 n=5)
UnmarshalDataset/code-2             82.25m ± ∞ ¹   81.41m ± ∞ ¹       ~ (p=0.151 n=5)
UnmarshalDataset/example-2          137.6µ ± ∞ ¹   139.2µ ± ∞ ¹       ~ (p=0.056 n=5)
Unmarshal/SimpleDocument/struct-2   499.1n ± ∞ ¹   507.9n ± ∞ ¹       ~ (p=0.310 n=5)
Unmarshal/SimpleDocument/map-2      701.8n ± ∞ ¹   693.3n ± ∞ ¹       ~ (p=0.056 n=5)
Unmarshal/ReferenceFile/struct-2    39.68µ ± ∞ ¹   39.95µ ± ∞ ¹       ~ (p=0.310 n=5)
Unmarshal/ReferenceFile/map-2       60.46µ ± ∞ ¹   60.12µ ± ∞ ¹       ~ (p=0.095 n=5)
Unmarshal/HugoFrontMatter-2         10.61µ ± ∞ ¹   10.59µ ± ∞ ¹       ~ (p=0.690 n=5)
Marshal/SimpleDocument/struct-2     361.9n ± ∞ ¹   365.7n ± ∞ ¹  +1.05% (p=0.008 n=5)
Marshal/SimpleDocument/map-2        498.0n ± ∞ ¹   494.0n ± ∞ ¹  -0.80% (p=0.008 n=5)
Marshal/ReferenceFile/struct-2      29.37µ ± ∞ ¹   29.25µ ± ∞ ¹       ~ (p=0.421 n=5)
Marshal/ReferenceFile/map-2         39.05µ ± ∞ ¹   38.43µ ± ∞ ¹  -1.59% (p=0.008 n=5)
Marshal/HugoFrontMatter-2           7.397µ ± ∞ ¹   7.282µ ± ∞ ¹       ~ (p=0.095 n=5)
geomean                             95.42µ         95.19µ        -0.23%
¹ need >= 6 samples for confidence interval at level 0.95

                                  │    old.txt    │               new.txt               │
                                  │      B/s      │      B/s       vs base              │
UnmarshalDataset/config-2           55.54Mi ± ∞ ¹   55.37Mi ± ∞ ¹       ~ (p=0.841 n=5)
UnmarshalDataset/canada-2           29.94Mi ± ∞ ¹   29.95Mi ± ∞ ¹       ~ (p=1.000 n=5)
UnmarshalDataset/citm_catalog-2     21.31Mi ± ∞ ¹   20.92Mi ± ∞ ¹       ~ (p=0.151 n=5)
UnmarshalDataset/twitter-2          43.38Mi ± ∞ ¹   44.81Mi ± ∞ ¹  +3.30% (p=0.008 n=5)
UnmarshalDataset/code-2             31.12Mi ± ∞ ¹   31.44Mi ± ∞ ¹       ~ (p=0.127 n=5)
UnmarshalDataset/example-2          56.15Mi ± ∞ ¹   55.48Mi ± ∞ ¹  -1.19% (p=0.048 n=5)
Unmarshal/SimpleDocument/struct-2   21.02Mi ± ∞ ¹   20.66Mi ± ∞ ¹       ~ (p=0.310 n=5)
Unmarshal/SimpleDocument/map-2      14.94Mi ± ∞ ¹   15.13Mi ± ∞ ¹       ~ (p=0.056 n=5)
Unmarshal/ReferenceFile/struct-2    126.0Mi ± ∞ ¹   125.1Mi ± ∞ ¹       ~ (p=0.310 n=5)
Unmarshal/ReferenceFile/map-2       82.67Mi ± ∞ ¹   83.14Mi ± ∞ ¹       ~ (p=0.095 n=5)
Unmarshal/HugoFrontMatter-2         49.10Mi ± ∞ ¹   49.15Mi ± ∞ ¹       ~ (p=0.690 n=5)
Marshal/SimpleDocument/struct-2     31.62Mi ± ∞ ¹   31.30Mi ± ∞ ¹  -1.03% (p=0.008 n=5)
Marshal/SimpleDocument/map-2        22.98Mi ± ∞ ¹   23.16Mi ± ∞ ¹  +0.79% (p=0.008 n=5)
Marshal/ReferenceFile/struct-2      66.80Mi ± ∞ ¹   67.06Mi ± ∞ ¹       ~ (p=0.397 n=5)
Marshal/ReferenceFile/map-2         48.98Mi ± ∞ ¹   49.77Mi ± ∞ ¹  +1.62% (p=0.008 n=5)
Marshal/HugoFrontMatter-2           67.42Mi ± ∞ ¹   68.49Mi ± ∞ ¹       ~ (p=0.095 n=5)
geomean                             41.21Mi         41.31Mi        +0.24%
¹ need >= 6 samples for confidence interval at level 0.95

                                  │    old.txt    │                new.txt                │
                                  │     B/op      │     B/op       vs base                │
UnmarshalDataset/config-2           5.502Mi ± ∞ ¹   5.502Mi ± ∞ ¹       ~ (p=1.000 n=5)
UnmarshalDataset/canada-2           76.31Mi ± ∞ ¹   76.31Mi ± ∞ ¹       ~ (p=0.524 n=5)
UnmarshalDataset/citm_catalog-2     32.90Mi ± ∞ ¹   32.90Mi ± ∞ ¹       ~ (p=0.548 n=5)
UnmarshalDataset/twitter-2          12.07Mi ± ∞ ¹   12.07Mi ± ∞ ¹       ~ (p=0.690 n=5)
UnmarshalDataset/code-2             20.29Mi ± ∞ ¹   20.29Mi ± ∞ ¹       ~ (p=0.389 n=5)
UnmarshalDataset/example-2          181.2Ki ± ∞ ¹   181.2Ki ± ∞ ¹       ~ (p=0.056 n=5)
Unmarshal/SimpleDocument/struct-2     805.0 ± ∞ ¹     805.0 ± ∞ ¹       ~ (p=1.000 n=5) ²
Unmarshal/SimpleDocument/map-2      1.106Ki ± ∞ ¹   1.106Ki ± ∞ ¹       ~ (p=1.000 n=5) ²
Unmarshal/ReferenceFile/struct-2    19.99Ki ± ∞ ¹   19.99Ki ± ∞ ¹       ~ (p=1.000 n=5) ²
Unmarshal/ReferenceFile/map-2       36.86Ki ± ∞ ¹   36.86Ki ± ∞ ¹       ~ (p=0.278 n=5)
Unmarshal/HugoFrontMatter-2         7.267Ki ± ∞ ¹   7.267Ki ± ∞ ¹       ~ (p=1.000 n=5) ²
Marshal/SimpleDocument/struct-2       240.0 ± ∞ ¹     240.0 ± ∞ ¹       ~ (p=1.000 n=5) ²
Marshal/SimpleDocument/map-2          272.0 ± ∞ ¹     272.0 ± ∞ ¹       ~ (p=1.000 n=5) ²
Marshal/ReferenceFile/struct-2      27.64Ki ± ∞ ¹   27.64Ki ± ∞ ¹       ~ (p=1.000 n=5) ²
Marshal/ReferenceFile/map-2         27.28Ki ± ∞ ¹   27.28Ki ± ∞ ¹       ~ (p=1.000 n=5) ²
Marshal/HugoFrontMatter-2           5.672Ki ± ∞ ¹   5.672Ki ± ∞ ¹       ~ (p=1.000 n=5) ²
geomean                             74.26Ki         74.26Ki        -0.00%
¹ need >= 6 samples for confidence interval at level 0.95
² all samples are equal

                                  │   old.txt    │               new.txt                │
                                  │  allocs/op   │  allocs/op    vs base                │
UnmarshalDataset/config-2           219.2k ± ∞ ¹   219.2k ± ∞ ¹       ~ (p=1.000 n=5)
UnmarshalDataset/canada-2           670.4k ± ∞ ¹   670.4k ± ∞ ¹       ~ (p=1.000 n=5) ²
UnmarshalDataset/citm_catalog-2     178.0k ± ∞ ¹   178.0k ± ∞ ¹       ~ (p=1.000 n=5)
UnmarshalDataset/twitter-2          54.84k ± ∞ ¹   54.84k ± ∞ ¹       ~ (p=0.802 n=5)
UnmarshalDataset/code-2             1.001M ± ∞ ¹   1.001M ± ∞ ¹       ~ (p=1.000 n=5) ²
UnmarshalDataset/example-2          1.306k ± ∞ ¹   1.306k ± ∞ ¹       ~ (p=1.000 n=5) ²
Unmarshal/SimpleDocument/struct-2    9.000 ± ∞ ¹    9.000 ± ∞ ¹       ~ (p=1.000 n=5) ²
Unmarshal/SimpleDocument/map-2       13.00 ± ∞ ¹    13.00 ± ∞ ¹       ~ (p=1.000 n=5) ²
Unmarshal/ReferenceFile/struct-2     160.0 ± ∞ ¹    160.0 ± ∞ ¹       ~ (p=1.000 n=5) ²
Unmarshal/ReferenceFile/map-2        619.0 ± ∞ ¹    619.0 ± ∞ ¹       ~ (p=1.000 n=5) ²
Unmarshal/HugoFrontMatter-2          141.0 ± ∞ ¹    141.0 ± ∞ ¹       ~ (p=1.000 n=5) ²
Marshal/SimpleDocument/struct-2      8.000 ± ∞ ¹    8.000 ± ∞ ¹       ~ (p=1.000 n=5) ²
Marshal/SimpleDocument/map-2         9.000 ± ∞ ¹    9.000 ± ∞ ¹       ~ (p=1.000 n=5) ²
Marshal/ReferenceFile/struct-2       317.0 ± ∞ ¹    317.0 ± ∞ ¹       ~ (p=1.000 n=5) ²
Marshal/ReferenceFile/map-2          429.0 ± ∞ ¹    429.0 ± ∞ ¹       ~ (p=1.000 n=5) ²
Marshal/HugoFrontMatter-2            85.00 ± ∞ ¹    85.00 ± ∞ ¹       ~ (p=1.000 n=5) ²
geomean                             1.060k         1.060k        +0.00%
¹ need >= 6 samples for confidence interval at level 0.95
² all samples are equal

Coverage

Results: v2 97.1% HEAD 97.1%
Delta: 0

[pelletier]

Looks good to me! I think the type check not being present is fine. If someone reports an issue about it we'll revisit it. Thank you for taking care of this.